Astronomy Picture of the Day

Astronomy Picture of the Day

Discover the cosmos!Each day a different image or photograph of our fascinating universe isfeatured, along with a brief explanation written by a professional astronomer.

2018 September 21

Explanation: Irregular galaxy NGC 55 is thought to be similar to the Large MagellanicCloud (LMC).But whilethe LMC is about 180,000 light-years awayand is a well known satellite of our own Milky Way Galaxy,NGC 55 is more like 6 million light-yearsdistantand is a member of theSculptor Galaxy Group.Classified as anirregular galaxy, indeep exposures the LMC itselfresembles a barred disk galaxy.Spanning about 50,000 light-years, NGC 55 isseen nearly edge-on though,presenting a flattened, narrow profile in contrastwith our face-on view of the LMC.Just as large star forming regions createemission nebulaein the LMC, NGC 55 is alsoseen to beproducing new stars.This highlydetailed galaxy portrait highlights a bright core crossed withdust clouds, telltale pinkish star forming regions, and young blue starclusters in NGC 55.

Authors & editors: Robert Nemiroff(MTU) &Jerry Bonnell (UMCP)NASA Official: Phillip NewmanSpecific rights apply.NASA WebPrivacy Policy and Important NoticesA service of:ASD atNASA /GSFC& Michigan Tech. U.

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Astronomy Picture of the Day

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Since the late 19th century astronomy has expanded to include astrophysics, the application of physical and chemical knowledge to an understanding of the nature of celestial objects and the physical processes that control their formation, evolution, and emission of radiation. In addition, the gases and dust particles around and between the stars have become the subjects of much research. Study of the nuclear reactions that provide the energy radiated by stars has shown how the diversity of atoms found in nature can be derived from a universe that, following the first few minutes of its existence, consisted only of hydrogen, helium, and a trace of lithium. Concerned with phenomena on the largest scale is cosmology, the study of the evolution of the universe. Astrophysics has transformed cosmology from a purely speculative activity to a modern science capable of predictions that can be tested.

Its great advances notwithstanding, astronomy is still subject to a major constraint: it is inherently an observational rather than an experimental science. Almost all measurements must be performed at great distances from the objects of interest, with no control over such quantities as their temperature, pressure, or chemical composition. There are a few exceptions to this limitationnamely, meteorites (most of which are from the asteroid belt, though some are from the Moon or Mars), rock and soil samples brought back from the Moon, samples of comet and asteroid dust returned by robotic spacecraft, and interplanetary dust particles collected in or above the stratosphere. These can be examined with laboratory techniques to provide information that cannot be obtained in any other way. In the future, space missions may return surface materials from Mars, or other objects, but much of astronomy appears otherwise confined to Earth-based observations augmented by observations from orbiting satellites and long-range space probes and supplemented by theory.

The solar system took shape 4.57 billion years ago, when it condensed within a large cloud of gas and dust. Gravitational attraction holds the planets in their elliptical orbits around the Sun. In addition to Earth, five major planets (Mercury, Venus, Mars, Jupiter, and Saturn) have been known from ancient times. Since then only two more have been discovered: Uranus by accident in 1781 and Neptune in 1846 after a deliberate search following a theoretical prediction based on observed irregularities in the orbit of Uranus. Pluto, discovered in 1930 after a search for a planet predicted to lie beyond Neptune, was considered a major planet until 2006, when it was redesignated a dwarf planet by the International Astronomical Union.

The average Earth-Sun distance, which originally defined the astronomical unit (AU), provides a convenient measure for distances within the solar system. The astronomical unit was originally defined by observations of the mean radius of Earths orbit but is now defined as 149,597,870.7 km (about 93 million miles). Mercury, at 0.4 AU, is the closest planet to the Sun, while Neptune, at 30.1 AU, is the farthest. Plutos orbit, with a mean radius of 39.5 AU, is sufficiently eccentric that at times it is closer to the Sun than is Neptune. The planes of the planetary orbits are all within a few degrees of the ecliptic, the plane that contains Earths orbit around the Sun. As viewed from far above Earths North Pole, all planets move in the same (counterclockwise) direction in their orbits.

Most of the mass of the solar system is concentrated in the Sun, with its 1.99 1033 grams. Together, all of the planets amount to 2.7 1030 grams (i.e., about one-thousandth of the Suns mass), and Jupiter alone accounts for 71 percent of this amount. The solar system also contains five known objects of intermediate size classified as dwarf planets and a very large number of much smaller objects collectively called small bodies. The small bodies, roughly in order of decreasing size, are the asteroids, or minor planets; comets, including Kuiper belt, Centaur, and Oort cloud objects; meteoroids; and interplanetary dust particles. Because of their starlike appearance when discovered, the largest of these bodies were termed asteroids, and that name is widely used, but, now that the rocky nature of these bodies is understood, their more descriptive name is minor planets.

The four inner, terrestrial planetsMercury, Venus, Earth, and Marsalong with the Moon have average densities in the range of 3.95.5 grams per cubic cm, setting them apart from the four outer, giant planetsJupiter, Saturn, Uranus, and Neptunewhose densities are all close to 1 gram per cubic cm, the density of water. The compositions of these two groups of planets must therefore be significantly different. This dissimilarity is thought to be attributable to conditions that prevailed during the early development of the solar system (see below Theories of origin). Planetary temperatures now range from around 170 C (330 F, 440 K) on Mercurys surface through the typical 15 C (60 F, 290 K) on Earth to 135 C (210 F, 140 K) on Jupiter near its cloud tops and down to 210 C (350 F, 60 K) near Neptunes cloud tops. These are average temperatures; large variations exist between dayside and nightside for planets closest to the Sun, except for Venus with its thick atmosphere.

The surfaces of the terrestrial planets and many satellites show extensive cratering, produced by high-speed impacts (see meteorite crater). On Earth, with its large quantities of water and an active atmosphere, many of these cosmic footprints have eroded, but remnants of very large craters can be seen in aerial and spacecraft photographs of the terrestrial surface. On Mercury, Mars, and the Moon, the absence of water and any significant atmosphere has left the craters unchanged for billions of years, apart from disturbances produced by infrequent later impacts. Volcanic activity has been an important force in the shaping of the surfaces of the Moon and the terrestrial planets. Seismic activity on the Moon has been monitored by means of seismometers left on its surface by Apollo astronauts and by Lunokhod robotic rovers. Cratering on the largest scale seems to have ceased about three billion years ago, although on the Moon there is clear evidence for a continued cosmic drizzle of small particles, with the larger objects churning (gardening) the lunar surface and the smallest producing microscopic impact pits in crystals in the lunar rocks.

All of the planets apart from the two closest to the Sun (Mercury and Venus) have natural satellites (moons) that are very diverse in appearance, size, and structure, as revealed in close-up observations from long-range space probes. The four outer dwarf planets have moons; Pluto has at least five moons, including one, Charon, fully half the size of Pluto itself. Over 200 asteroids and 80 Kuiper belt objects also have moons. Four planets (Jupiter, Saturn, Uranus, and Neptune), one dwarf planet (Haumea), and one Centaur object (Chariklo) have rings, disklike systems of small rocks and particles that orbit their parent bodies.

During the U.S. Apollo missions a total weight of 381.7 kg (841.5 pounds) of lunar material was collected; an additional 300 grams (0.66 pounds) was brought back by unmanned Soviet Luna vehicles. About 15 percent of the Apollo samples have been distributed for analysis, with the remainder stored at the NASA Johnson Space Center, Houston, Texas. The opportunity to employ a wide range of laboratory techniques on these lunar samples has revolutionized planetary science. The results of the analyses have enabled investigators to determine the composition and age of the lunar surface. Seismic observations have made it possible to probe the lunar interior. In addition, retroreflectors left on the Moons surface by Apollo astronauts have allowed high-power laser beams to be sent from Earth to the Moon and back, permitting scientists to monitor the Earth-Moon distance to an accuracy of a few centimetres. This experiment, which has provided data used in calculations of the dynamics of the Earth-Moon system, has shown that the separation of the two bodies is increasing by 4.4 cm (1.7 inches) each year. (For additional information on lunar studies, see Moon.)

Mercury is too hot to retain an atmosphere, but Venuss brilliant white appearance is the result of its being completely enveloped in thick clouds of carbon dioxide, impenetrable at visible wavelengths. Below the upper clouds, Venus has a hostile atmosphere containing clouds of sulfuric acid droplets. The cloud cover shields the planets surface from direct sunlight, but the energy that does filter through warms the surface, which then radiates at infrared wavelengths. The long-wavelength infrared radiation is trapped by the dense clouds such that an efficient greenhouse effect keeps the surface temperature near 465 C (870 F, 740 K). Radar, which can penetrate the thick Venusian clouds, has been used to map the planets surface. In contrast, the atmosphere of Mars is very thin and is composed mostly of carbon dioxide (95 percent), with very little water vapour; the planets surface pressure is only about 0.006 that of Earth. The outer planets have atmospheres composed largely of light gases, mainly hydrogen and helium.

Each planet rotates on its axis, and nearly all of them rotate in the same directioncounterclockwise as viewed from above the ecliptic. The two exceptions are Venus, which rotates in the clockwise direction beneath its cloud cover, and Uranus, which has its rotation axis very nearly in the plane of the ecliptic.

Some of the planets have magnetic fields. Earths field extends outward until it is disturbed by the solar windan outward flow of protons and electrons from the Sunwhich carries a magnetic field along with it. Through processes not yet fully understood, particles from the solar wind and galactic cosmic rays (high-speed particles from outside the solar system) populate two doughnut-shaped regions called the Van Allen radiation belts. The inner belt extends from about 1,000 to 5,000 km (600 to 3,000 miles) above Earths surface, and the outer from roughly 15,000 to 25,000 km (9,300 to 15,500 miles). In these belts, trapped particles spiral along paths that take them around Earth while bouncing back and forth between the Northern and Southern hemispheres, with their orbits controlled by Earths magnetic field. During periods of increased solar activity, these regions of trapped particles are disturbed, and some of the particles move down into Earths atmosphere, where they collide with atoms and molecules to produce auroras.

Jupiter has a magnetic field far stronger than Earths and many more trapped electrons, whose synchrotron radiation (electromagnetic radiation emitted by high-speed charged particles that are forced to move in curved paths, as under the influence of a magnetic field) is detectable from Earth. Bursts of increased radio emission are correlated with the position of Io, the innermost of the four Galilean moons of Jupiter. Saturn has a magnetic field that is much weaker than Jupiters, but it too has a region of trapped particles. Mercury has a weak magnetic field that is only about 1 percent as strong as Earths and shows no evidence of trapped particles. Uranus and Neptune have fields that are less than one-tenth the strength of Saturns and appear much more complex than that of Earth. No field has been detected around Venus or Mars.

More than 500,000 asteroids with well-established orbits are known, and thousands of additional objects are discovered each year. Hundreds of thousands more have been seen, but their orbits have not been as well determined. It is estimated that several million asteroids exist, but most are small, and their combined mass is estimated to be less than a thousandth that of Earth. Most of the asteroids have orbits close to the ecliptic and move in the asteroid belt, between 2.3 and 3.3 AU from the Sun. Because some asteroids travel in orbits that can bring them close to Earth, there is a possibility of a collision that could have devastating results (see Earth impact hazard).

Comets are considered to come from a vast reservoir, the Oort cloud, orbiting the Sun at distances of 20,00050,000 AU or more and containing trillions of icy objectslatent comet nucleiwith the potential to become active comets. Many comets have been observed over the centuries. Most make only a single pass through the inner solar system, but some are deflected by Jupiter or Saturn into orbits that allow them to return at predictable times. Halleys Comet is the best known of these periodic comets; its next return into the inner solar system is predicted for 2061. Many short-period comets are thought to come from the Kuiper belt, a region lying mainly between 30 AU and 50 AU from the Sunbeyond Neptunes orbit but including part of Plutosand housing perhaps hundreds of millions of comet nuclei. Very few comet masses have been well determined, but most are probably less than 1018 grams, one-billionth the mass of Earth.

Since the 1990s more than a thousand comet nuclei in the Kuiper belt have been observed with large telescopes; a few are about half the size of Pluto, and Pluto is the largest Kuiper belt object. Plutos orbital and physical characteristics had long caused it to be regarded as an anomaly among the planets. However, after the discovery of numerous other Pluto-like objects beyond Neptune, Pluto was seen to be no longer unique in its neighbourhood but rather a giant member of the local population. Consequently, in 2006 astronomers at the general assembly of the International Astronomical Union elected to create the new category of dwarf planets for objects with such qualifications. Pluto, Eris, and Ceres, the latter being the largest member of the asteroid belt, were given this distinction. Two other Kuiper belt objects, Makemake and Haumea, were also designated as dwarf planets.

Smaller than the observed asteroids and comets are the meteoroids, lumps of stony or metallic material believed to be mostly fragments of asteroids. Meteoroids vary from small rocks to boulders weighing a ton or more. A relative few have orbits that bring them into Earths atmosphere and down to the surface as meteorites. Most meteorites that have been collected on Earth are probably from asteroids. A few have been identified as being from the Moon, Mars, or the asteroid Vesta.

Meteorites are classified into three broad groups: stony (chondrites and achondrites; about 94 percent), iron (5 percent), and stony-iron (1 percent). Most meteoroids that enter the atmosphere heat up sufficiently to glow and appear as meteors, and the great majority of these vaporize completely or break up before they reach the surface. Many, perhaps most, meteors occur in showers (see meteor shower) and follow orbits that seem to be identical with those of certain comets, thus pointing to a cometary origin. For example, each May, when Earth crosses the orbit of Halleys Comet, the Eta Aquarid meteor shower occurs. Micrometeorites (interplanetary dust particles), the smallest meteoroidal particles, can be detected from Earth-orbiting satellites or collected by specially equipped aircraft flying in the stratosphere and returned for laboratory inspection. Since the late 1960s numerous meteorites have been found in the Antarctic on the surface of stranded ice flows (see Antarctic meteorites). Some meteorites contain microscopic crystals whose isotopic proportions are unique and appear to be dust grains that formed in the atmospheres of different stars.

The age of the solar system, taken to be close to 4.6 billion years, has been derived from measurements of radioactivity in meteorites, lunar samples, and Earths crust. Abundances of isotopes of uranium, thorium, and rubidium and their decay products, lead and strontium, are the measured quantities.

Assessment of the chemical composition of the solar system is based on data from Earth, the Moon, and meteorites as well as on the spectral analysis of light from the Sun and planets. In broad outline, the solar system abundances of the chemical elements decrease with increasing atomic weight. Hydrogen atoms are by far the most abundant, constituting 91 percent; helium is next, with 8.9 percent; and all other types of atoms together amount to only 0.1 percent.

The origin of Earth, the Moon, and the solar system as a whole is a problem that has not yet been settled in detail. The Sun probably formed by condensation of the central region of a large cloud of gas and dust, with the planets and other bodies of the solar system forming soon after, their composition strongly influenced by the temperature and pressure gradients in the evolving solar nebula. Less-volatile materials could condense into solids relatively close to the Sun to form the terrestrial planets. The abundant, volatile lighter elements could condense only at much greater distances to form the giant gas planets.

In the1990s astronomers confirmed that other stars have one or more planets revolving around them. Studies of these planetary systems have both supported and challenged astronomers theoretical models of how Earths solar system formed. Unlike the solar system, many extrasolar planetary systems have large gas giants like Jupiter orbiting very close to their stars, and in some cases these hot Jupiters are closer to their star than Mercury is to the Sun.

That so many gas giants, which form in the outer regions of their system, end up so close to their stars suggests that gas giants migrate and that such migration may have happened in the solar systems history. According to the Grand Tack hypothesis, Jupiter may have done so within a few million years of the solar systems formation. In this scenario, Jupiter is the first giant planet to form, at about 3 AU from the Sun. Drag from the protoplanetary disk causes it to fall inward to about 1.5 AU. However, by this time, Saturn begins to form at about 3 AU and captures Jupiter in a 3:2 resonance. (That is, for every three revolutions Jupiter makes, Saturn makes two.) The two planets migrate outward and clear away any material that would have gone to making Mars bigger. Mars should be bigger than Venus or Earth, but it is only half their size. The Grand Tack, in which Jupiter moves inward and then outward, explains Marss small size.

About 500 million years after the Grand Tack, according to the Nice Model (named after the French city where it was first proposed), after the four giant planetsJupiter, Saturn, Uranus, and Neptuneformed, they orbited 517 AU from the Sun. These planets were in a disk of smaller bodies called planetesimals and in orbital resonances with each other. About four billion years ago, gravitational interactions with the planetesimals increased the eccentricity of the planets orbits, driving them out of resonance. Saturn, Uranus and Neptune migrated outward, and Jupiter migrated slightly inward. (Uranus and Neptune may even have switched places.) This migration scattered the disk, causing the Late Heavy Bombardment. The final remnant of the disk became the Kuiper belt.

The origin of the planetary satellites is not entirely settled. As to the origin of the Moon, the opinion of astronomers long oscillated between theories that saw its origin and condensation as simultaneous with the formation of Earth and those that posited a separate origin for the Moon and its later capture by Earths gravitational field. Similarities and differences in abundances of the chemical elements and their isotopes on Earth and the Moon challenged each group of theories. Finally, in the 1980s a model emerged that gained the support of most lunar scientiststhat of a large impact on Earth and the expulsion of material that subsequently formed the Moon. (See Moon: Origin and evolution.) For the outer planets, with their multiple satellites, many very small and quite unlike one another, the picture is less clear. Some of these moons have relatively smooth icy surfaces, whereas others are heavily cratered; at least one, Jupiters Io, is volcanic. Some of the moons may have formed along with their parent planets, and others may have formed elsewhere and been captured.

The measurable quantities in stellar astrophysics include the externally observable features of the stars: distance, temperature, radiation spectrum and luminosity, composition (of the outer layers), diameter, mass, and variability in any of these. Theoretical astrophysicists use these observations to model the structure of stars and to devise theories for their formation and evolution. Positional information can be used for dynamical analysis, which yields estimates of stellar masses.

In a system dating back at least to the Greek astronomer-mathematician Hipparchus in the 2nd century bce, apparent stellar brightness (m) is measured in magnitudes. Magnitudes are now defined such that a first-magnitude star is 100 times brighter than a star of sixth magnitude. The human eye cannot see stars fainter than about sixth magnitude, but modern instruments used with large telescopes can record stars as faint as about 30th magnitude. By convention, the absolute magnitude (M) is defined as the magnitude that a star would appear to have if it were located at a standard distance of 10 parsecs. These quantities are related through the expression m M = 5 log10 r 5, in which r is the stars distance in parsecs.

The magnitude scale is anchored on a group of standard stars. An absolute measure of radiant power is luminosity, which is related to the absolute magnitude and usually expressed in ergs per second (ergs/sec). (Sometimes the luminosity is stated in terms of the solar luminosity, 3.86 1033 ergs/sec.) Luminosity can be calculated when m and r are known. Correction might be necessary for the interstellar absorption of starlight.

There are several methods for measuring a stars diameter. From the brightness and distance, the luminosity (L) can be calculated, and, from observations of the brightness at different wavelengths, the temperature (T) can be calculated. Because the radiation from many stars can be well approximated by a Planck blackbody spectrum (see Plancks radiation law), these measured quantities can be related through the expression L = 4R2T4, thus providing a means of calculating R, the stars radius. In this expression, is the Stefan-Boltzmann constant, 5.67 105 ergs/cm2K4sec, in which K is the temperature in kelvins. (The radius R refers to the stars photosphere, the region where the star becomes effectively opaque to outside observation.) Stellar angular diameters can be measured through interferometrythat is, the combining of several telescopes together to form a larger instrument that can resolve sizes smaller than those that an individual telescope can resolve. Alternatively, the intensity of the starlight can be monitored during occultation by the Moon, which produces diffraction fringes whose pattern depends on the angular diameter of the star. Stellar angular diameters of several milliarcseconds can be measured.

Many stars occur in binary systems (see binary star), in which the two partners orbit their mutual centre of mass. Such a system provides the best measurement of stellar masses. The period (P) of a binary system is related to the masses of the two stars (m1 and m2) and the orbital semimajor axis (mean radius; a) via Keplers third law: P2 = 42a3/G(m1 + m2). (G is the universal gravitational constant.) From diameters and masses, average values of the stellar density can be calculated and thence the central pressure. With the assumption of an equation of state, the central temperature can then be calculated. For example, in the Sun the central density is 158 grams per cubic cm; the pressure is calculated to be more than one billion times the pressure of Earths atmosphere at sea level and the temperature around 15 million K (27 million F). At this temperature, all atoms are ionized, and so the solar interior consists of a plasma, an ionized gas with hydrogen nuclei (i.e., protons), helium nuclei, and electrons as major constituents. A small fraction of the hydrogen nuclei possess sufficiently high speeds that, on colliding, their electrostatic repulsion is overcome, resulting in the formation, by means of a set of fusion reactions, of helium nuclei and a release of energy (see proton-proton cycle). Some of this energy is carried away by neutrinos, but most of it is carried by photons to the surface of the Sun to maintain its luminosity.

Other stars, both more and less massive than the Sun, have broadly similar structures, but the size, central pressure and temperature, and fusion rate are functions of the stars mass and composition. The stars and their internal fusion (and resulting luminosity) are held stable against collapse through a delicate balance between the inward pressure produced by gravitational attraction and the outward pressure supplied by the photons produced in the fusion reactions.

Stars that are in this condition of hydrostatic equilibrium are termed main-sequence stars, and they occupy a well-defined band on the Hertzsprung-Russell (H-R) diagram, in which luminosity is plotted against colour index or temperature. Spectral classification, based initially on the colour index, includes the major spectral types O, B, A, F, G, K and M, each subdivided into 10 parts (see star: Stellar spectra). Temperature is deduced from broadband spectral measurements in several standard wavelength intervals. Measurement of apparent magnitudes in two spectral regions, the B and V bands (centred on 4350 and 5550 angstroms, respectively), permits calculation of the colour index, CI = mB mV, from which the temperature can be calculated.

For a given temperature, there are stars that are much more luminous than main-sequence stars. Given the dependence of luminosity on the square of the radius and the fourth power of the temperature (R2T4 of the luminosity expression above), greater luminosity implies larger radius, and such stars are termed giant stars or supergiant stars. Conversely, stars with luminosities much less than those of main-sequence stars of the same temperature must be smaller and are termed white dwarf stars. Surface temperatures of white dwarfs typically range from 10,000 to 12,000 K (18,000 to 21,000 F), and they appear visually as white or blue-white.

The strength of spectral lines of the more abundant elements in a stars atmosphere allows additional subdivisions within a class. Thus, the Sun, a main-sequence star, is classified as G2 V, in which the V denotes main sequence. Betelgeuse, a red giant with a surface temperature about half that of the Sun but with a luminosity of about 10,000 solar units, is classified as M2 Iab. In this classification, the spectral type is M2, and the Iab indicates a giant, well above the main sequence on the H-R diagram.

The range of physically allowable masses for stars is very narrow. If the stars mass is too small, the central temperature will be too low to sustain fusion reactions. The theoretical minimum stellar mass is about 0.08 solar mass. An upper theoretical bound called the Eddington limit, of several hundred solar masses, has been suggested, but this value is not firmly defined. Stars as massive as this will have luminosities about one million times greater than that of the Sun.

A general model of star formation and evolution has been developed, and the major features seem to be established. A large cloud of gas and dust can contract under its own gravitational attraction if its temperature is sufficiently low. As gravitational energy is released, the contracting central material heats up until a point is reached at which the outward radiation pressure balances the inward gravitational pressure, and contraction ceases. Fusion reactions take over as the stars primary source of energy, and the star is then on the main sequence. The time to pass through these formative stages and onto the main sequence is less than 100 million years for a star with as much mass as the Sun. It takes longer for less massive stars and a much shorter time for those much more massive.

Once a star has reached its main-sequence stage, it evolves relatively slowly, fusing hydrogen nuclei in its core to form helium nuclei. Continued fusion not only releases the energy that is radiated but also results in nucleosynthesis, the production of heavier nuclei.

Stellar evolution has of necessity been followed through computer modeling, because the timescales for most stages are generally too extended for measurable changes to be observed, even over a period of many years. One exception is the supernova, the violently explosive finale of certain stars. Different types of supernovas can be distinguished by their spectral lines and by changes in luminosity during and after the outburst. In Type Ia, a white dwarf star attracts matter from a nearby companion; when the white dwarfs mass exceeds about 1.4 solar masses, the star implodes and is completely destroyed. Type II supernovas are not as luminous as Type Ia and are the final evolutionary stage of stars more massive than about eight solar masses. Type Ib and Ic supernovas are like Type II in that they are from the collapse of a massive star, but they do not retain their hydrogen envelope.

The nature of the final products of stellar evolution depends on stellar mass. Some stars pass through an unstable stage in which their dimensions, temperature, and luminosity change cyclically over periods of hours or days. These so-called Cepheid variables serve as standard candles for distance measurements (see above Determining astronomical distances). Some stars blow off their outer layers to produce planetary nebulas. The expanding material can be seen glowing in a thin shell as it disperses into the interstellar medium while the remnant core, initially with a surface temperature as high as 100,000 K (180,000 F), cools to become a white dwarf. The maximum stellar mass that can exist as a white dwarf is about 1.4 solar masses and is known as the Chandrasekhar limit. More-massive stars may end up as either neutron stars or black holes.

The average density of a white dwarf is calculated to exceed one million grams per cubic cm. Further compression is limited by a quantum condition called degeneracy (see degenerate gas), in which only certain energies are allowed for the electrons in the stars interior. Under sufficiently great pressure, the electrons are forced to combine with protons to form neutrons. The resulting neutron star will have a density in the range of 10141015 grams per cubic cm, comparable to the density within atomic nuclei. The behaviour of large masses having nuclear densities is not yet sufficiently understood to be able to set a limit on the maximum size of a neutron star, but it is thought to be less than three solar masses.

Still more-massive remnants of stellar evolution would have smaller dimensions and would be even denser that neutron stars. Such remnants are conceived to be black holes, objects so compact that no radiation can escape from within a characteristic distance called the Schwarzschild radius. This critical dimension is defined by Rs = 2GM/c2. (Rs is the Schwarzschild radius, G is the gravitational constant, M is the objects mass, and c is the speed of light.) For an object of three solar masses, the Schwarzschild radius would be about three kilometres. Radiation emitted from beyond the Schwarzschild radius can still escape and be detected.

Although no light can be detected coming from within a black hole, the presence of a black hole may be manifested through the effects of its gravitational field, as, for example, in a binary star system. If a black hole is paired with a normal visible star, it may pull matter from its companion toward itself. This matter is accelerated as it approaches the black hole and becomes so intensely heated that it radiates large amounts of X-rays from the periphery of the black hole before reaching the Schwarzschild radius. Some candidates for stellar black holes have been founde.g., the X-ray source Cygnus X-1. Each of them has an estimated mass clearly exceeding that allowable for a neutron star, a factor crucial in the identification of possible black holes. (Supermassive black holes that do not originate as individual stars are thought to exist at the centre of active galaxies; see below Study of other galaxies and related phenomena.)

Whereas the existence of stellar black holes has been strongly indicated, the existence of neutron stars was confirmed in 1968 when they were identified with the then newly discovered pulsars, objects characterized by the emission of radiation at short and extremely regular intervals, generally between 1 and 1,000 pulses per second and stable to better than a part per billion. Pulsars are considered to be rotating neutron stars, remnants of some supernovas.

Stars are not distributed randomly throughout space. Many stars are in systems consisting of two or three members separated by less than 1,000 AU. On a larger scale, star clusters may contain many thousands of stars. Galaxies are much larger systems of stars and usually include clouds of gas and dust.

The solar system is located within the Milky Way Galaxy, close to its equatorial plane and about 8 kiloparsecs from the galactic centre. The galactic diameter is about 30 kiloparsecs, as indicated by luminous matter. There is evidence, however, for nonluminous matterso-called dark matterextending out nearly twice this distance. The entire system is rotating such that, at the position of the Sun, the orbital speed is about 220 km per second (almost 500,000 miles per hour) and a complete circuit takes roughly 240 million years. Application of Keplers third law leads to an estimate for the galactic mass of about 100 billion solar masses. The rotational velocity can be measured from the Doppler shifts observed in the 21-cm emission line of neutral hydrogen and the lines of millimetre wavelengths from various molecules, especially carbon monoxide. At great distances from the galactic centre, the rotational velocity does not drop off as expected but rather increases slightly. This behaviour appears to require a much larger galactic mass than can be accounted for by the known (luminous) matter. Additional evidence for the presence of dark matter comes from a variety of other observations. The nature and extent of the dark matter (or missing mass) constitutes one of todays major astronomical puzzles.

There are about 100 billion stars in the Milky Way Galaxy. Star concentrations within the galaxy fall into three types: open clusters, globular clusters, and associations (see star cluster). Open clusters lie primarily in the disk of the galaxy; most contain between 50 and 1,000 stars within a region no more than 10 parsecs in diameter. Stellar associations tend to have somewhat fewer stars; moreover, the constituent stars are not as closely grouped as those in the clusters and are for the most part hotter. Globular clusters, which are widely scattered around the galaxy, may extend up to about 100 parsecs in diameter and may have as many as a million stars. The importance to astronomers of globular clusters lies in their use as indicators of the age of the galaxy. Because massive stars evolve more rapidly than do smaller stars, the age of a cluster can be estimated from its H-R diagram. In a young cluster the main sequence will be well populated, but in an old cluster the heavier stars will have evolved away from the main sequence. The extent of the depopulation of the main sequence provides an index of age. In this way, the oldest globular clusters have been found to be about 12.5 billion years old, which should therefore be the minimum age for the galaxy.

The interstellar medium, composed primarily of gas and dust, occupies the regions between the stars. On average, it contains less than one atom in each cubic centimetre, with about 1 percent of its mass in the form of minute dust grains. The gas, mostly hydrogen, has been mapped by means of its 21-cm emission line. The gas also contains numerous molecules. Some of these have been detected by the visible-wavelength absorption lines that they impose on the spectra of more-distant stars, while others have been identified by their own emission lines at millimetre wavelengths. Many of the interstellar molecules are found in giant molecular clouds, wherein complex organic molecules have been discovered.

In the vicinity of a very hot O- or B-type star, the intensity of ultraviolet radiation is sufficiently high to ionize the surrounding hydrogen out to a distance as great as 100 parsecs to produce an H II region, known as a Strmgren sphere. Such regions are strong and characteristic emitters of radiation at radio wavelengths, and their dimensions are well calibrated in terms of the luminosity of the central star. Using radio interferometers, astronomers are able to measure the angular diameters of H II regions even in some external galaxies and can thereby deduce the great distances to those remote systems. This method can be used for distances up to about 30 megaparsecs. (For additional information on H II regions, see nebula: Diffuse nebulae (H II regions).)

Interstellar dust grains scatter and absorb starlight, the effect being roughly inversely proportional to wavelength from the infrared to the near ultraviolet. As a result, stellar spectra tend to be reddened. Absorption typically amounts to about one magnitude per kiloparsec but varies considerably in different directions. Some dusty regions contain silicate materials, identified by a broad absorption feature around a wavelength of 10 m. Other prominent spectral features in the infrared range have been sometimes, but not conclusively, attributed to graphite grains and polycyclic aromatic hydrocarbons (PAHs).

Starlight often shows a small degree of polarization (a few percent), with the effect increasing with stellar distance. This is attributed to the scattering of the starlight from dust grains that have been partially aligned in a weak interstellar magnetic field. The strength of this field is estimated to be a few microgauss, very close to the strength inferred from observations of nonthermal cosmic radio noise. This radio background has been identified as synchrotron radiation, emitted by cosmic-ray electrons traveling at nearly the speed of light and moving along curved paths in the interstellar magnetic field. The spectrum of the cosmic radio noise is close to what is calculated on the basis of measurements of the cosmic rays near Earth.

Cosmic rays constitute another component of the interstellar medium. Cosmic rays that are detected in the vicinity of Earth comprise high-speed nuclei and electrons. Individual particle energies, expressed in electron volts (eV; 1 eV = 1.6 1012 erg), range with decreasing numbers from about 106 eV to more than 1020 eV. Among the nuclei, hydrogen nuclei are the most plentiful at 86 percent, helium nuclei next at 13 percent, and all other nuclei together at about 1 percent. Electrons are about 2 percent as abundant as the nuclear component. (The relative numbers of different nuclei vary somewhat with kinetic energy, while the electron proportion is strongly energy-dependent.)

A minority of cosmic rays detected in Earths vicinity are produced in the Sun, especially at times of increased solar activity (as indicated by sunspots and solar flares). The origin of galactic cosmic rays has not yet been conclusively identified, but they are thought to be produced in stellar processes such as supernova explosions, perhaps with additional acceleration occurring in the interstellar regions. (For additional information on interstellar matter, see Milky Way Galaxy: The general interstellar medium.)

The central region of the Milky Way Galaxy is so heavily obscured by dust that direct observation has become possible only with the development of astronomy at nonvisual wavelengthsnamely, radio, infrared, and, more recently, X-ray and gamma-ray wavelengths. Together, these observations have revealed a nuclear region of intense activity, with a large number of separate sources of emission and a great deal of dust. Detection of gamma-ray emission at a line energy of 511,000 eV, which corresponds to the annihilation of electrons and positrons (the antimatter counterpart of electrons), along with radio mapping of a region no more than 20 AU across, points to a very compact and energetic source, designated Sagittarius A*, at the centre of the galaxy. Sagittarius A* is a supermassive black hole with a mass equivalent to 4,310,000 Suns.

Galaxies are normally classified into three principal types according to their appearance: spiral, elliptical, and irregular. Galactic diameters are typically in the tens of kiloparsecs and the distances between galaxies typically in megaparsecs.

Spiral galaxiesof which the Milky Way system is a characteristic exampletend to be flattened, roughly circular systems with their constituent stars strongly concentrated along spiral arms. These arms are thought to be produced by traveling density waves, which compress and expand the galactic material. Between the spiral arms exists a diffuse interstellar medium of gas and dust, mostly at very low temperatures (below 100 K [280 F, 170 C]). Spiral galaxies are typically a few kiloparsecs in thickness; they have a central bulge and taper gradually toward the outer edges.

Ellipticals show none of the spiral features but are more densely packed stellar systems. They range in shape from nearly spherical to very flattened and contain little interstellar matter. Irregular galaxies number only a few percent of all stellar systems and exhibit none of the regular features associated with spirals or ellipticals.

Properties vary considerably among the different types of galaxies. Spirals typically have masses in the range of a billion to a trillion solar masses, with ellipticals having values from 10 times smaller to 10 times larger and the irregulars generally 10100 times smaller. Visual galactic luminosities show similar spreads among the three types, but the irregulars tend to be less luminous. In contrast, at radio wavelengths the maximum luminosity for spirals is usually 100,000 times less than for ellipticals or irregulars.

Quasars are objects whose spectra display very large redshifts, thus implying (in accordance with the Hubble law) that they lie at the greatest distances (see above Determining astronomical distances). They were discovered in 1963 but remained enigmatic for many years. They appear as starlike (i.e., very compact) sources of radio waveshence their initial designation as quasi-stellar radio sources, a term later shortened to quasars. They are now considered to be the exceedingly luminous cores of distant galaxies. These energetic cores, which emit copious quantities of X-rays and gamma rays, are termed active galactic nuclei (AGN) and include the object Cygnus A and the nuclei of a class of galaxies called Seyfert galaxies. They may be powered by the infall of matter into supermassive black holes.

The Milky Way Galaxy is one of the Local Group of galaxies, which contains about four dozen members and extends over a volume about two megaparsecs in diameter. Two of the closest members are the Magellanic Clouds, irregular galaxies about 50 kiloparsecs away. At about 740 kiloparsecs, the Andromeda Galaxy is one of the most distant in the Local Group. Some members of the group are moving toward the Milky Way system while others are traveling away from it. At greater distances, all galaxies are moving away from the Milky Way Galaxy. Their speeds (as determined from the redshifted wavelengths in their spectra) are generally proportional to their distances. The Hubble law relates these two quantities (see above Determining astronomical distances). In the absence of any other method, the Hubble law continues to be used for distance determinations to the farthest objectsthat is, galaxies and quasars for which redshifts can be measured.

Cosmology is the scientific study of the universe as a unified whole, from its earliest moments through its evolution to its ultimate fate. The currently accepted cosmological model is the big bang. In this picture, the expansion of the universe started in an intense explosion 13.8 billion years ago. In this primordial fireball, the temperature exceeded one trillion K, and most of the energy was in the form of radiation. As the expansion proceeded (accompanied by cooling), the role of the radiation diminished, and other physical processes dominated in turn. Thus, after about three minutes, the temperature had dropped to the one-billion-K range, making it possible for nuclear reactions of protons to take place and produce nuclei of deuterium and helium. (At the higher temperatures that prevailed earlier, these nuclei would have been promptly disrupted by high-energy photons.) With further expansion, the time between nuclear collisions had increased and the proportion of deuterium and helium nuclei had stabilized. After a few hundred thousand years, the temperature must have dropped sufficiently for electrons to remain attached to nuclei to constitute atoms. Galaxies are thought to have begun forming after a few million years, but this stage is very poorly understood. Star formation probably started much later, after at least a billion years, and the process continues today.

Observational support for this general model comes from several independent directions. The expansion has been documented by the redshifts observed in the spectra of galaxies. Furthermore, the radiation left over from the original fireball would have cooled with the expansion. Confirmation of this relic energy came in 1965 with one of the most striking cosmic discoveries of the 20th centurythe observation, at short radio wavelengths, of a widespread cosmic radiation corresponding to a temperature of almost 3 K (about 270 C [454 F]). The shape of the observed spectrum is an excellent fit with the theoretical Planck blackbody spectrum. (The present best value for this temperature is 2.735 K, but it is still called three-degree radiation or the cosmic microwave background.) The spectrum of this cosmic radio noise peaks at approximately a one-millimetre wavelength, which is in the far infrared, a difficult region to observe from Earth; however, the spectrum has been well mapped by the Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe, and Planck satellites. Additional support for the big bang theory comes from the observed cosmic abundances of deuterium and helium. Normal stellar nucleosynthesis cannot produce their measured quantities, which fit well with calculations of production during the early stages of the big bang.

Early surveys of the cosmic background radiation indicated that it is extremely uniform in all directions (isotropic). Calculations have shown that it is difficult to achieve this degree of isotropy unless there was a very early and rapid inflationary period before the expansion settled into its present mode. Nevertheless, the isotropy posed problems for models of galaxy formation. Galaxies originate from turbulent conditions that produce local fluctuations of density, toward which more matter would then be gravitationally attracted. Such density variations were difficult to reconcile with the isotropy required by observations of the 3 K radiation. This problem was solved when the COBE satellite was able to detect the minute fluctuations in the cosmic background from which the galaxies formed.

The very earliest stages of the big bang are less well understood. The conditions of temperature and pressure that prevailed prior to the first microsecond require the introduction of theoretical ideas of subatomic particle physics. Subatomic particles are usually studied in laboratories with giant accelerators, but the region of particle energies of potential significance to the question at hand lies beyond the range of accelerators currently available. Fortunately, some important conclusions can be drawn from the observed cosmic helium abundance, which is dependent on conditions in the early big bang. The observed helium abundance sets a limit on the number of families of certain types of subatomic particles that can exist.

The age of the universe can be calculated in several ways. Assuming the validity of the big bang model, one attempts to answer the question: How long has the universe been expanding in order to have reached its present size? The numbers relevant to calculating an answer are Hubbles constant (i.e., the current expansion rate), the density of matter in the universe, and the cosmological constant, which allows for change in the expansion rate. In 2003 a calculation based on a fresh determination of Hubbles constant yielded an age of 13.7 billion 200 million years, although the precise value depends on certain assumed details of the model used. Independent estimates of stellar ages have yielded values less than this, as would be expected, but other estimates, based on supernova distance measurements, have arrived at values of about 15 billion years, still consistent, within the errors. In the big bang model the age is proportional to the reciprocal of Hubbles constant, hence the importance of determining H as reliably as possible. For example, a value for H of 100 km/sec/Mpc would lead to an age less than that of many stars, a physically unacceptable result.

A small minority of astronomers have developed alternative cosmological theories that are seriously pursued. The overwhelming professional opinion, however, continues to support the big bang model.

Finally, there is the question of the future behaviour of the universe: Is it open? That is to say, will the expansion continue indefinitely? Or is it closed, such that the expansion will slow down and eventually reverse, resulting in contraction? (The final collapse of such a contracting universe is sometimes termed the big crunch.) The density of the universe seems to be at the critical density; that is, the universe is neither open nor closed but flat. So-called dark energy, a kind of repulsive force that is now believed to be a major component of the universe, appears to be the decisive factor in predictions of the long-term fate of the cosmos. If this energy is a cosmological constant (as proposed in 1917 by Albert Einstein to correct certain problems in his model of the universe), then the result would be a big chill. In this scenario, the universe would continue to expand, but its density would decrease. While old stars would burn out, new stars would no longer form. The universe would become cold and dark. The dark (nonluminous) matter component of the universe, whose composition remains unknown, is not considered sufficient to close the universe and cause it to collapse; it now appears to contribute only a fourth of the density needed for closure.

An additional factor in deciding the fate of the universe might be the mass of neutrinos. For decades the neutrino had been postulated to have zero mass, although there was no compelling theoretical reason for this to be so. From the observation of neutrinos generated in the Sun and other celestial sources such as supernovas, in cosmic-ray interactions with Earths atmosphere, and in particle accelerators, investigators have concluded that neutrinos have some mass, though only an extremely small fraction of the mass of an electron. Although there are vast numbers of neutrinos in the universe, the sum of such small neutrino masses appears insufficient to close the universe.

Read more here:

astronomy | Definition & Facts | Britannica.com

Department of Astronomy – University of Washington

Sarah Tuttle recently joined the UW Astronomy Department as an Assistant Professor, and is head of UWs new Space & Ground Instrumentation Laboratory. In her own words: I am primarily an instrumental astrophysicist working on novel hardware approaches, and build spectrographs to study the physical processes of galaxies. Im interested… Read more

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Department of Astronomy – University of Washington

Bad Astronomy – : Bad Astronomy

This is my last post for the Bad Astronomy Blog on Discover Magazine. As of today Monday, November 12, 2012 the blog has a new home at Slate magazine.

It has been my pleasure and honor to be a Discover blogger for more than four years. Still, I remember my science teacher in third grade quoting Heraclitus to us: “Nothing is permanent except change”. Thats true today, of course, and just as obviously in the Age of the Internet the velocity of that change is accelerating.

But in this case I hope the change isnt too shocking for you, dear BABloggees. All you have to do is switch a URL in your bookmarks or update your RSS feed (to do that, just copy that link address into your feed reader). Ill still be writing the same sort of material, Ill still make dumb puns, and Ill still be Tweeting, Facebooking, and GooglePlussing like mad.

To be clear: all the archives of my blog will be copied to Slate magazine, but will still have a home here at Discover. Id be obliged if you updated links to the new archive, but old links shouldnt break.

And so, I bid a fond adieu to Discover. What I said in my post announcing the move still holds true: I encourage everyone to read the fantastic collection of science blogs that live here, among the best such blogs in the world; fantastic company in which to be. And I hope you follow me to Slate.

Its a big Universe out there, roomy enough for all of us. And theres still a vast amount left to explore and understand.

Thanks.

Folks, its time. And an appropriate time: for my penultimate post here at Discover Magazine, Ive decided to show you my tattoo.

Ive been meaning to post this for a while, but there were a lot of behind-the-scenes issues getting permissions I wont bore you with. But by the time I was able to post this it was so long after I got inked it seemed a little silly. Still, Discover Magazine was the reason I got it, so it seems fair and fitting to post this now. And Ive dyeing to let yall know anyway.

As a brief recap, a few years ago I made a bet with then-Discover Magazine CEO, Henry Donahue: if I got 2 million page views in one month, and the magazine got 5 million total, wed both get tattoos. In March 2009 we did it! So Henry and I went about getting inked.

He got a pretty nifty Celtic fish on his shoulder. For mine, I decided to turn to you, my readers, for suggestions. And they poured in. I narrowed it down to a handful I liked, then made my decision. Henry and I thought it would be fun for me to try to get my tattoo on the TV show “L.A. Ink”, so I applied. They accepted! Discover Magazine generously offered to cover my expenses, and so a little while later I was on my way to Hollywood to get myself some ink.

Thats the basic story. So, without further ado, here it is: my tattoo!

Cool, huh? Its perfect, and just what I wanted! And how appropriate is it to get an asteroid burning up over the Earth? I know, the scales a bit off, but its a tattoo, not a scientific graphic in the Astronomical Journal. And I love the flames and the colors.

The actual clip never wound up getting aired on TLC, but they did create a fully-produced version and put it up on YouTube they have a higher res version on the TLC site. For those of you too lazy to click, here is the YouTube video version:

The first thing to note in the video is that while I seem upbeat I was actually screaming in pain inside my head. The whole thing took just under four hours, and the last quarter of that was where Dan was going over the flames again and again, shading in all the reds and oranges. The pain was, um, astonishing.

Still, I love the end result! If youre looking to get a full-color tattoo, you could do a lot worse than Dan Smith. Hes an excellent artist, and a friendly guy. If I were to get another tattoo which will never ever happen Id want him to do it.

Thank you Henry, thank you Dan, and thank you Discover Magazine for supporting this bit of fun. It was quite a ride, and I have a nice piece of art to show for it thatll last the rest of my life.

Related posts:

Big news: Bad Astronomy is moving to Slate magazine My secret nefarious inky plan revealed We who are about to dye Tat two

The Cascade range of volcanoes is pretty impressive to see from the ground. Stretching from California up to Washington, it includes famous mountains like Saint Helens, Hood, and Rainier. Ive seen many of these while driving in the area, and theyre even cooler from an airplane.

But I have to say, the view from the International Space Station might be best.

[Click to cascadienate.]

This shot was taken from the ISS on September 20, 2012, and shows the region around Mount Shasta, a 4300 meter peak in northern California. Its technically dormant it erupted last in 1786. In geologically recent history its erupted every 600 years or so, but thats not a precise schedule, so geologists keep an eye on it, as they do many of the peaks in the Cascades. As well they should.

To the west of the mountain (to the right in the picture, near the edge) is the much smaller Black Butte. I only point that out because you can see a highway winding around it to the right. Thats I5, a major north-south highway, and a few years back when my family lived in Northern California, I drove it on our way to and back from Oregon. Black Butte was a pretty impressive lava dome, looking exactly what you expect a volcano to look like. And looming in the distance was Shasta, but more standard mountainy looking. That appearance is, of course, quite deceiving.

I love volcanoes, and Im fascinated by them. Im hoping to visit some more very soon.. and Ill have some news about that, I think, in the near future.

Image credit: NASA

Do you like volcano pictures from space too? Heres a bunch of em!

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Speaking of Neil Tyson, if youre a fan of his youll be pleased to know that his show, Star Talk Radio, is now going to be part of the Nerdist Channel network! Thats actually a pretty big deal; Chris Hardwick has created this juggernaut of Nerdist and it reaches a lot of folks.

The new show is essentially a video version of the radio show. Chris interviewed Neil about it for The Nerdist website. If youre curious what itll be like, heres a video of a live Star Talk interview he did with several comedians (Hodgman! Schaal!) and Mike Massamino, a NASA astronaut:

Cool, eh? And maybe Ill have more news about this soon, too. Superman isnt the only guy who walks around in his underwear Neil has talked to.

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I love it when kids get excited enough about science to go out and do something about it. Thats why Im digging Jeffrey Tang whos 10 because he created the Astronomy For Kids podcast, where he talks about different astronomical things. The first podcast went up in February 2012 (“The Solar System”) and hes done others on Stars, the Moon, Saturn, and gravity. Theyre only a few minutes long, perfect for a kid to listen to, and the ones I listened to were accurate and covered the ground pretty well. Theyre also interesting and fun!

If you have a kid who likes science, I bet theyll like this podcast. And I can see these being played in schools, too. Who better to connect with kids than another kid?

[Today is Carl Sagans birthday, celebrated by lovers of science and rationality around the planet. I wrote the following post last year, but I think its still appropriate (and I updated his age). Happy birthday, Carl. Its a darker cosmos without you, but we still walk with the candle you lit for us.]

If Carl Sagan were still alive, hed be 78 years old today. Perhaps he wouldnt have been overly concerned with arbitrary time measurements, especially when based on the fickle way we define a “year”, but its human nature to look back at such integrally-divisible dates and Carl was very much a student of human nature.

Ive written about him so much in the past theres not much I can add right now, so I thought I would simply embed a video for you to watch but which one? Where James Randi eloquently and emotionally talks about his friendship with Carl? Or the wonderful first installment of Symphony of Science using my favorite quote by Carl? Or this amazing speech about how life seeks life?

But in the end, the choice is obvious. Carl Sagans essay, “Pale Blue Dot”, will, I think, stand the test of time, and will deservedly be considered one of the greatest passages ever written in the English language.

Happy birthday to Doctor Carl Sagan, Professor of Astronomy, scientist, skeptic, muse, and though he may not have thought of himself this way poet.

Ill leave you with this, something I wrote abut Carl a while back, when asked about what his greatest legacy is:

Sagans insight, his gift to us, is the knowledge that we all have the ability to examine the Universe with all the power of human curiosity, and we need not retreat from the answers we find.

Of all the amazing pictures returned from the moon by the Lunar Reconnaissance Orbiter and I may include the Apollo landing sites among them I think my favorites are the ones showing boulders that rolled down slopes.

Did I say rolled? I mean bounced!

[Click to enselenate.]

This shot from LRO shows the floor of crater Shuckburgh E, an impact crater about 9 km (~6 miles) across. The image shows a region about 655 meters (0.4 miles) across. The crater floor here is not level; its tilted up from left to right, and also has contours. Boulders dislodged for some reason (a seismic event, or a nearby impact) on the right have rolled down to the left and some actually skipped along, bouncing and bounding as they did.

The two biggest trails are dashed, indicating the boulders had a bit of a rollicking time before coming to rest. You can see both boulders at the left of the trails, where they came to a stop. Note that the sunlight is coming from the bottom of this picture, which can play tricks on perspective. I see the boulders looking almost like craters and the skidding trails they left like little mounds. If you flip the picture over it may look better to you.

As always, pictures like this are a strong reminder that even on the Moon, where time stretches long and processes are slow, changes do occur. Maybe not often, and maybe not recently, but given enough time you have to think of the Moon as a dynamic place.

Image credit: NASA/GSFC/Arizona State University

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Astronomers are discovering a lot of planets these days. The official count is 800+, with thousands of more candidates (unconfirmed but suspiciously planet-like).

Right now we give them alphabet soup names. Alpha Centauri Bb. HR 8799b (through HR8799 e). And of course, everyones favorite, 2MASS J04414489+2301513b.

These catalog names are useful, but less than public friendly. In science fiction we get Vulcan, Psychon, Arrakis, and other cool names. So why not in real life?

The folks at Uwingu asked themselves this very thing. Uwingu (pronounced oo-WIN-goo) is an astronomy and space startup company thats looking to fund scientific research and exploration. I wrote an intro to Uwingu back when it was soliciting funds to get initially rolling (happily, that goal was met). The idea is to sell goods and services to space enthusiasts, and use the proceeds toward doing real science. The folks in charge are professional astronomers and space scientists at the tops of their fields, people like Alan Stern and Pamela Gay. Full disclosure: I am on the Board of Advisors for Uwingu, an unpaid position, but Id write about it and support it anyway. These are top-notch scientists behind the project.

What does this have to do with the letter and number salad that is the current state of exoplanet names? As their first foray, the folks at Uwingu decided to let people create a suggested names list for these planets. For $0.99 a pop, you can submit a name you like to the database, and for another $0.99 you can vote for your favorite in the current list. Ill note these names are not official they are not assigned to specific planets, and only the International Astronomical Union can make these official (and mind you, theyre the ones who so elegantly handled the Pluto not being a planet issue (yes, thats sarcasm)). But, these names will be seen by planetary astronomers, and eventually those planets are going to need names. Why not yours?

I think this is a fun idea. There are currently nearly a hundred names in the database as I write this, but its expected to grow rapidly. If you think there should be a QonoS, Abydos, or even Alderaan in memoriam, of course then head over to Uwingu.

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Well now, this is an interesting discovery: astronomers have found what looks like a “super-Earth” a planet more massive than Earth but still smaller than a gas giant orbiting a nearby star at the right distance to have liquid water on it! Given that, it might might be Earthlike.

This is pretty cool news. Weve found planets like this before, but not very many! And it gets niftier: the planet has at least five siblings, all of which orbit its star closer than it does.

Now let me be clear: this is a planet candidate; it has not yet been confirmed. Reading the journal paper (PDF), though, the data look pretty good. It may yet turn out not to be real, but for the purpose of this blog post Ill just put this caveat here, call it a planet from here on out, and fairly warned be ye, says I.

The star is called HD 40307, and its a bit over 40 light years away (pretty close in galactic standards, but I wouldnt want to walk there). Its a K2.5 dwarf, which means its cooler, dimmer, and smaller than the Sun, but not by much. In other words, its reasonably Sun-like. By coincidence, it appears ot be about the age as the Sun, too: 4.5 billion years. It was observed using HARPS, the High Accuracy Radial Velocity Planet Searcher (I know, it should be HARVPS, but thats harvd to pronounce). This is an extremely sensitive instrument that looks for changes in the starlight as a planet (or planets) orbits a star. The gravity of the star causes the planet to orbit it, but the planet has gravity too. As it circles the star, the star makes a littler circle too (I like to think of it as two kids, one bigger than the other, clasping hands and swinging each other around; the lighter kid makes a big circle and the bigger kid makes a smaller circle). As the star makes its circle, half the time its approaching us and half the time its receding. This means its light is Doppler shifted, the same effect that makes a motorcycle engine drop in pitch as it passes you.

Massive planets tug on their star harder, so theyre easier to find this way. Also, a planet closer in has a shorter orbit, so you dont have to look as long to find it. But in the end, by measuring just how the star is Doppler shifted, you can get the mass and orbital period of the planet. Or planets.

In this case, HD 40307 was originally observed a little while back by HARPS, and three planets were found. But the data are public, so a team of astronomers grabbed it and used a more sensitive method to extract any planetary signatures from the data. They found the three previously-seen planets easily enough, but also found three more! One of them is from a planet that has (at least) seven times the mass of the Earth, and orbits with a 198 day period. Called HD 40307g (planets are named after their host star, with a lower case letter after starting with b), its in the “super-Earth” range: more massive than Earth, but less than, say Neptune (which is 17 times our mass).

We dont know how big the planet is, unfortunately. It might be dense and only a little bigger than Earth, or it could be big and puffy. But if its density and size are just so, it could easily have about the same surface gravity as Earth that is, if you stood on it, youd weight the same as you do now!

But the very interesting thing is that it orbits the star at a distance of about 90 million kilometers (55 million miles) closer to its star than is is to the Sun but thats good! The star is fainter and cooler than the Sun, remember. In fact, at this distance, the planet is right in the stars “habitable zone”, where the temperature is about right for liquid water to exist!

Thats exciting because of the prospect for life. Now, whenever I mention this I hear from people who get all huffy and say that we dont know you need water for life. Thats true, but look around. Water is common on Earth, and here we are. We dont know that you need water for life, but we do know that water is abundant and we need it. We dont know for sure of any other ways for life to form, so it makes sense to look where we understand things best. And that means liquid water.

Heres a diagram of the system as compared to our own:

Note the scales are a bit different, so that the habitable zones of the Sun and of HD 40307 line up better (remember, HD 40307g is actually closer to its star than Earth is to the Sun an AU is the distance of the Earth to the Sun, so HD 40307 is about 0.6 AU from its star). What makes me smile is that the new planet is actually better situated in its “Goldilocks Zone” than Earth is! Thats good news, actually: the orbit may be elliptical (the shape cant be determined from the types of observations made) but still stay entirely in the stars habitable zone.

And take a look at the system: the other planets all orbit closer to the star! We only have two inside Earths orbit in our solar system but all five of HD 40307s planets would fit comfortably inside Mercurys orbit. Amazing.

So this planet if it checks out as being real is one of only a few weve found in the right location for life as we know it. And some of those weve found already are gas giants (though they could have big moons where life could arise). So what this shows us is that the Earth isnt as out of the ordinary as we may have once thought: nature has lots of ways of putting planets the right distances from their stars for life.

Were edging closer all the time to finding that big goal: an Earth-sized, Earth-like planet orbiting a Sun-like star at the right distance for life. This planet is a actually a pretty good fit, but we just dont know enough about it (primarily its size). So Im still waiting. And given the numbers of stars weve observed, and the number of planets we found, as always I have to ask: has Earth II already been observed, and the data just waiting to be uncovered?

Image credits: ESO/M. Kornmesser; Tuomi et al.

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A few people including my pal Deric Hughes put together this non-partisan and nicely done video in honor of democracy:

If you like it, give it a thumbs-up on YouTube and Like it on FB.

And theyre right. As I wrote last night, there is much work to be done. I dont think we can or even should put our differences aside we need them to keep a check on runaway beliefs. But that doesnt mean we cant work together to move things forward.

X

Excerpt from:

Bad Astronomy – : Bad Astronomy

Bad Astronomy – : Bad Astronomy

This is my last post for the Bad Astronomy Blog on Discover Magazine. As of today Monday, November 12, 2012 the blog has a new home at Slate magazine.

It has been my pleasure and honor to be a Discover blogger for more than four years. Still, I remember my science teacher in third grade quoting Heraclitus to us: “Nothing is permanent except change”. Thats true today, of course, and just as obviously in the Age of the Internet the velocity of that change is accelerating.

But in this case I hope the change isnt too shocking for you, dear BABloggees. All you have to do is switch a URL in your bookmarks or update your RSS feed (to do that, just copy that link address into your feed reader). Ill still be writing the same sort of material, Ill still make dumb puns, and Ill still be Tweeting, Facebooking, and GooglePlussing like mad.

To be clear: all the archives of my blog will be copied to Slate magazine, but will still have a home here at Discover. Id be obliged if you updated links to the new archive, but old links shouldnt break.

And so, I bid a fond adieu to Discover. What I said in my post announcing the move still holds true: I encourage everyone to read the fantastic collection of science blogs that live here, among the best such blogs in the world; fantastic company in which to be. And I hope you follow me to Slate.

Its a big Universe out there, roomy enough for all of us. And theres still a vast amount left to explore and understand.

Thanks.

Folks, its time. And an appropriate time: for my penultimate post here at Discover Magazine, Ive decided to show you my tattoo.

Ive been meaning to post this for a while, but there were a lot of behind-the-scenes issues getting permissions I wont bore you with. But by the time I was able to post this it was so long after I got inked it seemed a little silly. Still, Discover Magazine was the reason I got it, so it seems fair and fitting to post this now. And Ive dyeing to let yall know anyway.

As a brief recap, a few years ago I made a bet with then-Discover Magazine CEO, Henry Donahue: if I got 2 million page views in one month, and the magazine got 5 million total, wed both get tattoos. In March 2009 we did it! So Henry and I went about getting inked.

He got a pretty nifty Celtic fish on his shoulder. For mine, I decided to turn to you, my readers, for suggestions. And they poured in. I narrowed it down to a handful I liked, then made my decision. Henry and I thought it would be fun for me to try to get my tattoo on the TV show “L.A. Ink”, so I applied. They accepted! Discover Magazine generously offered to cover my expenses, and so a little while later I was on my way to Hollywood to get myself some ink.

Thats the basic story. So, without further ado, here it is: my tattoo!

Cool, huh? Its perfect, and just what I wanted! And how appropriate is it to get an asteroid burning up over the Earth? I know, the scales a bit off, but its a tattoo, not a scientific graphic in the Astronomical Journal. And I love the flames and the colors.

The actual clip never wound up getting aired on TLC, but they did create a fully-produced version and put it up on YouTube they have a higher res version on the TLC site. For those of you too lazy to click, here is the YouTube video version:

The first thing to note in the video is that while I seem upbeat I was actually screaming in pain inside my head. The whole thing took just under four hours, and the last quarter of that was where Dan was going over the flames again and again, shading in all the reds and oranges. The pain was, um, astonishing.

Still, I love the end result! If youre looking to get a full-color tattoo, you could do a lot worse than Dan Smith. Hes an excellent artist, and a friendly guy. If I were to get another tattoo which will never ever happen Id want him to do it.

Thank you Henry, thank you Dan, and thank you Discover Magazine for supporting this bit of fun. It was quite a ride, and I have a nice piece of art to show for it thatll last the rest of my life.

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The Cascade range of volcanoes is pretty impressive to see from the ground. Stretching from California up to Washington, it includes famous mountains like Saint Helens, Hood, and Rainier. Ive seen many of these while driving in the area, and theyre even cooler from an airplane.

But I have to say, the view from the International Space Station might be best.

[Click to cascadienate.]

This shot was taken from the ISS on September 20, 2012, and shows the region around Mount Shasta, a 4300 meter peak in northern California. Its technically dormant it erupted last in 1786. In geologically recent history its erupted every 600 years or so, but thats not a precise schedule, so geologists keep an eye on it, as they do many of the peaks in the Cascades. As well they should.

To the west of the mountain (to the right in the picture, near the edge) is the much smaller Black Butte. I only point that out because you can see a highway winding around it to the right. Thats I5, a major north-south highway, and a few years back when my family lived in Northern California, I drove it on our way to and back from Oregon. Black Butte was a pretty impressive lava dome, looking exactly what you expect a volcano to look like. And looming in the distance was Shasta, but more standard mountainy looking. That appearance is, of course, quite deceiving.

I love volcanoes, and Im fascinated by them. Im hoping to visit some more very soon.. and Ill have some news about that, I think, in the near future.

Image credit: NASA

Do you like volcano pictures from space too? Heres a bunch of em!

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Speaking of Neil Tyson, if youre a fan of his youll be pleased to know that his show, Star Talk Radio, is now going to be part of the Nerdist Channel network! Thats actually a pretty big deal; Chris Hardwick has created this juggernaut of Nerdist and it reaches a lot of folks.

The new show is essentially a video version of the radio show. Chris interviewed Neil about it for The Nerdist website. If youre curious what itll be like, heres a video of a live Star Talk interview he did with several comedians (Hodgman! Schaal!) and Mike Massamino, a NASA astronaut:

Cool, eh? And maybe Ill have more news about this soon, too. Superman isnt the only guy who walks around in his underwear Neil has talked to.

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I love it when kids get excited enough about science to go out and do something about it. Thats why Im digging Jeffrey Tang whos 10 because he created the Astronomy For Kids podcast, where he talks about different astronomical things. The first podcast went up in February 2012 (“The Solar System”) and hes done others on Stars, the Moon, Saturn, and gravity. Theyre only a few minutes long, perfect for a kid to listen to, and the ones I listened to were accurate and covered the ground pretty well. Theyre also interesting and fun!

If you have a kid who likes science, I bet theyll like this podcast. And I can see these being played in schools, too. Who better to connect with kids than another kid?

[Today is Carl Sagans birthday, celebrated by lovers of science and rationality around the planet. I wrote the following post last year, but I think its still appropriate (and I updated his age). Happy birthday, Carl. Its a darker cosmos without you, but we still walk with the candle you lit for us.]

If Carl Sagan were still alive, hed be 78 years old today. Perhaps he wouldnt have been overly concerned with arbitrary time measurements, especially when based on the fickle way we define a “year”, but its human nature to look back at such integrally-divisible dates and Carl was very much a student of human nature.

Ive written about him so much in the past theres not much I can add right now, so I thought I would simply embed a video for you to watch but which one? Where James Randi eloquently and emotionally talks about his friendship with Carl? Or the wonderful first installment of Symphony of Science using my favorite quote by Carl? Or this amazing speech about how life seeks life?

But in the end, the choice is obvious. Carl Sagans essay, “Pale Blue Dot”, will, I think, stand the test of time, and will deservedly be considered one of the greatest passages ever written in the English language.

Happy birthday to Doctor Carl Sagan, Professor of Astronomy, scientist, skeptic, muse, and though he may not have thought of himself this way poet.

Ill leave you with this, something I wrote abut Carl a while back, when asked about what his greatest legacy is:

Sagans insight, his gift to us, is the knowledge that we all have the ability to examine the Universe with all the power of human curiosity, and we need not retreat from the answers we find.

Of all the amazing pictures returned from the moon by the Lunar Reconnaissance Orbiter and I may include the Apollo landing sites among them I think my favorites are the ones showing boulders that rolled down slopes.

Did I say rolled? I mean bounced!

[Click to enselenate.]

This shot from LRO shows the floor of crater Shuckburgh E, an impact crater about 9 km (~6 miles) across. The image shows a region about 655 meters (0.4 miles) across. The crater floor here is not level; its tilted up from left to right, and also has contours. Boulders dislodged for some reason (a seismic event, or a nearby impact) on the right have rolled down to the left and some actually skipped along, bouncing and bounding as they did.

The two biggest trails are dashed, indicating the boulders had a bit of a rollicking time before coming to rest. You can see both boulders at the left of the trails, where they came to a stop. Note that the sunlight is coming from the bottom of this picture, which can play tricks on perspective. I see the boulders looking almost like craters and the skidding trails they left like little mounds. If you flip the picture over it may look better to you.

As always, pictures like this are a strong reminder that even on the Moon, where time stretches long and processes are slow, changes do occur. Maybe not often, and maybe not recently, but given enough time you have to think of the Moon as a dynamic place.

Image credit: NASA/GSFC/Arizona State University

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Astronomers are discovering a lot of planets these days. The official count is 800+, with thousands of more candidates (unconfirmed but suspiciously planet-like).

Right now we give them alphabet soup names. Alpha Centauri Bb. HR 8799b (through HR8799 e). And of course, everyones favorite, 2MASS J04414489+2301513b.

These catalog names are useful, but less than public friendly. In science fiction we get Vulcan, Psychon, Arrakis, and other cool names. So why not in real life?

The folks at Uwingu asked themselves this very thing. Uwingu (pronounced oo-WIN-goo) is an astronomy and space startup company thats looking to fund scientific research and exploration. I wrote an intro to Uwingu back when it was soliciting funds to get initially rolling (happily, that goal was met). The idea is to sell goods and services to space enthusiasts, and use the proceeds toward doing real science. The folks in charge are professional astronomers and space scientists at the tops of their fields, people like Alan Stern and Pamela Gay. Full disclosure: I am on the Board of Advisors for Uwingu, an unpaid position, but Id write about it and support it anyway. These are top-notch scientists behind the project.

What does this have to do with the letter and number salad that is the current state of exoplanet names? As their first foray, the folks at Uwingu decided to let people create a suggested names list for these planets. For $0.99 a pop, you can submit a name you like to the database, and for another $0.99 you can vote for your favorite in the current list. Ill note these names are not official they are not assigned to specific planets, and only the International Astronomical Union can make these official (and mind you, theyre the ones who so elegantly handled the Pluto not being a planet issue (yes, thats sarcasm)). But, these names will be seen by planetary astronomers, and eventually those planets are going to need names. Why not yours?

I think this is a fun idea. There are currently nearly a hundred names in the database as I write this, but its expected to grow rapidly. If you think there should be a QonoS, Abydos, or even Alderaan in memoriam, of course then head over to Uwingu.

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Well now, this is an interesting discovery: astronomers have found what looks like a “super-Earth” a planet more massive than Earth but still smaller than a gas giant orbiting a nearby star at the right distance to have liquid water on it! Given that, it might might be Earthlike.

This is pretty cool news. Weve found planets like this before, but not very many! And it gets niftier: the planet has at least five siblings, all of which orbit its star closer than it does.

Now let me be clear: this is a planet candidate; it has not yet been confirmed. Reading the journal paper (PDF), though, the data look pretty good. It may yet turn out not to be real, but for the purpose of this blog post Ill just put this caveat here, call it a planet from here on out, and fairly warned be ye, says I.

The star is called HD 40307, and its a bit over 40 light years away (pretty close in galactic standards, but I wouldnt want to walk there). Its a K2.5 dwarf, which means its cooler, dimmer, and smaller than the Sun, but not by much. In other words, its reasonably Sun-like. By coincidence, it appears ot be about the age as the Sun, too: 4.5 billion years. It was observed using HARPS, the High Accuracy Radial Velocity Planet Searcher (I know, it should be HARVPS, but thats harvd to pronounce). This is an extremely sensitive instrument that looks for changes in the starlight as a planet (or planets) orbits a star. The gravity of the star causes the planet to orbit it, but the planet has gravity too. As it circles the star, the star makes a littler circle too (I like to think of it as two kids, one bigger than the other, clasping hands and swinging each other around; the lighter kid makes a big circle and the bigger kid makes a smaller circle). As the star makes its circle, half the time its approaching us and half the time its receding. This means its light is Doppler shifted, the same effect that makes a motorcycle engine drop in pitch as it passes you.

Massive planets tug on their star harder, so theyre easier to find this way. Also, a planet closer in has a shorter orbit, so you dont have to look as long to find it. But in the end, by measuring just how the star is Doppler shifted, you can get the mass and orbital period of the planet. Or planets.

In this case, HD 40307 was originally observed a little while back by HARPS, and three planets were found. But the data are public, so a team of astronomers grabbed it and used a more sensitive method to extract any planetary signatures from the data. They found the three previously-seen planets easily enough, but also found three more! One of them is from a planet that has (at least) seven times the mass of the Earth, and orbits with a 198 day period. Called HD 40307g (planets are named after their host star, with a lower case letter after starting with b), its in the “super-Earth” range: more massive than Earth, but less than, say Neptune (which is 17 times our mass).

We dont know how big the planet is, unfortunately. It might be dense and only a little bigger than Earth, or it could be big and puffy. But if its density and size are just so, it could easily have about the same surface gravity as Earth that is, if you stood on it, youd weight the same as you do now!

But the very interesting thing is that it orbits the star at a distance of about 90 million kilometers (55 million miles) closer to its star than is is to the Sun but thats good! The star is fainter and cooler than the Sun, remember. In fact, at this distance, the planet is right in the stars “habitable zone”, where the temperature is about right for liquid water to exist!

Thats exciting because of the prospect for life. Now, whenever I mention this I hear from people who get all huffy and say that we dont know you need water for life. Thats true, but look around. Water is common on Earth, and here we are. We dont know that you need water for life, but we do know that water is abundant and we need it. We dont know for sure of any other ways for life to form, so it makes sense to look where we understand things best. And that means liquid water.

Heres a diagram of the system as compared to our own:

Note the scales are a bit different, so that the habitable zones of the Sun and of HD 40307 line up better (remember, HD 40307g is actually closer to its star than Earth is to the Sun an AU is the distance of the Earth to the Sun, so HD 40307 is about 0.6 AU from its star). What makes me smile is that the new planet is actually better situated in its “Goldilocks Zone” than Earth is! Thats good news, actually: the orbit may be elliptical (the shape cant be determined from the types of observations made) but still stay entirely in the stars habitable zone.

And take a look at the system: the other planets all orbit closer to the star! We only have two inside Earths orbit in our solar system but all five of HD 40307s planets would fit comfortably inside Mercurys orbit. Amazing.

So this planet if it checks out as being real is one of only a few weve found in the right location for life as we know it. And some of those weve found already are gas giants (though they could have big moons where life could arise). So what this shows us is that the Earth isnt as out of the ordinary as we may have once thought: nature has lots of ways of putting planets the right distances from their stars for life.

Were edging closer all the time to finding that big goal: an Earth-sized, Earth-like planet orbiting a Sun-like star at the right distance for life. This planet is a actually a pretty good fit, but we just dont know enough about it (primarily its size). So Im still waiting. And given the numbers of stars weve observed, and the number of planets we found, as always I have to ask: has Earth II already been observed, and the data just waiting to be uncovered?

Image credits: ESO/M. Kornmesser; Tuomi et al.

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A few people including my pal Deric Hughes put together this non-partisan and nicely done video in honor of democracy:

If you like it, give it a thumbs-up on YouTube and Like it on FB.

And theyre right. As I wrote last night, there is much work to be done. I dont think we can or even should put our differences aside we need them to keep a check on runaway beliefs. But that doesnt mean we cant work together to move things forward.

X

Original post:

Bad Astronomy – : Bad Astronomy

Astronomy Picture of the Day

Astronomy Picture of the Day

Discover the cosmos!Each day a different image or photograph of our fascinating universe isfeatured, along with a brief explanation written by a professional astronomer.

2018 September 4

Explanation: What’s happened to the Moon?Nothing, but something has happened to the image of the Moon.The heat from a volcanic lava fountain in the foreground has warmed and madeturbulent the air nearby, causing passing light to refract differently than usual.The result is a lava plume that appears to be melting the Moon.The featured picture was taken as the full Sturgeon Moon was setting behind Mt. Etna as it erupted in Italy about one week ago.The picture is actually a composite of two images, one taken right after the other, with the same camera and lens. The first image was a quick exposure to capture details of the setting Moon, while the second exposure, taken after the Moon set a few minutes later, was longer so as to capture details of the faint lava jets.From our Earth, we can only see the Sun, Moon, planets, and stars as they appear through the distortion of the Earth’s atmosphere. This distortion can not only change the images of familiar orbs into unusual shapes, it can –unexpectedly at times — delay sunset and moonset by several minutes.

Authors & editors: Robert Nemiroff(MTU) &Jerry Bonnell (UMCP)NASA Official: Phillip NewmanSpecific rights apply.NASA WebPrivacy Policy and Important NoticesA service of:ASD atNASA /GSFC& Michigan Tech. U.

Follow this link:

Astronomy Picture of the Day

astronomy | Definition & Facts | Britannica.com

Since the late 19th century astronomy has expanded to include astrophysics, the application of physical and chemical knowledge to an understanding of the nature of celestial objects and the physical processes that control their formation, evolution, and emission of radiation. In addition, the gases and dust particles around and between the stars have become the subjects of much research. Study of the nuclear reactions that provide the energy radiated by stars has shown how the diversity of atoms found in nature can be derived from a universe that, following the first few minutes of its existence, consisted only of hydrogen, helium, and a trace of lithium. Concerned with phenomena on the largest scale is cosmology, the study of the evolution of the universe. Astrophysics has transformed cosmology from a purely speculative activity to a modern science capable of predictions that can be tested.

Its great advances notwithstanding, astronomy is still subject to a major constraint: it is inherently an observational rather than an experimental science. Almost all measurements must be performed at great distances from the objects of interest, with no control over such quantities as their temperature, pressure, or chemical composition. There are a few exceptions to this limitationnamely, meteorites (most of which are from the asteroid belt, though some are from the Moon or Mars), rock and soil samples brought back from the Moon, samples of comet and asteroid dust returned by robotic spacecraft, and interplanetary dust particles collected in or above the stratosphere. These can be examined with laboratory techniques to provide information that cannot be obtained in any other way. In the future, space missions may return surface materials from Mars, or other objects, but much of astronomy appears otherwise confined to Earth-based observations augmented by observations from orbiting satellites and long-range space probes and supplemented by theory.

The solar system took shape 4.57 billion years ago, when it condensed within a large cloud of gas and dust. Gravitational attraction holds the planets in their elliptical orbits around the Sun. In addition to Earth, five major planets (Mercury, Venus, Mars, Jupiter, and Saturn) have been known from ancient times. Since then only two more have been discovered: Uranus by accident in 1781 and Neptune in 1846 after a deliberate search following a theoretical prediction based on observed irregularities in the orbit of Uranus. Pluto, discovered in 1930 after a search for a planet predicted to lie beyond Neptune, was considered a major planet until 2006, when it was redesignated a dwarf planet by the International Astronomical Union.

The average Earth-Sun distance, which originally defined the astronomical unit (AU), provides a convenient measure for distances within the solar system. The astronomical unit was originally defined by observations of the mean radius of Earths orbit but is now defined as 149,597,870.7 km (about 93 million miles). Mercury, at 0.4 AU, is the closest planet to the Sun, while Neptune, at 30.1 AU, is the farthest. Plutos orbit, with a mean radius of 39.5 AU, is sufficiently eccentric that at times it is closer to the Sun than is Neptune. The planes of the planetary orbits are all within a few degrees of the ecliptic, the plane that contains Earths orbit around the Sun. As viewed from far above Earths North Pole, all planets move in the same (counterclockwise) direction in their orbits.

Most of the mass of the solar system is concentrated in the Sun, with its 1.99 1033 grams. Together, all of the planets amount to 2.7 1030 grams (i.e., about one-thousandth of the Suns mass), and Jupiter alone accounts for 71 percent of this amount. The solar system also contains five known objects of intermediate size classified as dwarf planets and a very large number of much smaller objects collectively called small bodies. The small bodies, roughly in order of decreasing size, are the asteroids, or minor planets; comets, including Kuiper belt, Centaur, and Oort cloud objects; meteoroids; and interplanetary dust particles. Because of their starlike appearance when discovered, the largest of these bodies were termed asteroids, and that name is widely used, but, now that the rocky nature of these bodies is understood, their more descriptive name is minor planets.

The four inner, terrestrial planetsMercury, Venus, Earth, and Marsalong with the Moon have average densities in the range of 3.95.5 grams per cubic cm, setting them apart from the four outer, giant planetsJupiter, Saturn, Uranus, and Neptunewhose densities are all close to 1 gram per cubic cm, the density of water. The compositions of these two groups of planets must therefore be significantly different. This dissimilarity is thought to be attributable to conditions that prevailed during the early development of the solar system (see below Theories of origin). Planetary temperatures now range from around 170 C (330 F, 440 K) on Mercurys surface through the typical 15 C (60 F, 290 K) on Earth to 135 C (210 F, 140 K) on Jupiter near its cloud tops and down to 210 C (350 F, 60 K) near Neptunes cloud tops. These are average temperatures; large variations exist between dayside and nightside for planets closest to the Sun, except for Venus with its thick atmosphere.

The surfaces of the terrestrial planets and many satellites show extensive cratering, produced by high-speed impacts (see meteorite crater). On Earth, with its large quantities of water and an active atmosphere, many of these cosmic footprints have eroded, but remnants of very large craters can be seen in aerial and spacecraft photographs of the terrestrial surface. On Mercury, Mars, and the Moon, the absence of water and any significant atmosphere has left the craters unchanged for billions of years, apart from disturbances produced by infrequent later impacts. Volcanic activity has been an important force in the shaping of the surfaces of the Moon and the terrestrial planets. Seismic activity on the Moon has been monitored by means of seismometers left on its surface by Apollo astronauts and by Lunokhod robotic rovers. Cratering on the largest scale seems to have ceased about three billion years ago, although on the Moon there is clear evidence for a continued cosmic drizzle of small particles, with the larger objects churning (gardening) the lunar surface and the smallest producing microscopic impact pits in crystals in the lunar rocks.

All of the planets apart from the two closest to the Sun (Mercury and Venus) have natural satellites (moons) that are very diverse in appearance, size, and structure, as revealed in close-up observations from long-range space probes. The four outer dwarf planets have moons; Pluto has at least five moons, including one, Charon, fully half the size of Pluto itself. Over 200 asteroids and 80 Kuiper belt objects also have moons. Four planets (Jupiter, Saturn, Uranus, and Neptune), one dwarf planet (Haumea), and one Centaur object (Chariklo) have rings, disklike systems of small rocks and particles that orbit their parent bodies.

During the U.S. Apollo missions a total weight of 381.7 kg (841.5 pounds) of lunar material was collected; an additional 300 grams (0.66 pounds) was brought back by unmanned Soviet Luna vehicles. About 15 percent of the Apollo samples have been distributed for analysis, with the remainder stored at the NASA Johnson Space Center, Houston, Texas. The opportunity to employ a wide range of laboratory techniques on these lunar samples has revolutionized planetary science. The results of the analyses have enabled investigators to determine the composition and age of the lunar surface. Seismic observations have made it possible to probe the lunar interior. In addition, retroreflectors left on the Moons surface by Apollo astronauts have allowed high-power laser beams to be sent from Earth to the Moon and back, permitting scientists to monitor the Earth-Moon distance to an accuracy of a few centimetres. This experiment, which has provided data used in calculations of the dynamics of the Earth-Moon system, has shown that the separation of the two bodies is increasing by 4.4 cm (1.7 inches) each year. (For additional information on lunar studies, see Moon.)

Mercury is too hot to retain an atmosphere, but Venuss brilliant white appearance is the result of its being completely enveloped in thick clouds of carbon dioxide, impenetrable at visible wavelengths. Below the upper clouds, Venus has a hostile atmosphere containing clouds of sulfuric acid droplets. The cloud cover shields the planets surface from direct sunlight, but the energy that does filter through warms the surface, which then radiates at infrared wavelengths. The long-wavelength infrared radiation is trapped by the dense clouds such that an efficient greenhouse effect keeps the surface temperature near 465 C (870 F, 740 K). Radar, which can penetrate the thick Venusian clouds, has been used to map the planets surface. In contrast, the atmosphere of Mars is very thin and is composed mostly of carbon dioxide (95 percent), with very little water vapour; the planets surface pressure is only about 0.006 that of Earth. The outer planets have atmospheres composed largely of light gases, mainly hydrogen and helium.

Each planet rotates on its axis, and nearly all of them rotate in the same directioncounterclockwise as viewed from above the ecliptic. The two exceptions are Venus, which rotates in the clockwise direction beneath its cloud cover, and Uranus, which has its rotation axis very nearly in the plane of the ecliptic.

Some of the planets have magnetic fields. Earths field extends outward until it is disturbed by the solar windan outward flow of protons and electrons from the Sunwhich carries a magnetic field along with it. Through processes not yet fully understood, particles from the solar wind and galactic cosmic rays (high-speed particles from outside the solar system) populate two doughnut-shaped regions called the Van Allen radiation belts. The inner belt extends from about 1,000 to 5,000 km (600 to 3,000 miles) above Earths surface, and the outer from roughly 15,000 to 25,000 km (9,300 to 15,500 miles). In these belts, trapped particles spiral along paths that take them around Earth while bouncing back and forth between the Northern and Southern hemispheres, with their orbits controlled by Earths magnetic field. During periods of increased solar activity, these regions of trapped particles are disturbed, and some of the particles move down into Earths atmosphere, where they collide with atoms and molecules to produce auroras.

Jupiter has a magnetic field far stronger than Earths and many more trapped electrons, whose synchrotron radiation (electromagnetic radiation emitted by high-speed charged particles that are forced to move in curved paths, as under the influence of a magnetic field) is detectable from Earth. Bursts of increased radio emission are correlated with the position of Io, the innermost of the four Galilean moons of Jupiter. Saturn has a magnetic field that is much weaker than Jupiters, but it too has a region of trapped particles. Mercury has a weak magnetic field that is only about 1 percent as strong as Earths and shows no evidence of trapped particles. Uranus and Neptune have fields that are less than one-tenth the strength of Saturns and appear much more complex than that of Earth. No field has been detected around Venus or Mars.

More than 500,000 asteroids with well-established orbits are known, and thousands of additional objects are discovered each year. Hundreds of thousands more have been seen, but their orbits have not been as well determined. It is estimated that several million asteroids exist, but most are small, and their combined mass is estimated to be less than a thousandth that of Earth. Most of the asteroids have orbits close to the ecliptic and move in the asteroid belt, between 2.3 and 3.3 AU from the Sun. Because some asteroids travel in orbits that can bring them close to Earth, there is a possibility of a collision that could have devastating results (see Earth impact hazard).

Comets are considered to come from a vast reservoir, the Oort cloud, orbiting the Sun at distances of 20,00050,000 AU or more and containing trillions of icy objectslatent comet nucleiwith the potential to become active comets. Many comets have been observed over the centuries. Most make only a single pass through the inner solar system, but some are deflected by Jupiter or Saturn into orbits that allow them to return at predictable times. Halleys Comet is the best known of these periodic comets; its next return into the inner solar system is predicted for 2061. Many short-period comets are thought to come from the Kuiper belt, a region lying mainly between 30 AU and 50 AU from the Sunbeyond Neptunes orbit but including part of Plutosand housing perhaps hundreds of millions of comet nuclei. Very few comet masses have been well determined, but most are probably less than 1018 grams, one-billionth the mass of Earth.

Since the 1990s more than a thousand comet nuclei in the Kuiper belt have been observed with large telescopes; a few are about half the size of Pluto, and Pluto is the largest Kuiper belt object. Plutos orbital and physical characteristics had long caused it to be regarded as an anomaly among the planets. However, after the discovery of numerous other Pluto-like objects beyond Neptune, Pluto was seen to be no longer unique in its neighbourhood but rather a giant member of the local population. Consequently, in 2006 astronomers at the general assembly of the International Astronomical Union elected to create the new category of dwarf planets for objects with such qualifications. Pluto, Eris, and Ceres, the latter being the largest member of the asteroid belt, were given this distinction. Two other Kuiper belt objects, Makemake and Haumea, were also designated as dwarf planets.

Smaller than the observed asteroids and comets are the meteoroids, lumps of stony or metallic material believed to be mostly fragments of asteroids. Meteoroids vary from small rocks to boulders weighing a ton or more. A relative few have orbits that bring them into Earths atmosphere and down to the surface as meteorites. Most meteorites that have been collected on Earth are probably from asteroids. A few have been identified as being from the Moon, Mars, or the asteroid Vesta.

Meteorites are classified into three broad groups: stony (chondrites and achondrites; about 94 percent), iron (5 percent), and stony-iron (1 percent). Most meteoroids that enter the atmosphere heat up sufficiently to glow and appear as meteors, and the great majority of these vaporize completely or break up before they reach the surface. Many, perhaps most, meteors occur in showers (see meteor shower) and follow orbits that seem to be identical with those of certain comets, thus pointing to a cometary origin. For example, each May, when Earth crosses the orbit of Halleys Comet, the Eta Aquarid meteor shower occurs. Micrometeorites (interplanetary dust particles), the smallest meteoroidal particles, can be detected from Earth-orbiting satellites or collected by specially equipped aircraft flying in the stratosphere and returned for laboratory inspection. Since the late 1960s numerous meteorites have been found in the Antarctic on the surface of stranded ice flows (see Antarctic meteorites). Some meteorites contain microscopic crystals whose isotopic proportions are unique and appear to be dust grains that formed in the atmospheres of different stars.

The age of the solar system, taken to be close to 4.6 billion years, has been derived from measurements of radioactivity in meteorites, lunar samples, and Earths crust. Abundances of isotopes of uranium, thorium, and rubidium and their decay products, lead and strontium, are the measured quantities.

Assessment of the chemical composition of the solar system is based on data from Earth, the Moon, and meteorites as well as on the spectral analysis of light from the Sun and planets. In broad outline, the solar system abundances of the chemical elements decrease with increasing atomic weight. Hydrogen atoms are by far the most abundant, constituting 91 percent; helium is next, with 8.9 percent; and all other types of atoms together amount to only 0.1 percent.

The origin of Earth, the Moon, and the solar system as a whole is a problem that has not yet been settled in detail. The Sun probably formed by condensation of the central region of a large cloud of gas and dust, with the planets and other bodies of the solar system forming soon after, their composition strongly influenced by the temperature and pressure gradients in the evolving solar nebula. Less-volatile materials could condense into solids relatively close to the Sun to form the terrestrial planets. The abundant, volatile lighter elements could condense only at much greater distances to form the giant gas planets.

In the1990s astronomers confirmed that other stars have one or more planets revolving around them. Studies of these planetary systems have both supported and challenged astronomers theoretical models of how Earths solar system formed. Unlike the solar system, many extrasolar planetary systems have large gas giants like Jupiter orbiting very close to their stars, and in some cases these hot Jupiters are closer to their star than Mercury is to the Sun.

That so many gas giants, which form in the outer regions of their system, end up so close to their stars suggests that gas giants migrate and that such migration may have happened in the solar systems history. According to the Grand Tack hypothesis, Jupiter may have done so within a few million years of the solar systems formation. In this scenario, Jupiter is the first giant planet to form, at about 3 AU from the Sun. Drag from the protoplanetary disk causes it to fall inward to about 1.5 AU. However, by this time, Saturn begins to form at about 3 AU and captures Jupiter in a 3:2 resonance. (That is, for every three revolutions Jupiter makes, Saturn makes two.) The two planets migrate outward and clear away any material that would have gone to making Mars bigger. Mars should be bigger than Venus or Earth, but it is only half their size. The Grand Tack, in which Jupiter moves inward and then outward, explains Marss small size.

About 500 million years after the Grand Tack, according to the Nice Model (named after the French city where it was first proposed), after the four giant planetsJupiter, Saturn, Uranus, and Neptuneformed, they orbited 517 AU from the Sun. These planets were in a disk of smaller bodies called planetesimals and in orbital resonances with each other. About four billion years ago, gravitational interactions with the planetesimals increased the eccentricity of the planets orbits, driving them out of resonance. Saturn, Uranus and Neptune migrated outward, and Jupiter migrated slightly inward. (Uranus and Neptune may even have switched places.) This migration scattered the disk, causing the Late Heavy Bombardment. The final remnant of the disk became the Kuiper belt.

The origin of the planetary satellites is not entirely settled. As to the origin of the Moon, the opinion of astronomers long oscillated between theories that saw its origin and condensation as simultaneous with the formation of Earth and those that posited a separate origin for the Moon and its later capture by Earths gravitational field. Similarities and differences in abundances of the chemical elements and their isotopes on Earth and the Moon challenged each group of theories. Finally, in the 1980s a model emerged that gained the support of most lunar scientiststhat of a large impact on Earth and the expulsion of material that subsequently formed the Moon. (See Moon: Origin and evolution.) For the outer planets, with their multiple satellites, many very small and quite unlike one another, the picture is less clear. Some of these moons have relatively smooth icy surfaces, whereas others are heavily cratered; at least one, Jupiters Io, is volcanic. Some of the moons may have formed along with their parent planets, and others may have formed elsewhere and been captured.

The measurable quantities in stellar astrophysics include the externally observable features of the stars: distance, temperature, radiation spectrum and luminosity, composition (of the outer layers), diameter, mass, and variability in any of these. Theoretical astrophysicists use these observations to model the structure of stars and to devise theories for their formation and evolution. Positional information can be used for dynamical analysis, which yields estimates of stellar masses.

In a system dating back at least to the Greek astronomer-mathematician Hipparchus in the 2nd century bce, apparent stellar brightness (m) is measured in magnitudes. Magnitudes are now defined such that a first-magnitude star is 100 times brighter than a star of sixth magnitude. The human eye cannot see stars fainter than about sixth magnitude, but modern instruments used with large telescopes can record stars as faint as about 30th magnitude. By convention, the absolute magnitude (M) is defined as the magnitude that a star would appear to have if it were located at a standard distance of 10 parsecs. These quantities are related through the expression m M = 5 log10 r 5, in which r is the stars distance in parsecs.

The magnitude scale is anchored on a group of standard stars. An absolute measure of radiant power is luminosity, which is related to the absolute magnitude and usually expressed in ergs per second (ergs/sec). (Sometimes the luminosity is stated in terms of the solar luminosity, 3.86 1033 ergs/sec.) Luminosity can be calculated when m and r are known. Correction might be necessary for the interstellar absorption of starlight.

There are several methods for measuring a stars diameter. From the brightness and distance, the luminosity (L) can be calculated, and, from observations of the brightness at different wavelengths, the temperature (T) can be calculated. Because the radiation from many stars can be well approximated by a Planck blackbody spectrum (see Plancks radiation law), these measured quantities can be related through the expression L = 4R2T4, thus providing a means of calculating R, the stars radius. In this expression, is the Stefan-Boltzmann constant, 5.67 105 ergs/cm2K4sec, in which K is the temperature in kelvins. (The radius R refers to the stars photosphere, the region where the star becomes effectively opaque to outside observation.) Stellar angular diameters can be measured through interferometrythat is, the combining of several telescopes together to form a larger instrument that can resolve sizes smaller than those that an individual telescope can resolve. Alternatively, the intensity of the starlight can be monitored during occultation by the Moon, which produces diffraction fringes whose pattern depends on the angular diameter of the star. Stellar angular diameters of several milliarcseconds can be measured.

Many stars occur in binary systems (see binary star), in which the two partners orbit their mutual centre of mass. Such a system provides the best measurement of stellar masses. The period (P) of a binary system is related to the masses of the two stars (m1 and m2) and the orbital semimajor axis (mean radius; a) via Keplers third law: P2 = 42a3/G(m1 + m2). (G is the universal gravitational constant.) From diameters and masses, average values of the stellar density can be calculated and thence the central pressure. With the assumption of an equation of state, the central temperature can then be calculated. For example, in the Sun the central density is 158 grams per cubic cm; the pressure is calculated to be more than one billion times the pressure of Earths atmosphere at sea level and the temperature around 15 million K (27 million F). At this temperature, all atoms are ionized, and so the solar interior consists of a plasma, an ionized gas with hydrogen nuclei (i.e., protons), helium nuclei, and electrons as major constituents. A small fraction of the hydrogen nuclei possess sufficiently high speeds that, on colliding, their electrostatic repulsion is overcome, resulting in the formation, by means of a set of fusion reactions, of helium nuclei and a release of energy (see proton-proton cycle). Some of this energy is carried away by neutrinos, but most of it is carried by photons to the surface of the Sun to maintain its luminosity.

Other stars, both more and less massive than the Sun, have broadly similar structures, but the size, central pressure and temperature, and fusion rate are functions of the stars mass and composition. The stars and their internal fusion (and resulting luminosity) are held stable against collapse through a delicate balance between the inward pressure produced by gravitational attraction and the outward pressure supplied by the photons produced in the fusion reactions.

Stars that are in this condition of hydrostatic equilibrium are termed main-sequence stars, and they occupy a well-defined band on the Hertzsprung-Russell (H-R) diagram, in which luminosity is plotted against colour index or temperature. Spectral classification, based initially on the colour index, includes the major spectral types O, B, A, F, G, K and M, each subdivided into 10 parts (see star: Stellar spectra). Temperature is deduced from broadband spectral measurements in several standard wavelength intervals. Measurement of apparent magnitudes in two spectral regions, the B and V bands (centred on 4350 and 5550 angstroms, respectively), permits calculation of the colour index, CI = mB mV, from which the temperature can be calculated.

For a given temperature, there are stars that are much more luminous than main-sequence stars. Given the dependence of luminosity on the square of the radius and the fourth power of the temperature (R2T4 of the luminosity expression above), greater luminosity implies larger radius, and such stars are termed giant stars or supergiant stars. Conversely, stars with luminosities much less than those of main-sequence stars of the same temperature must be smaller and are termed white dwarf stars. Surface temperatures of white dwarfs typically range from 10,000 to 12,000 K (18,000 to 21,000 F), and they appear visually as white or blue-white.

The strength of spectral lines of the more abundant elements in a stars atmosphere allows additional subdivisions within a class. Thus, the Sun, a main-sequence star, is classified as G2 V, in which the V denotes main sequence. Betelgeuse, a red giant with a surface temperature about half that of the Sun but with a luminosity of about 10,000 solar units, is classified as M2 Iab. In this classification, the spectral type is M2, and the Iab indicates a giant, well above the main sequence on the H-R diagram.

The range of physically allowable masses for stars is very narrow. If the stars mass is too small, the central temperature will be too low to sustain fusion reactions. The theoretical minimum stellar mass is about 0.08 solar mass. An upper theoretical bound called the Eddington limit, of several hundred solar masses, has been suggested, but this value is not firmly defined. Stars as massive as this will have luminosities about one million times greater than that of the Sun.

A general model of star formation and evolution has been developed, and the major features seem to be established. A large cloud of gas and dust can contract under its own gravitational attraction if its temperature is sufficiently low. As gravitational energy is released, the contracting central material heats up until a point is reached at which the outward radiation pressure balances the inward gravitational pressure, and contraction ceases. Fusion reactions take over as the stars primary source of energy, and the star is then on the main sequence. The time to pass through these formative stages and onto the main sequence is less than 100 million years for a star with as much mass as the Sun. It takes longer for less massive stars and a much shorter time for those much more massive.

Once a star has reached its main-sequence stage, it evolves relatively slowly, fusing hydrogen nuclei in its core to form helium nuclei. Continued fusion not only releases the energy that is radiated but also results in nucleosynthesis, the production of heavier nuclei.

Stellar evolution has of necessity been followed through computer modeling, because the timescales for most stages are generally too extended for measurable changes to be observed, even over a period of many years. One exception is the supernova, the violently explosive finale of certain stars. Different types of supernovas can be distinguished by their spectral lines and by changes in luminosity during and after the outburst. In Type Ia, a white dwarf star attracts matter from a nearby companion; when the white dwarfs mass exceeds about 1.4 solar masses, the star implodes and is completely destroyed. Type II supernovas are not as luminous as Type Ia and are the final evolutionary stage of stars more massive than about eight solar masses. Type Ib and Ic supernovas are like Type II in that they are from the collapse of a massive star, but they do not retain their hydrogen envelope.

The nature of the final products of stellar evolution depends on stellar mass. Some stars pass through an unstable stage in which their dimensions, temperature, and luminosity change cyclically over periods of hours or days. These so-called Cepheid variables serve as standard candles for distance measurements (see above Determining astronomical distances). Some stars blow off their outer layers to produce planetary nebulas. The expanding material can be seen glowing in a thin shell as it disperses into the interstellar medium while the remnant core, initially with a surface temperature as high as 100,000 K (180,000 F), cools to become a white dwarf. The maximum stellar mass that can exist as a white dwarf is about 1.4 solar masses and is known as the Chandrasekhar limit. More-massive stars may end up as either neutron stars or black holes.

The average density of a white dwarf is calculated to exceed one million grams per cubic cm. Further compression is limited by a quantum condition called degeneracy (see degenerate gas), in which only certain energies are allowed for the electrons in the stars interior. Under sufficiently great pressure, the electrons are forced to combine with protons to form neutrons. The resulting neutron star will have a density in the range of 10141015 grams per cubic cm, comparable to the density within atomic nuclei. The behaviour of large masses having nuclear densities is not yet sufficiently understood to be able to set a limit on the maximum size of a neutron star, but it is thought to be less than three solar masses.

Still more-massive remnants of stellar evolution would have smaller dimensions and would be even denser that neutron stars. Such remnants are conceived to be black holes, objects so compact that no radiation can escape from within a characteristic distance called the Schwarzschild radius. This critical dimension is defined by Rs = 2GM/c2. (Rs is the Schwarzschild radius, G is the gravitational constant, M is the objects mass, and c is the speed of light.) For an object of three solar masses, the Schwarzschild radius would be about three kilometres. Radiation emitted from beyond the Schwarzschild radius can still escape and be detected.

Although no light can be detected coming from within a black hole, the presence of a black hole may be manifested through the effects of its gravitational field, as, for example, in a binary star system. If a black hole is paired with a normal visible star, it may pull matter from its companion toward itself. This matter is accelerated as it approaches the black hole and becomes so intensely heated that it radiates large amounts of X-rays from the periphery of the black hole before reaching the Schwarzschild radius. Some candidates for stellar black holes have been founde.g., the X-ray source Cygnus X-1. Each of them has an estimated mass clearly exceeding that allowable for a neutron star, a factor crucial in the identification of possible black holes. (Supermassive black holes that do not originate as individual stars are thought to exist at the centre of active galaxies; see below Study of other galaxies and related phenomena.)

Whereas the existence of stellar black holes has been strongly indicated, the existence of neutron stars was confirmed in 1968 when they were identified with the then newly discovered pulsars, objects characterized by the emission of radiation at short and extremely regular intervals, generally between 1 and 1,000 pulses per second and stable to better than a part per billion. Pulsars are considered to be rotating neutron stars, remnants of some supernovas.

Stars are not distributed randomly throughout space. Many stars are in systems consisting of two or three members separated by less than 1,000 AU. On a larger scale, star clusters may contain many thousands of stars. Galaxies are much larger systems of stars and usually include clouds of gas and dust.

The solar system is located within the Milky Way Galaxy, close to its equatorial plane and about 8 kiloparsecs from the galactic centre. The galactic diameter is about 30 kiloparsecs, as indicated by luminous matter. There is evidence, however, for nonluminous matterso-called dark matterextending out nearly twice this distance. The entire system is rotating such that, at the position of the Sun, the orbital speed is about 220 km per second (almost 500,000 miles per hour) and a complete circuit takes roughly 240 million years. Application of Keplers third law leads to an estimate for the galactic mass of about 100 billion solar masses. The rotational velocity can be measured from the Doppler shifts observed in the 21-cm emission line of neutral hydrogen and the lines of millimetre wavelengths from various molecules, especially carbon monoxide. At great distances from the galactic centre, the rotational velocity does not drop off as expected but rather increases slightly. This behaviour appears to require a much larger galactic mass than can be accounted for by the known (luminous) matter. Additional evidence for the presence of dark matter comes from a variety of other observations. The nature and extent of the dark matter (or missing mass) constitutes one of todays major astronomical puzzles.

There are about 100 billion stars in the Milky Way Galaxy. Star concentrations within the galaxy fall into three types: open clusters, globular clusters, and associations (see star cluster). Open clusters lie primarily in the disk of the galaxy; most contain between 50 and 1,000 stars within a region no more than 10 parsecs in diameter. Stellar associations tend to have somewhat fewer stars; moreover, the constituent stars are not as closely grouped as those in the clusters and are for the most part hotter. Globular clusters, which are widely scattered around the galaxy, may extend up to about 100 parsecs in diameter and may have as many as a million stars. The importance to astronomers of globular clusters lies in their use as indicators of the age of the galaxy. Because massive stars evolve more rapidly than do smaller stars, the age of a cluster can be estimated from its H-R diagram. In a young cluster the main sequence will be well populated, but in an old cluster the heavier stars will have evolved away from the main sequence. The extent of the depopulation of the main sequence provides an index of age. In this way, the oldest globular clusters have been found to be about 12.5 billion years old, which should therefore be the minimum age for the galaxy.

The interstellar medium, composed primarily of gas and dust, occupies the regions between the stars. On average, it contains less than one atom in each cubic centimetre, with about 1 percent of its mass in the form of minute dust grains. The gas, mostly hydrogen, has been mapped by means of its 21-cm emission line. The gas also contains numerous molecules. Some of these have been detected by the visible-wavelength absorption lines that they impose on the spectra of more-distant stars, while others have been identified by their own emission lines at millimetre wavelengths. Many of the interstellar molecules are found in giant molecular clouds, wherein complex organic molecules have been discovered.

In the vicinity of a very hot O- or B-type star, the intensity of ultraviolet radiation is sufficiently high to ionize the surrounding hydrogen out to a distance as great as 100 parsecs to produce an H II region, known as a Strmgren sphere. Such regions are strong and characteristic emitters of radiation at radio wavelengths, and their dimensions are well calibrated in terms of the luminosity of the central star. Using radio interferometers, astronomers are able to measure the angular diameters of H II regions even in some external galaxies and can thereby deduce the great distances to those remote systems. This method can be used for distances up to about 30 megaparsecs. (For additional information on H II regions, see nebula: Diffuse nebulae (H II regions).)

Interstellar dust grains scatter and absorb starlight, the effect being roughly inversely proportional to wavelength from the infrared to the near ultraviolet. As a result, stellar spectra tend to be reddened. Absorption typically amounts to about one magnitude per kiloparsec but varies considerably in different directions. Some dusty regions contain silicate materials, identified by a broad absorption feature around a wavelength of 10 m. Other prominent spectral features in the infrared range have been sometimes, but not conclusively, attributed to graphite grains and polycyclic aromatic hydrocarbons (PAHs).

Starlight often shows a small degree of polarization (a few percent), with the effect increasing with stellar distance. This is attributed to the scattering of the starlight from dust grains that have been partially aligned in a weak interstellar magnetic field. The strength of this field is estimated to be a few microgauss, very close to the strength inferred from observations of nonthermal cosmic radio noise. This radio background has been identified as synchrotron radiation, emitted by cosmic-ray electrons traveling at nearly the speed of light and moving along curved paths in the interstellar magnetic field. The spectrum of the cosmic radio noise is close to what is calculated on the basis of measurements of the cosmic rays near Earth.

Cosmic rays constitute another component of the interstellar medium. Cosmic rays that are detected in the vicinity of Earth comprise high-speed nuclei and electrons. Individual particle energies, expressed in electron volts (eV; 1 eV = 1.6 1012 erg), range with decreasing numbers from about 106 eV to more than 1020 eV. Among the nuclei, hydrogen nuclei are the most plentiful at 86 percent, helium nuclei next at 13 percent, and all other nuclei together at about 1 percent. Electrons are about 2 percent as abundant as the nuclear component. (The relative numbers of different nuclei vary somewhat with kinetic energy, while the electron proportion is strongly energy-dependent.)

A minority of cosmic rays detected in Earths vicinity are produced in the Sun, especially at times of increased solar activity (as indicated by sunspots and solar flares). The origin of galactic cosmic rays has not yet been conclusively identified, but they are thought to be produced in stellar processes such as supernova explosions, perhaps with additional acceleration occurring in the interstellar regions. (For additional information on interstellar matter, see Milky Way Galaxy: The general interstellar medium.)

The central region of the Milky Way Galaxy is so heavily obscured by dust that direct observation has become possible only with the development of astronomy at nonvisual wavelengthsnamely, radio, infrared, and, more recently, X-ray and gamma-ray wavelengths. Together, these observations have revealed a nuclear region of intense activity, with a large number of separate sources of emission and a great deal of dust. Detection of gamma-ray emission at a line energy of 511,000 eV, which corresponds to the annihilation of electrons and positrons (the antimatter counterpart of electrons), along with radio mapping of a region no more than 20 AU across, points to a very compact and energetic source, designated Sagittarius A*, at the centre of the galaxy. Sagittarius A* is a supermassive black hole with a mass equivalent to 4,310,000 Suns.

Galaxies are normally classified into three principal types according to their appearance: spiral, elliptical, and irregular. Galactic diameters are typically in the tens of kiloparsecs and the distances between galaxies typically in megaparsecs.

Spiral galaxiesof which the Milky Way system is a characteristic exampletend to be flattened, roughly circular systems with their constituent stars strongly concentrated along spiral arms. These arms are thought to be produced by traveling density waves, which compress and expand the galactic material. Between the spiral arms exists a diffuse interstellar medium of gas and dust, mostly at very low temperatures (below 100 K [280 F, 170 C]). Spiral galaxies are typically a few kiloparsecs in thickness; they have a central bulge and taper gradually toward the outer edges.

Ellipticals show none of the spiral features but are more densely packed stellar systems. They range in shape from nearly spherical to very flattened and contain little interstellar matter. Irregular galaxies number only a few percent of all stellar systems and exhibit none of the regular features associated with spirals or ellipticals.

Properties vary considerably among the different types of galaxies. Spirals typically have masses in the range of a billion to a trillion solar masses, with ellipticals having values from 10 times smaller to 10 times larger and the irregulars generally 10100 times smaller. Visual galactic luminosities show similar spreads among the three types, but the irregulars tend to be less luminous. In contrast, at radio wavelengths the maximum luminosity for spirals is usually 100,000 times less than for ellipticals or irregulars.

Quasars are objects whose spectra display very large redshifts, thus implying (in accordance with the Hubble law) that they lie at the greatest distances (see above Determining astronomical distances). They were discovered in 1963 but remained enigmatic for many years. They appear as starlike (i.e., very compact) sources of radio waveshence their initial designation as quasi-stellar radio sources, a term later shortened to quasars. They are now considered to be the exceedingly luminous cores of distant galaxies. These energetic cores, which emit copious quantities of X-rays and gamma rays, are termed active galactic nuclei (AGN) and include the object Cygnus A and the nuclei of a class of galaxies called Seyfert galaxies. They may be powered by the infall of matter into supermassive black holes.

The Milky Way Galaxy is one of the Local Group of galaxies, which contains about four dozen members and extends over a volume about two megaparsecs in diameter. Two of the closest members are the Magellanic Clouds, irregular galaxies about 50 kiloparsecs away. At about 740 kiloparsecs, the Andromeda Galaxy is one of the most distant in the Local Group. Some members of the group are moving toward the Milky Way system while others are traveling away from it. At greater distances, all galaxies are moving away from the Milky Way Galaxy. Their speeds (as determined from the redshifted wavelengths in their spectra) are generally proportional to their distances. The Hubble law relates these two quantities (see above Determining astronomical distances). In the absence of any other method, the Hubble law continues to be used for distance determinations to the farthest objectsthat is, galaxies and quasars for which redshifts can be measured.

Cosmology is the scientific study of the universe as a unified whole, from its earliest moments through its evolution to its ultimate fate. The currently accepted cosmological model is the big bang. In this picture, the expansion of the universe started in an intense explosion 13.8 billion years ago. In this primordial fireball, the temperature exceeded one trillion K, and most of the energy was in the form of radiation. As the expansion proceeded (accompanied by cooling), the role of the radiation diminished, and other physical processes dominated in turn. Thus, after about three minutes, the temperature had dropped to the one-billion-K range, making it possible for nuclear reactions of protons to take place and produce nuclei of deuterium and helium. (At the higher temperatures that prevailed earlier, these nuclei would have been promptly disrupted by high-energy photons.) With further expansion, the time between nuclear collisions had increased and the proportion of deuterium and helium nuclei had stabilized. After a few hundred thousand years, the temperature must have dropped sufficiently for electrons to remain attached to nuclei to constitute atoms. Galaxies are thought to have begun forming after a few million years, but this stage is very poorly understood. Star formation probably started much later, after at least a billion years, and the process continues today.

Observational support for this general model comes from several independent directions. The expansion has been documented by the redshifts observed in the spectra of galaxies. Furthermore, the radiation left over from the original fireball would have cooled with the expansion. Confirmation of this relic energy came in 1965 with one of the most striking cosmic discoveries of the 20th centurythe observation, at short radio wavelengths, of a widespread cosmic radiation corresponding to a temperature of almost 3 K (about 270 C [454 F]). The shape of the observed spectrum is an excellent fit with the theoretical Planck blackbody spectrum. (The present best value for this temperature is 2.735 K, but it is still called three-degree radiation or the cosmic microwave background.) The spectrum of this cosmic radio noise peaks at approximately a one-millimetre wavelength, which is in the far infrared, a difficult region to observe from Earth; however, the spectrum has been well mapped by the Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe, and Planck satellites. Additional support for the big bang theory comes from the observed cosmic abundances of deuterium and helium. Normal stellar nucleosynthesis cannot produce their measured quantities, which fit well with calculations of production during the early stages of the big bang.

Early surveys of the cosmic background radiation indicated that it is extremely uniform in all directions (isotropic). Calculations have shown that it is difficult to achieve this degree of isotropy unless there was a very early and rapid inflationary period before the expansion settled into its present mode. Nevertheless, the isotropy posed problems for models of galaxy formation. Galaxies originate from turbulent conditions that produce local fluctuations of density, toward which more matter would then be gravitationally attracted. Such density variations were difficult to reconcile with the isotropy required by observations of the 3 K radiation. This problem was solved when the COBE satellite was able to detect the minute fluctuations in the cosmic background from which the galaxies formed.

The very earliest stages of the big bang are less well understood. The conditions of temperature and pressure that prevailed prior to the first microsecond require the introduction of theoretical ideas of subatomic particle physics. Subatomic particles are usually studied in laboratories with giant accelerators, but the region of particle energies of potential significance to the question at hand lies beyond the range of accelerators currently available. Fortunately, some important conclusions can be drawn from the observed cosmic helium abundance, which is dependent on conditions in the early big bang. The observed helium abundance sets a limit on the number of families of certain types of subatomic particles that can exist.

The age of the universe can be calculated in several ways. Assuming the validity of the big bang model, one attempts to answer the question: How long has the universe been expanding in order to have reached its present size? The numbers relevant to calculating an answer are Hubbles constant (i.e., the current expansion rate), the density of matter in the universe, and the cosmological constant, which allows for change in the expansion rate. In 2003 a calculation based on a fresh determination of Hubbles constant yielded an age of 13.7 billion 200 million years, although the precise value depends on certain assumed details of the model used. Independent estimates of stellar ages have yielded values less than this, as would be expected, but other estimates, based on supernova distance measurements, have arrived at values of about 15 billion years, still consistent, within the errors. In the big bang model the age is proportional to the reciprocal of Hubbles constant, hence the importance of determining H as reliably as possible. For example, a value for H of 100 km/sec/Mpc would lead to an age less than that of many stars, a physically unacceptable result.

A small minority of astronomers have developed alternative cosmological theories that are seriously pursued. The overwhelming professional opinion, however, continues to support the big bang model.

Finally, there is the question of the future behaviour of the universe: Is it open? That is to say, will the expansion continue indefinitely? Or is it closed, such that the expansion will slow down and eventually reverse, resulting in contraction? (The final collapse of such a contracting universe is sometimes termed the big crunch.) The density of the universe seems to be at the critical density; that is, the universe is neither open nor closed but flat. So-called dark energy, a kind of repulsive force that is now believed to be a major component of the universe, appears to be the decisive factor in predictions of the long-term fate of the cosmos. If this energy is a cosmological constant (as proposed in 1917 by Albert Einstein to correct certain problems in his model of the universe), then the result would be a big chill. In this scenario, the universe would continue to expand, but its density would decrease. While old stars would burn out, new stars would no longer form. The universe would become cold and dark. The dark (nonluminous) matter component of the universe, whose composition remains unknown, is not considered sufficient to close the universe and cause it to collapse; it now appears to contribute only a fourth of the density needed for closure.

An additional factor in deciding the fate of the universe might be the mass of neutrinos. For decades the neutrino had been postulated to have zero mass, although there was no compelling theoretical reason for this to be so. From the observation of neutrinos generated in the Sun and other celestial sources such as supernovas, in cosmic-ray interactions with Earths atmosphere, and in particle accelerators, investigators have concluded that neutrinos have some mass, though only an extremely small fraction of the mass of an electron. Although there are vast numbers of neutrinos in the universe, the sum of such small neutrino masses appears insufficient to close the universe.

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astronomy | Definition & Facts | Britannica.com

Astronomy – Wikipedia

Not to be confused with astrology, the pseudoscience.

Astronomy (from Greek: ) is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets; the phenomena include supernova explosions, gamma ray bursts, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth’s atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole.[1]

Astronomy is one of the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, Maya, and many ancient indigenous peoples of the Americas performed methodical observations of the night sky. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars, but professional astronomy is now often considered to be synonymous with astrophysics.[2]

Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, which is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results.

Astronomy is one of the few sciences in which amateurs still play an active role, especially in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets.

Astronomy (from the Greek from astron, “star” and – -nomia from nomos, “law” or “culture”) means “law of the stars” (or “culture of the stars” depending on the translation). Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects.[5] Although the two fields share a common origin, they are now entirely distinct.[6]

Generally, either the term “astronomy” or “astrophysics” may be used to refer to this subject.[7][8][9] Based on strict dictionary definitions, “astronomy” refers to “the study of objects and matter outside the Earth’s atmosphere and of their physical and chemical properties”[10] and “astrophysics” refers to the branch of astronomy dealing with “the behavior, physical properties, and dynamic processes of celestial objects and phenomena.”[11] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, “astronomy” may be used to describe the qualitative study of the subject, whereas “astrophysics” is used to describe the physics-oriented version of the subject.[12] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[7] Few fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use “astronomy” and “astrophysics,” partly depending on whether the department is historically affiliated with a physics department,[8] and many professional astronomers have physics rather than astronomy degrees.[9] Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy and Astrophysics.

In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.[13]

Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in Mesopotamia, Greece, Persia, India, China, Egypt, and Central America, astronomical observatories were assembled, and ideas on the nature of the Universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Universe, or the Ptolemaic system, named after Ptolemy.[14]

A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations.[15] The Babylonians discovered that lunar eclipses recurred in a repeating cycle known as a saros.[16]

Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena.[17] In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a heliocentric model of the solar system.[18] In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe.[19] Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy.[20] The Antikythera mechanism (c. 15080 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.[21]

During the Middle Ages, astronomy was mostly stagnant in medieval Europe, at least until the 13th century. However, astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century.[22][23][24] In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian astronomer Azophi in his Book of Fixed Stars.[25] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and the Chinese astronomers in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[26][27] It is also believed that the ruins at Great Zimbabwe and Timbuktu[28] may have housed an astronomical observatory.[29] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.[30][31][32][33]

The Roman Catholic Church gave more financial and social support to the study of astronomy for over six centuries, from the recovery of ancient learning during the late Middle Ages into the Enlightenment, than any other, and, probably, all other, institutions. Among the Church’s motives was finding the date for Easter.[34]

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended by Galileo Galilei and expanded upon by Johannes Kepler.

Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down.[35] It was left to Newton’s invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope.[36]

The English astronomer John Flamsteed catalogued over 3000 stars.[37] Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.[38] The distance to a star was announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.[39]

During the 1819th centuries, the study of the three-body problem by Euler, Clairaut, and D’Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrange and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.[40]

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 181415, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth’s own Sun, but with a wide range of temperatures, masses, and sizes.[26]

The existence of the Earth’s galaxy, the Milky Way, as a separate group of stars, was only proved in the 20th century, along with the existence of “external” galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe.[41] Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars, which have been used to explain such observed phenomena as quasars, pulsars, blazars, and radio galaxies. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang, which is heavily supported by evidence provided by cosmic microwave background radiation, Hubble’s law, and the cosmological abundances of elements. Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere.[citation needed] In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September.[42][43]

Our main source of information about celestial bodies and other objects is visible light more generally electromagnetic radiation.[44] Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth’s surface, while other parts are only observable from either high altitudes or outside the Earth’s atmosphere. Specific information on these subfields is given below.

Radio astronomy uses radiation outside the visible range with wavelengths greater than approximately one millimeter.[45] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.[45]

Although some radio waves are emitted directly by astronomical objects, a product of thermal emission, most of the radio emission that is observed is the result of synchrotron radiation, which is produced when electrons orbit magnetic fields.[45] Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21cm, are observable at radio wavelengths.[12][45]

A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.[12][45]

Infrared astronomy is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous Galactic protostars and their host star clusters.[47][48]With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space.[49] Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.[50]

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.[51] Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 to 7000 (400 nm to 700nm),[51] that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 (10 to 320nm).[45] Light at those wavelengths are absorbed by the Earth’s atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.[45] However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary.[45]

X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 107 (10million) kelvins, and thermal emission from thick gases above 107 Kelvin.[45] Since X-rays are absorbed by the Earth’s atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.[45]

Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.[45] The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth’s atmosphere.[52]

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.[45]

In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.

In neutrino astronomy, astronomers use heavily shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun, but 24 neutrinos were also detected from supernova 1987A.[45] Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth’s atmosphere, result in a cascade of secondary particles which can be detected by current observatories.[53] Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth’s atmosphere.[45]

Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on 14 September 2015, observing gravitational waves from a binary black hole.[54] A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.[55][56]

The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.[57][58]

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters or potential collisions of the Earth with those objects.[59]

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allows astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy.[60]

During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting those stars.[61]

Theoretical astronomers use several tools including analytical models and computational numerical simulations; each has its particular advantages. Analytical models of a process are generally better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.[62][63]

Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and model’s results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.

Phenomena modeled by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale distribution of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.

A few examples of this process:

Dark matter and dark energy are the current leading topics in astronomy,[64] as their discovery and controversy originated during the study of the galaxies.

At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year oscillation in sunspot number. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.[65]

The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.[66] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.[67]

The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. Above that is the convection zone where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots.[65]

A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause. As the solar wind passes the Earth, it interacts with the Earth’s magnetic field (magnetosphere) and deflects the solar wind, but traps some creating the Van Allen radiation belts that envelop the Earth. The aurora are created when solar wind particles are guided by the magnetic flux lines into the Earth’s polar regions where the lines the descend into the atmosphere.[68]

Planetary science is the study of the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The Solar System has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.[69]

The Solar System is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus, and Neptune.[70] Beyond Neptune lies the Kuiper Belt, and finally the Oort Cloud, which may extend as far as a light-year.

The planets were formed 4.6 billion years ago in the protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided and one such collision may have formed the Moon.[71]

Once a planet reaches sufficient mass, the materials of different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.[72]

A planet or moon’s interior heat is produced from the collisions that created the body, by the decay of radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.[73]

The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.[74] Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.[75]

Almost all elements heavier than hydrogen and helium were created inside the cores of stars.[74]

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting red giant formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.[76]

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae;[77] while smaller stars blow off their outer layers and leave behind the inert core in the form of a white dwarf. The ejection of the outer layers forms a planetary nebula.[78] The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.[79] Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.[80] Planetary nebulae and supernovae distribute the “metals” produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.[81]

Our solar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters.[82]

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[75]

As the more massive stars appear, they transform the cloud into an H II region (ionized atomic hydrogen) of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.[83]

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.[84]

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies, their morphology (description) and classification, the observation of active galaxies, and at a larger scale, the groups and clusters of galaxies. Finally, the latter is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.[85]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may have been formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a super-massive black hole that is emitting radiation from in-falling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.[86]

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids between.[87]

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Cosmology (from the Greek (kosmos) “world, universe” and (logos) “word, study” or literally “logic”) could be considered the study of the Universe as a whole.

Observations of the large-scale structure of the Universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our Universe began at a single point in time, and thereafter expanded over the course of 13.8 billion years[88] to its present condition.[89] The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.[89]

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Astronomy – Wikipedia

Alternative medicine – Wikipedia

Alternative medicineAM, complementary and alternative medicine (CAM), complementary medicine, heterodox medicine, integrative medicine (IM), complementary and integrative medicine (CIM), new-age medicine, unconventional medicine, unorthodox medicineHow alternative treatments “work”:a) Misinterpreted natural course the individual gets better without treatment.b) Placebo effect or false treatment effect an individual receives “alternative therapy” and is convinced it will help. The conviction makes them more likely to get better.c) Nocebo effect an individual is convinced that standard treatment will not work, and that alternative treatment will work. This decreases the likelihood standard treatment will work, while the placebo effect of the “alternative” remains. d) No adverse effects Standard treatment is replaced with “alternative” treatment, getting rid of adverse effects, but also of improvement. e) Interference Standard treatment is “complemented” with something that interferes with its effect. This can both cause worse effect, but also decreased (or even increased) side effects, which may be interpreted as “helping”.Researchers such as epidemiologists, clinical statisticians and pharmacologists use clinical trials to tease out such effects, allowing doctors to offer only that which has been shown to work. “Alternative treatments” often refuse to use trials or make it deliberately hard to do so.

Alternative medicine, fringe medicine, pseudomedicine or simply questionable medicine is the use and promotion of practices which are unproven, disproven, impossible to prove, or excessively harmful in relation to their effect in the attempt to achieve the healing effects of medicine. They differ from experimental medicine in that the latter employs responsible investigation, and accepts results that show it to be ineffective. The scientific consensus is that alternative therapies either do not, or cannot, work. In some cases laws of nature are violated by their basic claims; in some the treatment is so much worse that its use is unethical. Alternative practices, products, and therapies range from only ineffective to having known harmful and toxic effects.

Alternative therapies may be credited for perceived improvement through placebo effects, decreased use or effect of medical treatment (and therefore either decreased side effects; or nocebo effects towards standard treatment), or the natural course of the condition or disease. Alternative treatment is not the same as experimental treatment or traditional medicine, although both can be misused in ways that are alternative. Alternative or complementary medicine is dangerous because it may discourage people from getting the best possible treatment, and may lead to a false understanding of the body and of science.

Alternative medicine is used by a significant number of people, though its popularity is often overstated. Large amounts of funding go to testing alternative medicine, with more than US$2.5 billion spent by the United States government alone. Almost none show any effect beyond that of false treatment, and most studies showing any effect have been statistical flukes. Alternative medicine is a highly profitable industry, with a strong lobby. This fact is often overlooked by media or intentionally kept hidden, with alternative practice being portrayed positively when compared to “big pharma”. The lobby has successfully pushed for alternative therapies to be subject to far less regulation than conventional medicine. Alternative therapies may even be allowed to promote use when there is demonstrably no effect, only a tradition of use. Regulation and licensing of alternative medicine and health care providers varies between and within countries. Despite laws making it illegal to market or promote alternative therapies for use in cancer treatment, many practitioners promote them. Alternative medicine is criticized for taking advantage of the weakest members of society. For example, the United States National Institutes of Health department studying alternative medicine, currently named National Center for Complementary and Integrative Health, was established as the Office of Alternative Medicine and was renamed the National Center for Complementary and Alternative Medicine before obtaining its current name. Therapies are often framed as “natural” or “holistic”, in apparent opposition to conventional medicine which is “artificial” and “narrow in scope”, statements which are intentionally misleading. When used together with functional medical treatment, alternative therapies do not “complement” (improve the effect of, or mitigate the side effects of) treatment. Significant drug interactions caused by alternative therapies may instead negatively impact functional treatment, making it less effective, notably in cancer.

Alternative diagnoses and treatments are not part of medicine, or of science-based curricula in medical schools, nor are they used in any practice based on scientific knowledge or experience. Alternative therapies are often based on religious belief, tradition, superstition, belief in supernatural energies, pseudoscience, errors in reasoning, propaganda, fraud, or lies. Alternative medicine is based on misleading statements, quackery, pseudoscience, antiscience, fraud, and poor scientific methodology. Promoting alternative medicine has been called dangerous and unethical. Testing alternative medicine that has no scientific basis has been called a waste of scarce research resources. Critics state that “there is really no such thing as alternative medicine, just medicine that works and medicine that doesn’t”, that the very idea of “alternative” treatments is paradoxical, as any treatment proven to work is by definition “medicine”.

Alternative medicine is defined loosely as a set of products, practices, and theories that are believed or perceived by their users to have the healing effects of medicine,[n 1][n 2] but whose effectiveness has not been clearly established using scientific methods,[n 1][n 3][4][5][6][7] or whose theory and practice is not part of biomedicine,[n 2][n 4][n 5][n 6] or whose theories or practices are directly contradicted by scientific evidence or scientific principles used in biomedicine.[4][5][11] “Biomedicine” or “medicine” is that part of medical science that applies principles of biology, physiology, molecular biology, biophysics, and other natural sciences to clinical practice, using scientific methods to establish the effectiveness of that practice. Unlike medicine,[n 4] an alternative product or practice does not originate from using scientific methods, but may instead be based on hearsay, religion, tradition, superstition, belief in supernatural energies, pseudoscience, errors in reasoning, propaganda, fraud, or other unscientific sources.[n 3][1][4][5]

In General Guidelines for Methodologies on Research and Evaluation of Traditional Medicine, published in 2000 by the World Health Organization (WHO), complementary and alternative medicine were defined as a broad set of health care practices that are not part of that country’s own tradition and are not integrated into the dominant health care system.[12]

The expression also refers to a diverse range of related and unrelated products, practices, and theories ranging from biologically plausible practices and products and practices with some evidence, to practices and theories that are directly contradicted by basic science or clear evidence, and products that have been conclusively proven to be ineffective or even toxic and harmful.[n 2][14][15]

The terms alternative medicine, complementary medicine, integrative medicine, holistic medicine, natural medicine, unorthodox medicine, fringe medicine, unconventional medicine, and new age medicine are used interchangeably as having the same meaning and are almost synonymous in most contexts.[16][17][18][19]

The meaning of the term “alternative” in the expression “alternative medicine”, is not that it is an effective alternative to medical science, although some alternative medicine promoters may use the loose terminology to give the appearance of effectiveness.[4][20] Loose terminology may also be used to suggest meaning that a dichotomy exists when it does not, e.g., the use of the expressions “western medicine” and “eastern medicine” to suggest that the difference is a cultural difference between the Asiatic east and the European west, rather than that the difference is between evidence-based medicine and treatments that do not work.[4]

Complementary medicine (CM) or integrative medicine (IM) is when alternative medicine is used together with functional medical treatment, in a belief that it improves the effect of treatments.[n 7][1][22][23][24] However, significant drug interactions caused by alternative therapies may instead negatively influence treatment, making treatments less effective, notably cancer therapy.[25][26] Both terms refer to use of alternative medical treatments alongside conventional medicine,[27][28][29] an example of which is use of acupuncture (sticking needles in the body to influence the flow of a supernatural energy), along with using science-based medicine, in the belief that the acupuncture increases the effectiveness or “complements” the science-based medicine.[29]

CAM is an abbreviation of the phrase complementary and alternative medicine.[30][31] It has also been called sCAM or SCAM with the addition of “so-called” or “supplements”.[32][33]

Allopathic medicine or allopathy is an expression commonly used by homeopaths and proponents of other forms of alternative medicine to refer to mainstream medicine. It was used to describe the traditional European practice of heroic medicine,[34] but later continued to be used to describe anything that was not homeopathy.[34]

Allopathy refers to the use of pharmacologically active agents or physical interventions to treat or suppress symptoms or pathophysiologic processes of diseases or conditions.[35] The German version of the word, allopathisch, was coined in 1810 by the creator of homeopathy, Samuel Hahnemann (17551843).[36] The word was coined from allo- (different) and -pathic (relating to a disease or to a method of treatment).[37] In alternative medicine circles the expression “allopathic medicine” is still used to refer to “the broad category of medical practice that is sometimes called Western medicine, biomedicine, evidence-based medicine, or modern medicine” (see the article on scientific medicine).[38]

Use of the term remains common among homeopaths and has spread to other alternative medicine practices. The meaning implied by the label has never been accepted by conventional medicine and is considered pejorative.[39] More recently, some sources have used the term “allopathic”, particularly American sources wishing to distinguish between Doctors of Medicine (MD) and Doctors of Osteopathic Medicine (DO) in the United States.[36][40] William Jarvis, an expert on alternative medicine and public health,[41] states that “although many modern therapies can be construed to conform to an allopathic rationale (e.g., using a laxative to relieve constipation), standard medicine has never paid allegiance to an allopathic principle” and that the label “allopath” was from the start “considered highly derisive by regular medicine”.[42]

Many conventional medical treatments do not fit the nominal definition of allopathy, as they seek to prevent illness, or remove its cause.[43][44]

CAM is an abbreviation of complementary and alternative medicine.[30][31] It has also been called sCAM or SCAM with the addition of “so-called” or “supplements”.[32][33] The words balance and holism are often used, claiming to take into account a “whole” person, in contrast to the supposed reductionism of medicine. Due to its many names the field has been criticized for intense rebranding of what are essentially the same practices: as soon as one name is declared synonymous with quackery, a new name is chosen.[16]

Traditional medicine refers to the pre-scientific practices of a certain culture, contrary to what is typically practiced in other cultures where medical science dominates.

“Eastern medicine” typically refers to the traditional medicines of Asia where conventional bio-medicine penetrated much later.

The words balance and holism are often used alongside complementary or integrative medicine, claiming to take into account a “whole” person, in contrast to the supposed reductionism of medicine. Due to its many names the field has been criticized for intense rebranding of what are essentially the same practices.[16]

Prominent members of the science[45][46] and biomedical science community[3] say that it is not meaningful to define an alternative medicine that is separate from a conventional medicine, that the expressions “conventional medicine”, “alternative medicine”, “complementary medicine”, “integrative medicine”, and “holistic medicine” do not refer to any medicine at all.[45][3][46][47]

Others in both the biomedical and CAM communities say that CAM cannot be precisely defined because of the diversity of theories and practices it includes, and because the boundaries between CAM and biomedicine overlap, are porous, and change. The expression “complementary and alternative medicine” (CAM) resists easy definition because the health systems and practices it refers to are diffuse, and its boundaries poorly defined.[14][n 8] Healthcare practices categorized as alternative may differ in their historical origin, theoretical basis, diagnostic technique, therapeutic practice and in their relationship to the medical mainstream. Some alternative therapies, including traditional Chinese medicine (TCM) and Ayurveda, have antique origins in East or South Asia and are entirely alternative medical systems;[52] others, such as homeopathy and chiropractic, have origins in Europe or the United States and emerged in the eighteenth and nineteenth centuries. Some, such as osteopathy and chiropractic, employ manipulative physical methods of treatment; others, such as meditation and prayer, are based on mind-body interventions. Treatments considered alternative in one location may be considered conventional in another.[55] Thus, chiropractic is not considered alternative in Denmark and likewise osteopathic medicine is no longer thought of as an alternative therapy in the United States.[55]

Critics say the expression is deceptive because it implies there is an effective alternative to science-based medicine, and that complementary is deceptive because it implies that the treatment increases the effectiveness of (complements) science-based medicine, while alternative medicines that have been tested nearly always have no measurable positive effect compared to a placebo.[4][56][57][58]

One common feature of all definitions of alternative medicine is its designation as “other than” conventional medicine. For example, the widely referenced descriptive definition of complementary and alternative medicine devised by the US National Center for Complementary and Integrative Health (NCCIH) of the National Institutes of Health (NIH), states that it is “a group of diverse medical and health care systems, practices, and products that are not generally considered part of conventional medicine”.[61] For conventional medical practitioners, it does not necessarily follow that either it or its practitioners would no longer be considered alternative.[n 9]

Some definitions seek to specify alternative medicine in terms of its social and political marginality to mainstream healthcare.[64] This can refer to the lack of support that alternative therapies receive from the medical establishment and related bodies regarding access to research funding, sympathetic coverage in the medical press, or inclusion in the standard medical curriculum.[64] In 1993, the British Medical Association (BMA), one among many professional organizations who have attempted to define alternative medicine, stated that it[n 10] referred to “…those forms of treatment which are not widely used by the conventional healthcare professions, and the skills of which are not taught as part of the undergraduate curriculum of conventional medical and paramedical healthcare courses”.[65] In a US context, an influential definition coined in 1993 by the Harvard-based physician,[66] David M. Eisenberg,[67] characterized alternative medicine “as interventions neither taught widely in medical schools nor generally available in US hospitals”.[68] These descriptive definitions are inadequate in the present-day when some conventional doctors offer alternative medical treatments and CAM introductory courses or modules can be offered as part of standard undergraduate medical training;[69] alternative medicine is taught in more than 50 per cent of US medical schools and increasingly US health insurers are willing to provide reimbursement for CAM therapies. In 1999, 7.7% of US hospitals reported using some form of CAM therapy; this proportion had risen to 37.7% by 2008.[71]

An expert panel at a conference hosted in 1995 by the US Office for Alternative Medicine (OAM),[72][n 11] devised a theoretical definition[72] of alternative medicine as “a broad domain of healing resources… other than those intrinsic to the politically dominant health system of a particular society or culture in a given historical period”.[74] This definition has been widely adopted by CAM researchers,[72] cited by official government bodies such as the UK Department of Health,[75] attributed as the definition used by the Cochrane Collaboration,[76] and, with some modification,[dubious discuss] was preferred in the 2005 consensus report of the US Institute of Medicine, Complementary and Alternative Medicine in the United States.[n 2]

The 1995 OAM conference definition, an expansion of Eisenberg’s 1993 formulation, is silent regarding questions of the medical effectiveness of alternative therapies.[77] Its proponents hold that it thus avoids relativism about differing forms of medical knowledge and, while it is an essentially political definition, this should not imply that the dominance of mainstream biomedicine is solely due to political forces.[77] According to this definition, alternative and mainstream medicine can only be differentiated with reference to what is “intrinsic to the politically dominant health system of a particular society of culture”.[78] However, there is neither a reliable method to distinguish between cultures and subcultures, nor to attribute them as dominant or subordinate, nor any accepted criteria to determine the dominance of a cultural entity.[78] If the culture of a politically dominant healthcare system is held to be equivalent to the perspectives of those charged with the medical management of leading healthcare institutions and programs, the definition fails to recognize the potential for division either within such an elite or between a healthcare elite and the wider population.[78]

Normative definitions distinguish alternative medicine from the biomedical mainstream in its provision of therapies that are unproven, unvalidated, or ineffective and support of theories with no recognized scientific basis. These definitions characterize practices as constituting alternative medicine when, used independently or in place of evidence-based medicine, they are put forward as having the healing effects of medicine, but are not based on evidence gathered with the scientific method.[1][3][27][28][61][80] Exemplifying this perspective, a 1998 editorial co-authored by Marcia Angell, a former editor of The New England Journal of Medicine, argued that:

It is time for the scientific community to stop giving alternative medicine a free ride. There cannot be two kinds of medicine conventional and alternative. There is only medicine that has been adequately tested and medicine that has not, medicine that works and medicine that may or may not work. Once a treatment has been tested rigorously, it no longer matters whether it was considered alternative at the outset. If it is found to be reasonably safe and effective, it will be accepted. But assertions, speculation, and testimonials do not substitute for evidence. Alternative treatments should be subjected to scientific testing no less rigorous than that required for conventional treatments.[3]

This line of division has been subject to criticism, however, as not all forms of standard medical practice have adequately demonstrated evidence of benefit,[n 4][81] and it is also unlikely in most instances that conventional therapies, if proven to be ineffective, would ever be classified as CAM.[72]

Similarly, the public information website maintained by the National Health and Medical Research Council (NHMRC) of the Commonwealth of Australia uses the acronym “CAM” for a wide range of health care practices, therapies, procedures and devices not within the domain of conventional medicine. In the Australian context this is stated to include acupuncture; aromatherapy; chiropractic; homeopathy; massage; meditation and relaxation therapies; naturopathy; osteopathy; reflexology, traditional Chinese medicine; and the use of vitamin supplements.[83]

The Danish National Board of Health’s “Council for Alternative Medicine” (Sundhedsstyrelsens Rd for Alternativ Behandling (SRAB)), an independent institution under the National Board of Health (Danish: Sundhedsstyrelsen), uses the term “alternative medicine” for:

Proponents of an evidence-base for medicine[n 12][86][87][88][89] such as the Cochrane Collaboration (founded in 1993 and from 2011 providing input for WHO resolutions) take a position that all systematic reviews of treatments, whether “mainstream” or “alternative”, ought to be held to the current standards of scientific method.[90] In a study titled Development and classification of an operational definition of complementary and alternative medicine for the Cochrane Collaboration (2011) it was proposed that indicators that a therapy is accepted include government licensing of practitioners, coverage by health insurance, statements of approval by government agencies, and recommendation as part of a practice guideline; and that if something is currently a standard, accepted therapy, then it is not likely to be widely considered as CAM.[72]

Alternative medicine consists of a wide range of health care practices, products, and therapies. The shared feature is a claim to heal that is not based on the scientific method. Alternative medicine practices are diverse in their foundations and methodologies.[61] Alternative medicine practices may be classified by their cultural origins or by the types of beliefs upon which they are based.[1][4][11][61] Methods may incorporate or be based on traditional medicinal practices of a particular culture, folk knowledge, superstition, spiritual beliefs, belief in supernatural energies (antiscience), pseudoscience, errors in reasoning, propaganda, fraud, new or different concepts of health and disease, and any bases other than being proven by scientific methods.[1][4][5][11] Different cultures may have their own unique traditional or belief based practices developed recently or over thousands of years, and specific practices or entire systems of practices.

Alternative medicine, such as using naturopathy or homeopathy in place of conventional medicine, is based on belief systems not grounded in science.[61]

Alternative medical systems may be based on traditional medicine practices, such as traditional Chinese medicine (TCM), Ayurveda in India, or practices of other cultures around the world.[61] Some useful applications of traditional medicines have been researched and accepted within ordinary medicine, however the underlying belief systems are seldom scientific and are not accepted.

Traditional medicine is considered alternative when it is used outside its home region; or when it is used together with or instead of known functional treatment; or when it can be reasonably expected that the patient or practitioner knows or should know that it will not work such as knowing that the practice is based on superstition.

Since ancient times, in many parts of the world a number of herbs reputed to possess abortifacient properties have been used in folk medicine. Among these are: tansy, pennyroyal, black cohosh, and the now-extinct silphium.[101]:4447, 6263, 15455, 23031 Historian of science Ann Hibner Koblitz has written of the probable protoscientific origins of this folk knowledge in observation of farm animals. Women who knew that grazing on certain plants would cause an animal to abort (with negative economic consequences for the farm) would be likely to try out those plants on themselves in order to avoid an unwanted pregnancy.[102]:120

However, modern users of these plants often lack knowledge of the proper preparation and dosage. The historian of medicine John Riddle has spoken of the “broken chain of knowledge” caused by urbanization and modernization,[101]:167205 and Koblitz has written that “folk knowledge about effective contraception techniques often disappears over time or becomes inextricably mixed with useless or harmful practices.”[102]:vii The ill-informed or indiscriminant use of herbs as abortifacients can cause serious and even lethal side-effects.[103][104]

Bases of belief may include belief in existence of supernatural energies undetected by the science of physics, as in biofields, or in belief in properties of the energies of physics that are inconsistent with the laws of physics, as in energy medicine.[61]

Substance based practices use substances found in nature such as herbs, foods, non-vitamin supplements and megavitamins, animal and fungal products, and minerals, including use of these products in traditional medical practices that may also incorporate other methods.[61][119][120] Examples include healing claims for nonvitamin supplements, fish oil, Omega-3 fatty acid, glucosamine, echinacea, flaxseed oil, and ginseng.[121] Herbal medicine, or phytotherapy, includes not just the use of plant products, but may also include the use of animal and mineral products.[119] It is among the most commercially successful branches of alternative medicine, and includes the tablets, powders and elixirs that are sold as “nutritional supplements”.[119] Only a very small percentage of these have been shown to have any efficacy, and there is little regulation as to standards and safety of their contents.[119] This may include use of known toxic substances, such as use of the poison lead in traditional Chinese medicine.[121]

A US agency, National Center on Complementary and Integrative Health (NCCIH), has created a classification system for branches of complementary and alternative medicine that divides them into five major groups. These groups have some overlap, and distinguish two types of energy medicine: veritable which involves scientifically observable energy (including magnet therapy, colorpuncture and light therapy) and putative, which invokes physically undetectable or unverifiable energy.[125] None of these energies have any evidence to support that they effect the body in any positive or health promoting way.[34]

The history of alternative medicine may refer to the history of a group of diverse medical practices that were collectively promoted as “alternative medicine” beginning in the 1970s, to the collection of individual histories of members of that group, or to the history of western medical practices that were labeled “irregular practices” by the western medical establishment.[4][126][127][128][129] It includes the histories of complementary medicine and of integrative medicine. Before the 1970s, western practitioners that were not part of the increasingly science-based medical establishment were referred to “irregular practitioners”, and were dismissed by the medical establishment as unscientific and as practicing quackery.[126][127] Until the 1970s, irregular practice became increasingly marginalized as quackery and fraud, as western medicine increasingly incorporated scientific methods and discoveries, and had a corresponding increase in success of its treatments.[128] In the 1970s, irregular practices were grouped with traditional practices of nonwestern cultures and with other unproven or disproven practices that were not part of biomedicine, with the entire group collectively marketed and promoted under the single expression “alternative medicine”.[4][126][127][128][130]

Use of alternative medicine in the west began to rise following the counterculture movement of the 1960s, as part of the rising new age movement of the 1970s.[4][131][132] This was due to misleading mass marketing of “alternative medicine” being an effective “alternative” to biomedicine, changing social attitudes about not using chemicals and challenging the establishment and authority of any kind, sensitivity to giving equal measure to beliefs and practices of other cultures (cultural relativism), and growing frustration and desperation by patients about limitations and side effects of science-based medicine.[4][127][128][129][130][132][133] At the same time, in 1975, the American Medical Association, which played the central role in fighting quackery in the United States, abolished its quackery committee and closed down its Department of Investigation.[126]:xxi[133] By the early to mid 1970s the expression “alternative medicine” came into widespread use, and the expression became mass marketed as a collection of “natural” and effective treatment “alternatives” to science-based biomedicine.[4][133][134][135] By 1983, mass marketing of “alternative medicine” was so pervasive that the British Medical Journal (BMJ) pointed to “an apparently endless stream of books, articles, and radio and television programmes urge on the public the virtues of (alternative medicine) treatments ranging from meditation to drilling a hole in the skull to let in more oxygen”.[133]

Mainly as a result of reforms following the Flexner Report of 1910[136] medical education in established medical schools in the US has generally not included alternative medicine as a teaching topic.[n 14] Typically, their teaching is based on current practice and scientific knowledge about: anatomy, physiology, histology, embryology, neuroanatomy, pathology, pharmacology, microbiology and immunology.[138] Medical schools’ teaching includes such topics as doctor-patient communication, ethics, the art of medicine,[139] and engaging in complex clinical reasoning (medical decision-making).[140] Writing in 2002, Snyderman and Weil remarked that by the early twentieth century the Flexner model had helped to create the 20th-century academic health center, in which education, research, and practice were inseparable. While this had much improved medical practice by defining with increasing certainty the pathophysiological basis of disease, a single-minded focus on the pathophysiological had diverted much of mainstream American medicine from clinical conditions that were not well understood in mechanistic terms, and were not effectively treated by conventional therapies.[141]

By 2001 some form of CAM training was being offered by at least 75 out of 125 medical schools in the US.[142] Exceptionally, the School of Medicine of the University of Maryland, Baltimore includes a research institute for integrative medicine (a member entity of the Cochrane Collaboration).[90][143] Medical schools are responsible for conferring medical degrees, but a physician typically may not legally practice medicine until licensed by the local government authority. Licensed physicians in the US who have attended one of the established medical schools there have usually graduated Doctor of Medicine (MD).[144] All states require that applicants for MD licensure be graduates of an approved medical school and complete the United States Medical Licensing Exam (USMLE).[144]

There is a general scientific consensus that alternative therapies lack the requisite scientific validation, and their effectiveness is either unproved or disproved.[1][4][145][146] Many of the claims regarding the efficacy of alternative medicines are controversial, since research on them is frequently of low quality and methodologically flawed. Selective publication bias, marked differences in product quality and standardisation, and some companies making unsubstantiated claims call into question the claims of efficacy of isolated examples where there is evidence for alternative therapies.[148]

The Scientific Review of Alternative Medicine points to confusions in the general population a person may attribute symptomatic relief to an otherwise-ineffective therapy just because they are taking something (the placebo effect); the natural recovery from or the cyclical nature of an illness (the regression fallacy) gets misattributed to an alternative medicine being taken; a person not diagnosed with science-based medicine may never originally have had a true illness diagnosed as an alternative disease category.[149]

Edzard Ernst characterized the evidence for many alternative techniques as weak, nonexistent, or negative[150] and in 2011 published his estimate that about 7.4% were based on “sound evidence”, although he believes that may be an overestimate.[151] Ernst has concluded that 95% of the alternative treatments he and his team studied, including acupuncture, herbal medicine, homeopathy, and reflexology, are “statistically indistinguishable from placebo treatments”, but he also believes there is something that conventional doctors can usefully learn from the chiropractors and homeopath: this is the therapeutic value of the placebo effect, one of the strangest phenomena in medicine.[152][153]

In 2003, a project funded by the CDC identified 208 condition-treatment pairs, of which 58% had been studied by at least one randomized controlled trial (RCT), and 23% had been assessed with a meta-analysis.[154] According to a 2005 book by a US Institute of Medicine panel, the number of RCTs focused on CAM has risen dramatically.

As of 2005[update], the Cochrane Library had 145 CAM-related Cochrane systematic reviews and 340 non-Cochrane systematic reviews. An analysis of the conclusions of only the 145 Cochrane reviews was done by two readers. In 83% of the cases, the readers agreed. In the 17% in which they disagreed, a third reader agreed with one of the initial readers to set a rating. These studies found that, for CAM, 38.4% concluded positive effect or possibly positive (12.4%), 4.8% concluded no effect, 0.7% concluded harmful effect, and 56.6% concluded insufficient evidence. An assessment of conventional treatments found that 41.3% concluded positive or possibly positive effect, 20% concluded no effect, 8.1% concluded net harmful effects, and 21.3% concluded insufficient evidence. However, the CAM review used the more developed 2004 Cochrane database, while the conventional review used the initial 1998 Cochrane database.

In the same way as for conventional therapies, drugs, and interventions, it can be difficult to test the efficacy of alternative medicine in clinical trials. In instances where an established, effective, treatment for a condition is already available, the Helsinki Declaration states that withholding such treatment is unethical in most circumstances. Use of standard-of-care treatment in addition to an alternative technique being tested may produce confounded or difficult-to-interpret results.[156]

Cancer researcher Andrew J. Vickers has stated:

Contrary to much popular and scientific writing, many alternative cancer treatments have been investigated in good-quality clinical trials, and they have been shown to be ineffective. The label “unproven” is inappropriate for such therapies; it is time to assert that many alternative cancer therapies have been “disproven”.[157]

A research methods expert and author of Snake Oil Science, R. Barker Bausell, has stated that “it’s become politically correct to investigate nonsense.”[158] There are concerns that just having NIH support is being used to give unfounded “legitimacy to treatments that are not legitimate.”[159]

Use of placebos to achieve a placebo effect in integrative medicine has been criticized as, “…diverting research time, money, and other resources from more fruitful lines of investigation in order to pursue a theory that has no basis in biology.”[57][58]

Another critic has argued that academic proponents of integrative medicine sometimes recommend misleading patients by using known placebo treatments to achieve a placebo effect.[n 15] However, a 2010 survey of family physicians found that 56% of respondents said they had used a placebo in clinical practice as well. Eighty-five percent of respondents believed placebos can have both psychological and physical benefits.[161]

Integrative medicine has been criticized in that its practitioners, trained in science-based medicine, deliberately mislead patients by pretending placebos are not. “quackademic medicine” is a pejorative term used for integrative medicine, which medical professionals consider an infiltration of quackery into academic science-based medicine.[58]

An analysis of trends in the criticism of complementary and alternative medicine (CAM) in five prestigious American medical journals during the period of reorganization within medicine (19651999) was reported as showing that the medical profession had responded to the growth of CAM in three phases, and that in each phase, changes in the medical marketplace had influenced the type of response in the journals.[162] Changes included relaxed medical licensing, the development of managed care, rising consumerism, and the establishment of the USA Office of Alternative Medicine (later National Center for Complementary and Alternative Medicine, currently National Center for Complementary and Integrative Health).[n 16] In the “condemnation” phase, from the late 1960s to the early 1970s, authors had ridiculed, exaggerated the risks, and petitioned the state to contain CAM; in the “reassessment” phase (mid-1970s through early 1990s), when increased consumer utilization of CAM was prompting concern, authors had pondered whether patient dissatisfaction and shortcomings in conventional care contributed to the trend; in the “integration” phase of the 1990s physicians began learning to work around or administer CAM, and the subjugation of CAM to scientific scrutiny had become the primary means of control.[citation needed]

Practitioners of complementary medicine usually discuss and advise patients as to available alternative therapies. Patients often express interest in mind-body complementary therapies because they offer a non-drug approach to treating some health conditions.[164]

In addition to the social-cultural underpinnings of the popularity of alternative medicine, there are several psychological issues that are critical to its growth. One of the most critical is the placebo effect a well-established observation in medicine.[165] Related to it are similar psychological effects, such as the will to believe,[166] cognitive biases that help maintain self-esteem and promote harmonious social functioning,[166] and the post hoc, ergo propter hoc fallacy.[166]

The popularity of complementary & alternative medicine (CAM) may be related to other factors that Edzard Ernst mentioned in an interview in The Independent:

Why is it so popular, then? Ernst blames the providers, customers and the doctors whose neglect, he says, has created the opening into which alternative therapists have stepped. “People are told lies. There are 40 million websites and 39.9 million tell lies, sometimes outrageous lies. They mislead cancer patients, who are encouraged not only to pay their last penny but to be treated with something that shortens their lives. “At the same time, people are gullible. It needs gullibility for the industry to succeed. It doesn’t make me popular with the public, but it’s the truth.[167]

Paul Offit proposed that “alternative medicine becomes quackery” in four ways: by recommending against conventional therapies that are helpful, promoting potentially harmful therapies without adequate warning, draining patients’ bank accounts, or by promoting “magical thinking.”[45]

Authors have speculated on the socio-cultural and psychological reasons for the appeal of alternative medicines among the minority using them in lieu of conventional medicine. There are several socio-cultural reasons for the interest in these treatments centered on the low level of scientific literacy among the public at large and a concomitant increase in antiscientific attitudes and new age mysticism.[166] Related to this are vigorous marketing[168] of extravagant claims by the alternative medical community combined with inadequate media scrutiny and attacks on critics.[166][169]

There is also an increase in conspiracy theories toward conventional medicine and pharmaceutical companies, mistrust of traditional authority figures, such as the physician, and a dislike of the current delivery methods of scientific biomedicine, all of which have led patients to seek out alternative medicine to treat a variety of ailments.[169] Many patients lack access to contemporary medicine, due to a lack of private or public health insurance, which leads them to seek out lower-cost alternative medicine.[170] Medical doctors are also aggressively marketing alternative medicine to profit from this market.[168]

Patients can be averse to the painful, unpleasant, and sometimes-dangerous side effects of biomedical treatments. Treatments for severe diseases such as cancer and HIV infection have well-known, significant side-effects. Even low-risk medications such as antibiotics can have potential to cause life-threatening anaphylactic reactions in a very few individuals. Many medications may cause minor but bothersome symptoms such as cough or upset stomach. In all of these cases, patients may be seeking out alternative treatments to avoid the adverse effects of conventional treatments.[166][169]

Complementary and alternative medicine (CAM) has been described as a broad domain of healing resources that encompasses all health systems, modalities, and practices and their accompanying theories and beliefs, other than those intrinsic to the politically dominant health system of a particular society or culture in a given historical period. CAM includes all such practices and ideas self-defined by their users as preventing or treating illness or promoting health and well-being. Boundaries within CAM and between the CAM domain and that of the dominant system are not always sharp or fixed.[72][dubious discuss]

According to recent research, the increasing popularity of the CAM needs to be explained by moral convictions or lifestyle choices rather than by economic reasoning.[171]

In developing nations, access to essential medicines is severely restricted by lack of resources and poverty. Traditional remedies, often closely resembling or forming the basis for alternative remedies, may comprise primary healthcare or be integrated into the healthcare system. In Africa, traditional medicine is used for 80% of primary healthcare, and in developing nations as a whole over one-third of the population lack access to essential medicines.[172]

Some have proposed adopting a prize system to reward medical research.[173] However, public funding for research exists. Increasing the funding for research on alternative medicine techniques is the purpose of the US National Center for Complementary and Alternative Medicine. NCCIH and its predecessor, the Office of Alternative Medicine, have spent more than US$2.5 billion on such research since 1992; this research has largely not demonstrated the efficacy of alternative treatments.[158][174][175][176]

That alternative medicine has been on the rise “in countries where Western science and scientific method generally are accepted as the major foundations for healthcare, and ‘evidence-based’ practice is the dominant paradigm” was described as an “enigma” in the Medical Journal of Australia.[177]

In the United States, the 1974 Child Abuse Prevention and Treatment Act (CAPTA) required that for states to receive federal money, they had to grant religious exemptions to child neglect and abuse laws regarding religion-based healing practices.[178] Thirty-one states have child-abuse religious exemptions.[179]

The use of alternative medicine in the US has increased,[1][180] with a 50 percent increase in expenditures and a 25 percent increase in the use of alternative therapies between 1990 and 1997 in America.[180] Americans spend many billions on the therapies annually.[180] Most Americans used CAM to treat and/or prevent musculoskeletal conditions or other conditions associated with chronic or recurring pain.[170] In America, women were more likely than men to use CAM, with the biggest difference in use of mind-body therapies including prayer specifically for health reasons”.[170] In 2008, more than 37% of American hospitals offered alternative therapies, up from 27 percent in 2005, and 25% in 2004.[181][182] More than 70% of the hospitals offering CAM were in urban areas.[182]

A survey of Americans found that 88 percent thought that “there are some good ways of treating sickness that medical science does not recognize”.[1] Use of magnets was the most common tool in energy medicine in America, and among users of it, 58 percent described it as at least “sort of scientific”, when it is not at all scientific.[1] In 2002, at least 60 percent of US medical schools have at least some class time spent teaching alternative therapies.[1] “Therapeutic touch”, was taught at more than 100 colleges and universities in 75 countries before the practice was debunked by a nine-year-old child for a school science project.[1][118]

The most common CAM therapies used in the US in 2002 were prayer (45%), herbalism (19%), breathing meditation (12%), meditation (8%), chiropractic medicine (8%), yoga (56%), body work (5%), diet-based therapy (4%), progressive relaxation (3%), mega-vitamin therapy (3%) and Visualization (2%)[170][183]

In Britain, the most often used alternative therapies were Alexander technique, Aromatherapy, Bach and other flower remedies, Body work therapies including massage, Counseling stress therapies, hypnotherapy, Meditation, Reflexology, Shiatsu, Ayurvedic medicine, Nutritional medicine, and Yoga.[184] Ayurvedic medicine remedies are mainly plant based with some use of animal materials. Safety concerns include the use of herbs containing toxic compounds and the lack of quality control in Ayurvedic facilities.[112][114]

According to the National Health Service (England), the most commonly used complementary and alternative medicines (CAM) supported by the NHS in the UK are: acupuncture, aromatherapy, chiropractic, homeopathy, massage, osteopathy and clinical hypnotherapy.[186]

Complementary therapies are often used in palliative care or by practitioners attempting to manage chronic pain in patients. Integrative medicine is considered more acceptable in the interdisciplinary approach used in palliative care than in other areas of medicine. “From its early experiences of care for the dying, palliative care took for granted the necessity of placing patient values and lifestyle habits at the core of any design and delivery of quality care at the end of life. If the patient desired complementary therapies, and as long as such treatments provided additional support and did not endanger the patient, they were considered acceptable.”[187] The non-pharmacologic interventions of complementary medicine can employ mind-body interventions designed to “reduce pain and concomitant mood disturbance and increase quality of life.”[188]

In Austria and Germany complementary and alternative medicine is mainly in the hands of doctors with MDs,[30] and half or more of the American alternative practitioners are licensed MDs.[189] In Germany herbs are tightly regulated: half are prescribed by doctors and covered by health insurance.[190]

Some professions of complementary/traditional/alternative medicine, such as chiropractic, have achieved full regulation in North America and other parts of the world and are regulated in a manner similar to that governing science-based medicine. In contrast, other approaches may be partially recognized and others have no regulation at all. Regulation and licensing of alternative medicine ranges widely from country to country, and state to state.

Government bodies in the US and elsewhere have published information or guidance about alternative medicine. The U.S. Food and Drug Administration (FDA), has issued online warnings for consumers about medication health fraud.[192] This includes a section on Alternative Medicine Fraud,[193] such as a warning that Ayurvedic products generally have not been approved by the FDA before marketing.[194]

Many of the claims regarding the safety and efficacy of alternative medicine are controversial. Some alternative treatments have been associated with unexpected side effects, which can be fatal.[195]

A commonly voiced concerns about complementary alternative medicine (CAM) is the way it’s regulated. There have been significant developments in how CAMs should be assessed prior to re-sale in the United Kingdom and the European Union (EU) in the last 2 years. Despite this, it has been suggested that current regulatory bodies have been ineffective in preventing deception of patients as many companies have re-labelled their drugs to avoid the new laws.[196] There is no general consensus about how to balance consumer protection (from false claims, toxicity, and advertising) with freedom to choose remedies.

Advocates of CAM suggest that regulation of the industry will adversely affect patients looking for alternative ways to manage their symptoms, even if many of the benefits may represent the placebo affect.[197] Some contend that alternative medicines should not require any more regulation than over-the-counter medicines that can also be toxic in overdose (such as paracetamol).[198]

Forms of alternative medicine that are biologically active can be dangerous even when used in conjunction with conventional medicine. Examples include immuno-augmentation therapy, shark cartilage, bioresonance therapy, oxygen and ozone therapies, and insulin potentiation therapy. Some herbal remedies can cause dangerous interactions with chemotherapy drugs, radiation therapy, or anesthetics during surgery, among other problems.[31] An anecdotal example of these dangers was reported by Associate Professor Alastair MacLennan of Adelaide University, Australia regarding a patient who almost bled to death on the operating table after neglecting to mention that she had been taking “natural” potions to “build up her strength” before the operation, including a powerful anticoagulant that nearly caused her death.[199]

To ABC Online, MacLennan also gives another possible mechanism:

And lastly [sic] there’s the cynicism and disappointment and depression that some patients get from going on from one alternative medicine to the next, and they find after three months the placebo effect wears off, and they’re disappointed and they move on to the next one, and they’re disappointed and disillusioned, and that can create depression and make the eventual treatment of the patient with anything effective difficult, because you may not get compliance, because they’ve seen the failure so often in the past.[200]

Conventional treatments are subjected to testing for undesired side-effects, whereas alternative treatments, in general, are not subjected to such testing at all. Any treatment whether conventional or alternative that has a biological or psychological effect on a patient may also have potential to possess dangerous biological or psychological side-effects. Attempts to refute this fact with regard to alternative treatments sometimes use the appeal to nature fallacy, i.e., “That which is natural cannot be harmful.” Specific groups of patients such as patients with impaired hepatic or renal function are more susceptible to side effects of alternative remedies.[201][202]

An exception to the normal thinking regarding side-effects is Homeopathy. Since 1938, the U.S. Food and Drug Administration (FDA) has regulated homeopathic products in “several significantly different ways from other drugs.”[203] Homeopathic preparations, termed “remedies”, are extremely dilute, often far beyond the point where a single molecule of the original active (and possibly toxic) ingredient is likely to remain. They are, thus, considered safe on that count, but “their products are exempt from good manufacturing practice requirements related to expiration dating and from finished product testing for identity and strength”, and their alcohol concentration may be much higher than allowed in conventional drugs.[203]

Those having experienced or perceived success with one alternative therapy for a minor ailment may be convinced of its efficacy and persuaded to extrapolate that success to some other alternative therapy for a more serious, possibly life-threatening illness.[204] For this reason, critics argue that therapies that rely on the placebo effect to define success are very dangerous. According to mental health journalist Scott Lilienfeld in 2002, “unvalidated or scientifically unsupported mental health practices can lead individuals to forgo effective treatments” and refers to this as “opportunity cost”. Individuals who spend large amounts of time and money on ineffective treatments may be left with precious little of either, and may forfeit the opportunity to obtain treatments that could be more helpful. In short, even innocuous treatments can indirectly produce negative outcomes.[205] Between 2001 and 2003, four children died in Australia because their parents chose ineffective naturopathic, homeopathic, or other alternative medicines and diets rather than conventional therapies.[206]

There have always been “many therapies offered outside of conventional cancer treatment centers and based on theories not found in biomedicine. These alternative cancer cures have often been described as ‘unproven,’ suggesting that appropriate clinical trials have not been conducted and that the therapeutic value of the treatment is unknown.” However, “many alternative cancer treatments have been investigated in good-quality clinical trials, and they have been shown to be ineffective….The label ‘unproven’ is inappropriate for such therapies; it is time to assert that many alternative cancer therapies have been ‘disproven’.”[157]

Edzard Ernst has stated:

…any alternative cancer cure is bogus by definition. There will never be an alternative cancer cure. Why? Because if something looked halfway promising, then mainstream oncology would scrutinize it, and if there is anything to it, it would become mainstream almost automatically and very quickly. All curative “alternative cancer cures” are based on false claims, are bogus, and, I would say, even criminal.[207]

“CAM”, meaning “complementary and alternative medicine”, is not as well researched as conventional medicine, which undergoes intense research before release to the public.[208] Funding for research is also sparse making it difficult to do further research for effectiveness of CAM.[209] Most funding for CAM is funded by government agencies.[208] Proposed research for CAM are rejected by most private funding agencies because the results of research are not reliable.[208] The research for CAM has to meet certain standards from research ethics committees, which most CAM researchers find almost impossible to meet.[208] Even with the little research done on it, CAM has not been proven to be effective.[210]

Steven Novella, a neurologist at Yale School of Medicine, wrote that government funded studies of integrating alternative medicine techniques into the mainstream are “used to lend an appearance of legitimacy to treatments that are not legitimate.”[159] Marcia Angell considered that critics felt that healthcare practices should be classified based solely on scientific evidence, and if a treatment had been rigorously tested and found safe and effective, science-based medicine will adopt it regardless of whether it was considered “alternative” to begin with.[3] It is possible for a method to change categories (proven vs. unproven), based on increased knowledge of its effectiveness or lack thereof. A prominent supporter of this position is George D. Lundberg, former editor of the Journal of the American Medical Association (JAMA).[47]

Writing in 1999 in CA: A Cancer Journal for Clinicians Barrie R. Cassileth mentioned a 1997 letter to the US Senate Subcommittee on Public Health and Safety, which had deplored the lack of critical thinking and scientific rigor in OAM-supported research, had been signed by four Nobel Laureates and other prominent scientists. (This was supported by the National Institutes of Health (NIH).)[211]

In March 2009 a staff writer for the Washington Post reported that the impending national discussion about broadening access to health care, improving medical practice and saving money was giving a group of scientists an opening to propose shutting down the National Center for Complementary and Alternative Medicine. They quoted one of these scientists, Steven Salzberg, a genome researcher and computational biologist at the University of Maryland, as saying “One of our concerns is that NIH is funding pseudoscience.” They noted that the vast majority of studies were based on fundamental misunderstandings of physiology and disease, and had shown little or no effect.[159]

Writers such as Carl Sagan, a noted astrophysicist, advocate of scientific skepticism and the author of The Demon-Haunted World: Science as a Candle in the Dark (1996), have lambasted the lack of empirical evidence to support the existence of the putative energy fields on which these therapies are predicated.

Sampson has also pointed out that CAM tolerated contradiction without thorough reason and experiment.[212] Barrett has pointed out that there is a policy at the NIH of never saying something doesn’t work only that a different version or dose might give different results.[158] Barrett also expressed concern that, just because some “alternatives” have merit, there is the impression that the rest deserve equal consideration and respect even though most are worthless, since they are all classified under the one heading of alternative medicine.[213]

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82-year-old polio survivor Mona Randolph uses one of only three “iron lungs” known to still be in use in the U.S. The iron lung, which was invented in 1920s, was often used on polio patients who were unable to breathe after the virus paralyzed muscle groups in the chest. Six nights a week, Randolph sleeps up to her neck in a noisy, airtight, 75-year-old iron tube.

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Category:Alternative medicine – Wikipedia

Alternative medicine encompasses methods used in both complementary medicine and alternative medicine, known collectively as complementary and alternative medicine (CAM). These methods are used in place of (“alternative to”), or in addition to (“complementary to”), conventional medical treatments. The terms are primarily used in the western world, and include several traditional medicine techniques practiced throughout the world.

If you add something to this category it should also be added to list of forms of alternative medicine.

This category has the following 10 subcategories, out of 10 total.

The following 106 pages are in this category, out of 106 total. This list may not reflect recent changes (learn more).

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Category:Alternative medicine – Wikipedia

Astronomy – Wikipedia

Not to be confused with astrology, the pseudoscience.

Astronomy (from Greek: ) is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets; the phenomena include supernova explosions, gamma ray bursts, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth’s atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole.[1]

Astronomy is one of the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, Maya, and many ancient indigenous peoples of the Americas performed methodical observations of the night sky. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars, but professional astronomy is now often considered to be synonymous with astrophysics.[2]

Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, which is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results.

Astronomy is one of the few sciences in which amateurs still play an active role, especially in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets.

Astronomy (from the Greek from astron, “star” and – -nomia from nomos, “law” or “culture”) means “law of the stars” (or “culture of the stars” depending on the translation). Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects.[5] Although the two fields share a common origin, they are now entirely distinct.[6]

Generally, either the term “astronomy” or “astrophysics” may be used to refer to this subject.[7][8][9] Based on strict dictionary definitions, “astronomy” refers to “the study of objects and matter outside the Earth’s atmosphere and of their physical and chemical properties”[10] and “astrophysics” refers to the branch of astronomy dealing with “the behavior, physical properties, and dynamic processes of celestial objects and phenomena.”[11] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, “astronomy” may be used to describe the qualitative study of the subject, whereas “astrophysics” is used to describe the physics-oriented version of the subject.[12] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[7] Few fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use “astronomy” and “astrophysics,” partly depending on whether the department is historically affiliated with a physics department,[8] and many professional astronomers have physics rather than astronomy degrees.[9] Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy and Astrophysics.

In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.[13]

Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in Mesopotamia, Greece, Persia, India, China, Egypt, and Central America, astronomical observatories were assembled, and ideas on the nature of the Universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Universe, or the Ptolemaic system, named after Ptolemy.[14]

A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations.[15] The Babylonians discovered that lunar eclipses recurred in a repeating cycle known as a saros.[16]

Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena.[17] In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a heliocentric model of the solar system.[18] In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe.[19] Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy.[20] The Antikythera mechanism (c. 15080 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.[21]

During the Middle Ages, astronomy was mostly stagnant in medieval Europe, at least until the 13th century. However, astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century.[22][23][24] In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian astronomer Azophi in his Book of Fixed Stars.[25] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and the Chinese astronomers in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[26][27] It is also believed that the ruins at Great Zimbabwe and Timbuktu[28] may have housed an astronomical observatory.[29] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.[30][31][32][33]

The Roman Catholic Church gave more financial and social support to the study of astronomy for over six centuries, from the recovery of ancient learning during the late Middle Ages into the Enlightenment, than any other, and, probably, all other, institutions. Among the Church’s motives was finding the date for Easter.[34]

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo used telescopes to enhance his observations.[35]

Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down.[36] It was left to Newton’s invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope.[35]

The English astronomer John Flamsteed catalogued over 3000 stars.[37] Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.[38] The distance to a star was announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.[39]

During the 1819th centuries, the study of the three-body problem by Euler, Clairaut, and D’Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrange and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.[40]

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 181415, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth’s own Sun, but with a wide range of temperatures, masses, and sizes.[26]

The existence of the Earth’s galaxy, the Milky Way, as a separate group of stars, was only proved in the 20th century, along with the existence of “external” galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe.[41] Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars, which have been used to explain such observed phenomena as quasars, pulsars, blazars, and radio galaxies. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang, which is heavily supported by evidence provided by cosmic microwave background radiation, Hubble’s law, and the cosmological abundances of elements. Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere.[citation needed] In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September.[42][43]

Our main source of information about celestial bodies and other objects is visible light more generally electromagnetic radiation.[44] Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth’s surface, while other parts are only observable from either high altitudes or outside the Earth’s atmosphere. Specific information on these subfields is given below.

Radio astronomy uses radiation outside the visible range with wavelengths greater than approximately one millimeter.[45] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.[45]

Although some radio waves are emitted directly by astronomical objects, a product of thermal emission, most of the radio emission that is observed is the result of synchrotron radiation, which is produced when electrons orbit magnetic fields.[45] Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21cm, are observable at radio wavelengths.[12][45]

A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.[12][45]

Infrared astronomy is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous Galactic protostars and their host star clusters.[47][48]With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space.[49] Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.[50]

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.[51] Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 to 7000 (400 nm to 700nm),[51] that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 (10 to 320nm).[45] Light at those wavelengths are absorbed by the Earth’s atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.[45] However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary.[45]

X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 107 (10million) kelvins, and thermal emission from thick gases above 107 Kelvin.[45] Since X-rays are absorbed by the Earth’s atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.[45]

Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.[45] The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth’s atmosphere.[52]

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.[45]

In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.

In neutrino astronomy, astronomers use heavily shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun, but 24 neutrinos were also detected from supernova 1987A.[45] Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth’s atmosphere, result in a cascade of secondary particles which can be detected by current observatories.[53] Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth’s atmosphere.[45]

Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on 14 September 2015, observing gravitational waves from a binary black hole.[54] A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.[55][56]

The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.[57][58]

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters or potential collisions of the Earth with those objects.[59]

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allows astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy.[60]

During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting those stars.[61]

Theoretical astronomers use several tools including analytical models and computational numerical simulations; each has its particular advantages. Analytical models of a process are generally better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.[62][63]

Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and model’s results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.

Phenomena modeled by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale distribution of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.

A few examples of this process:

Dark matter and dark energy are the current leading topics in astronomy,[64] as their discovery and controversy originated during the study of the galaxies.

At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year oscillation in sunspot number. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.[65]

The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.[66] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.[67]

The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. Above that is the convection zone where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots.[65]

A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause. As the solar wind passes the Earth, it interacts with the Earth’s magnetic field (magnetosphere) and deflects the solar wind, but traps some creating the Van Allen radiation belts that envelop the Earth. The aurora are created when solar wind particles are guided by the magnetic flux lines into the Earth’s polar regions where the lines the descend into the atmosphere.[68]

Planetary science is the study of the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The Solar System has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.[69]

The Solar System is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus, and Neptune.[70] Beyond Neptune lies the Kuiper Belt, and finally the Oort Cloud, which may extend as far as a light-year.

The planets were formed 4.6 billion years ago in the protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided and one such collision may have formed the Moon.[71]

Once a planet reaches sufficient mass, the materials of different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.[72]

A planet or moon’s interior heat is produced from the collisions that created the body, by the decay of radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.[73]

The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.[74] Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.[75]

Almost all elements heavier than hydrogen and helium were created inside the cores of stars.[74]

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting red giant formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.[76]

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae;[77] while smaller stars blow off their outer layers and leave behind the inert core in the form of a white dwarf. The ejection of the outer layers forms a planetary nebula.[78] The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.[79] Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.[80] Planetary nebulae and supernovae distribute the “metals” produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.[81]

Our solar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters.[82]

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[75]

As the more massive stars appear, they transform the cloud into an H II region (ionized atomic hydrogen) of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.[83]

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.[84]

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies, their morphology (description) and classification, the observation of active galaxies, and at a larger scale, the groups and clusters of galaxies. Finally, the latter is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.[85]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may have been formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a super-massive black hole that is emitting radiation from in-falling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.[86]

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids between.[87]

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Cosmology (from the Greek (kosmos) “world, universe” and (logos) “word, study” or literally “logic”) could be considered the study of the Universe as a whole.

Observations of the large-scale structure of the Universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our Universe began at a single point in time, and thereafter expanded over the course of 13.8 billion years[88] to its present condition.[89] The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.[89]

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Astronomy – Wikipedia

Astronomy News & Current Events | Sky & Telescope

Sometimes, it’s just as exciting to watch a celestial object fade or disappear as it is to see it explode. We celebrate the “return” of a mysterious variable star and prepare for Pluto to occult a star.

Plan for optimal conditions for the years Perseid meteor shower, early in the mornings of August 12th and 13th.

Astronomers have discovered a hiccup in stellar luminosities that may point to a new understanding of stellar structure.

New results show that the Medusae Fossae Formation, a set of broad and enigmatic plains, provides most of the fine dust found everywhere on the Martian surface.

As told in this month’s astronomy podcast, August offers excellent viewing conditions for the always-flashy Perseid meteor shower and a chance to see four bright planets at once.

An X-ray telescope recently installed on the International Space Station has provided a detailed look at a black hole feeding off its companion star.

Scientists studying microscopic minerals found in a meteorite have discovered evidence of the feistiness of the toddler Sun.

Astronomers have seen a stars light redden as it passed our galaxys central black hole, just as general relativity predicts.

A radar instrument on one of the oldest operational Mars orbiters has discovered possible evidence of liquid water on Mars.

Not only will the Moon will be totally eclipsed this Friday, but Mars will be at opposition and shine in tandem with the red Moon all night!

HaloSat, a mini-satellite recently deployed from the International Space Station, is on the hunt for the universe’s missing matter.

A new study says our neighbor galaxys big stellar halo and stellar evolution are due to a major collision that ended 2 billion years ago.

Wonder what to see now that Mars is at its biggest and brightest? Here are expert tips for when, where, and how to look.

A combination of neutrino detection and observations across the full spectrum of light has pinpointed a cosmic accelerator for the first time, revitalizing multi-messenger astronomy.

A new study explores the impacts of stars that age past the main sequence and evolve into red giants on the planets orbiting around them by looking at the orbits of gas giants closely circling evolved stars.

Catch up on this week in astronomy news with a trio of images: a pair of near-Earth asteroid twins, ground-based views that beat Hubble’s, and the tumultuous galactic center.

Over the past 18 months, astronomers have painstakingly tracked a dozen tiny moons that they found circling the giant planet Jupiter.

Comet PanSTARRS (C/2017 S3) has erupted again! Now bright enough to see in binoculars, it might become a naked-eye object if it survives until perihelion.

On the night ofJuly 27, 2018,the longest total lunar eclipse for the next 105 years will be visibleacross parts of Europe, Africa, Asia and Australia.Six months later, onJanuary 20, 2019,there will be the ‘Great American’ lunar eclipse, where totality is visible across all 50 states.

A new National Academies study assesses NASA’s efforts to protect neighboring worlds from contamination and recommends ways the space agency could do a better job.

Theres no hiding changes in Earths atmosphere over the seasons are a dead giveaway to the fact that Earth hosts life. Now a new study explores whether we might use atmospheric seasonality like Earths to detect life on other planets. Looking for Change Most of the searches for life beyond our planet focus on

With opposition only weeks away, will the current global dust storm finally break? We look at the prospects.

Hanging dramatically in the west during twilight next Sunday evening (July 15, 2018) will be a bright star and crescent: Venus and the Moon. The cosmic couple will be quite the eye-catcher if your sky is clear.

The expedition off the coast of Washington state performs a first, recovering meteorite fragments from a documented fall.

Astronomers using a new instrument on the Very Large Telescope in Chile have directly imaged a newborn planet.

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Astronomy News & Current Events | Sky & Telescope

Astronomy – definition of astronomy by The Free Dictionary

the branch of astronomy that studies meteors.

the branch of astronomy that studies aerolites, or stony meteors.

the ratio between the light reflected from a surface and the total light falling upon that surface, as the albedo of the moon.

the point in the orbit of a heavenly body where it is farthest from the sun. Cf. perihelion.

in an orbit around a moon, the point furthest from the moon. Cf. perilune.

the astronomical studies of the planet Mars. areologist, n. areologic, areological, adj.

Rare. a constellation or small group of unrelated stars. asterismal, adj.

the art of navigating in space. Cf. astronavigation. astrogator, n.

the theory of the evolution of heavenly bodies.

a geological specialty that studies celestial bodies.

the branch of astronomy that studies the fixed stars.

a scientific analysis and mapping of the stars and planets. astrographic, adj.

the worship of the heavenly bodies. Also called Sabaism. astrolater, n.

1. a form of divination involving studying the stars.2. Rare. astrology. Also called sideromancy. astromancer, n. astromantic, adj.

the branch of astronomy that studies the dimensions of heavenly bodies, especially the measurements made to determine the positions and orbits of various stars. astrometric, astrometrical, adj.

the science of space travel, concerned with both the construction and the operation of vehicles that travel through interplanetary or interstellar space. Also called cosmonautics. astronautic, astronautical, adj. astronaut, n.

a type of navigation involving observations of the apparent positions of heavenly bodies. Also called celestial navigation, celo-navigation. astronavigator, n.

the science that studies the stars and other features of the material universe beyond the earths atmosphere. astronomer, n. astronomical, adj.

a person strongly attracted to knowledge about the stars. astrophilic, adj.

a form of photography used to record astronomical phenomena.

the branch of astronomy concerned with the origin, and the chemical and physical nature of heavenly bodies. astrophysicist, n.

astronavigation. Also called celo-navigation.

the study of stars through a telescope in which the star appears as a ring of light, in order to observe the stars scintillation. chromatoscope, n.

the fundamental theoretical basis of modern astronomy, first demonstrated in the early 16th century by Copernicus, who showed that the earth and the other planets orbit around the sun. Cf. Ptolemaism.

Obsolete, an instrument, like an astrolabe, used for astronomical observations.

astronautics. cosmonaut. n. cosmonautical, adj.

an instrument used in astronomy to show the apparent movement of the sun.

an instrument originally designed for measuring the suns diameter, now used for measuring the angular distance between stars.

the practice of measuring the angular distance between stars by means of a heliometer. heliometric, heliometrical, adj.

the period between the old moon and the new when the moon is invisible each month. interlunar, adj.

an imaginary great circle in the sphere of the heavens, passing through the poles and the zenith and nadir of any point and intersecting the equator at right angles. See also 178. GEOGRAPHY. meridian, meridional, adj.

the entire system of galaxies, including the Milky Way. metagalactic, adj.

the periodic oscillation that can be observed in the precession of the earths axis and the precession of the equinoxes. See also 133. EARTH. nutational, adj.

the inclination of the earths equator or the angle between the plane of the earths orbit and the plane of the equator (2327). Also called obliquity of the ecliptic. See also 133. EARTH. obliquitous, adj.

the process of one heavenly body disappearing behind another as viewed by an observer.

a false moon, in reality a bright spot or a luminous ring surrounding the moon.

the point in the orbit of a heavenly body where it is nearest the sun. Also spelled perihelium. Cf. aphelion.

perihelion.

in orbit around a moon, the point nearest the moon. Cf. apolune.

Rare. a work or treatise on astronomy or celestial bodies.

1. a representation of the planetary system, particularly one in which the movements of the planets are simulated by projectors.2. a room or building housing such an apparatus.

the branch of astronomy that studies the planets. planetologist, n. planetologic, planetological, adj.

a map showing half or more of the sphere of the heavens, indicating which part is visible at what hour from a given location. planispheric, planispherical, adj.

the complicated demonstration of Ptolemy, 2nd-century geographer and astronomer, that the earth is the fixed center of the universe around which the sun and the other planets revolve; now discredited. Cf. Copemicanism.

a supporter of the Ptolemaic explanation of planetary motions.

the branch of astronomy that studies radio frequencies emitted by the sun, planets, and other celestial bodies.

astrolatry.

the combination or configuration of the aspects of the planets and other heavenly bodies.

the scientific analysis and mapping of the moons physical features. selenographer, selenographist, n. selenographic, selenographical, adj.

the branch of astronomy that studies the moon. selenologist, n. selenologic, selenological, adj.

a form of divination involving observations of the stars. Also called astromancy. sideromancer, n. sideromantic, adj.

an abnormal fear of the stars.

Obsolete, astronomy.

the branch of astronomy that deals with the description of the heavens by constructing maps and charts, especially of the fixed stars. Also called uranology. uranographer, uranographist, n. uranographic, uranographical, adj.

1. a written description of the heavens and celestial bodies.2. another term for astronomy.

1. a treatise recording the positions and magnitudes of heavenly bodies.2. the science of measuring the real or apparent distances of heavenly bodies from Earth. uranometrical, adj.

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Astronomy – definition of astronomy by The Free Dictionary