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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

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 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 24

Explanation: Two small robots have begun hopping around the surface of asteroid Ryugu.The rovers, each the size of a small frying pan, move around the low gravity of kilometer-sized 162173 Ryugu by hopping, staying aloft for about 15 minutes and typically landing again several meters away. On Saturday, Rover 1A returned an early picture of its new home world, on the left, during one of its first hops.On Friday, lander MINERVA-II-1 detached from its mothership Hayabusa2, dropped Rovers 1A and 1B, and then landed on Ryugu.Studying Ryugu could tell humanity not only about Ryugu’s surface and interior, but about what materials were available in the early Solar System for the development of life. Two more hopping rovers are planned for release, and Hayabusa2 itself is scheduled to collect a surface sample from Ryugu and return it to Earth for detailed analysis before 2021.

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

Astronomy News & Current Events | Sky & Telescope

The Milky Way’s two largest companion galaxies may have once been a threesome but new data from the Gaia satellite leaves the satellites’ history an open question.

TESS finds its first exoplanet a super-Earth around bright nearby star Pi Mensae and astronomers watched an asteroid hide a galaxy to get the details on the asteroid’s size, shape, and orbit.

An unexpected pattern in the Milky Way’s disk of stars points to a recent whack from another galaxy.

After a 10-day lockdown to cooperate with a criminal investigation, Sunspot Solar Observatory is back to looking at the Sun.

A recent analysis of data from NASA’s Dawn spacecraft reveals the role of cryovolcanism past and likely present on the giant asteroid Ceres.

Thirty years ago, Gene Roddenberry, of Star Trek fame, and three astronomers made the case that the orange-hued star 40 Eridani A ought to host Vulcan, Mr. Spock’s home. Now, a robotic survey has discovered a planet around that very star.

Starbirth and stardeath light up a nearby galaxy while faraway galaxies twist and bend in these new images from NASA’s Hubble and Chandra space observatories.

Last weekend the 10th edition of what has become a major stargazing event drew thousands of visitors to Mount Desert Island in Maine.

A new finding suggests that LIGOs neutron-star merger was a typical gamma-ray burst after all.

A new technique gives astronomers a closer look at what makes some stellar carnage so incredibly luminous.

Revised data changed expectations for a star pair that was supposed to merge in 2022.

Cassini’s legacy sheds more light on the strange mystery of Saturn’s northern polar hexagon.

Juno observations reveal that Jupiters magnetic field has a wacky plume.

How did supermassive black holes form? Two studies discovered dozens of middling-mass black holes in dwarf galaxies to fuel an ongoing debate.

Scientists predicted the shape of the solar corona as it would be seen during the August 21, 2017, total solar eclipse. Observations confirmed that they got the broad strokes right.

As told in this month’s astronomy podcast, Venus is disappearing in the west after sunset. So September offers you a final chance to see four bright planets at once.

Astronomers have a precise new mass measurement for Beta Pictoris b, a young gas giant still in the throes of formation 63 light-years from Earth.

NASA’s Osiris-REX asteroid sample return mission spies target Bennu for the first time. Now the spacecraft is setting up for its close approach in December.

The famed Arecibo Observatory has faced down several funding challenges in recent years, and a hurricane to boot, but now a new project is making the radio dish more relevant to astronomy than ever.

After more than a decade of tantalizing but inconclusive hints, new research shows convincingly that patches of water ice lie exposed on the floors of many permanently shadowed lunar craters.

The Opportunity rover fell silent in June after nearly 15 years of work on the Red Planet. Now the dust storm that prevented its batteries from charging is clearing.

In astronomy news this week: A stunning just-released photo of last year’s eclipse, 15,000 galaxies revealed in Hubble’s new ultraviolet view of the deep sky, and watching star formation in action in the spiral galaxy M74.

New observations provide solid evidence of heavy metals in a gas giant exoplanets atmosphere.

A team of scientists has captured evidence that PDS 70b, the first directly imaged instance of early planet formation, is actively accreting material, and theyve measured the rate at which its growing.

Astronomers have discovered auroras around a set of brown dwarfs including one that wanders the galaxy by itself indicating surprisingly strong magnetic fields in these failed stars.

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Astronomy News & Current Events | Sky & Telescope

Astronomy (magazine) – Wikipedia

Astronomy (ISSN0091-6358) is a monthly American magazine about astronomy. Targeting amateur astronomers for its readers, it contains columns on sky viewing, reader-submitted astrophotographs, and articles on astronomy and astrophysics that are readable by nonscientists.

Astronomy is a magazine about the science and hobby of astronomy. Based near Milwaukee in Waukesha, Wisconsin, it is produced by Kalmbach Publishing. Astronomys readers include those interested in astronomy, and those who want to know about sky events, observing techniques, astrophotography, and amateur astronomy in general.

Astronomy was founded in 1973 by Stephen A. Walther, a graduate of the University of WisconsinStevens Point and amateur astronomer. The first issue, August 1973, consisted of 48 pages with five feature articles and information about what to see in the sky that month. Issues contained astrophotos and illustrations created by astronomical artists. Walther had worked part time as a planetarium lecturer at the University of WisconsinMilwaukee and developed an interest in photographing constellations at an early age. Although even in childhood he was interested to obsession in Astronomy, he did so poorly in mathematics that his mother despaired that he would ever be able to earn a living. However he graduated in Journalism from the University of Wisconsin Stevens Point, and as a senior class project he created a business plan for a magazine for amateur astronomers. With the help of his brother David, he was able to bring the magazine to fruition.[citation needed]. He died in 1977.

AstroMedia Corp., the company Walther had founded to publish Astronomy, brought in Richard Berry as editor. Berry also created the offshoot Odyssey, aimed at young readers, and the specialized Telescope Making. In 1985, Milwaukee hobby publisher Kalmbach bought Astronomy.

In 1992, Richard Berry left the magazine and Robert Burnham took over as chief editor. Kalmbach discontinued Deep Sky and Telescope Making magazines and sold Odyssey. In 1996 Bonnie Gordon, now a professor at Central Arizona College, assumed the editorship. David J. Eicher, the creator of “Deep Sky,” became chief editor in 2002.

The Astronomy staff also produces other publications. These have included Explore the Universe; Beginners Guide to Astronomy; Origin and Fate of the Universe; Mars: Explore the Red Planet’s Past, Present, and Future; Atlas of the Stars; Cosmos; and 50 Greatest Mysteries of the Universe. There also was, for a time in the mid-2000s, a Brazilian edition published by Duetto Editora called Astronomy Brasil. However, due mainly to low circulation numbers, Duetto ceased its publication in September 2007.

Astronomy publishes articles about the hobby and science of astronomy. Generally, the front half of the magazine reports on professional science, while the back half of the magazine presents items of interest to hobbyists. Science articles cover such topics as cosmology, space exploration, exobiology, research conducted by professional-class observatories, and individual professional astronomers. Each issue of Astronomy contains a foldout star map showing the evening sky for the current month and the positions of planets, and some comets.

The magazine has regular columnists. They include science writer Bob Berman, who writes a column called Bob Bermans Strange Universe. Stephen James OMeara writes Stephen James OMearas Secret Sky, which covers observing tips and stories relating to deep-sky objects, planets, and comets. Glenn Chaple writes “Glenn Chaples Observing Basics”, a beginners column. Phil Harrington writes “Phil Harringtons Binocular Universe”, about observing with binoculars. “Telescope Insider” interviews people who are a part of the telescope-manufacturing industry.

In each issue of Astronomy Magazine, readers will find star and planet charts, telescope observing tips and techniques, and advice on taking photography of the night sky.[2] The magazine also publishes reader-submitted photos in a gallery, lists astronomy-related events, letters from readers, news, and announcements of new products.

Astronomy may include special sections bound into the magazine, such as booklets or posters. Recent examples have included a Messier Catalog booklet, poster showing comet C/2006 P1 (McNaught) and historical comets, a Skyguide listing upcoming sky events, a Telescope Buyer’s Guide; a poster titled “Atlas of Extrasolar Planets”; and a poster showing the life cycles of stars.

Astronomy is the largest circulation astronomy magazine, with monthly circulation of 114,080.[3] The majority of its readers are in the United States, but it is also circulated in Canada and internationally.[4]

Its major competitor is Sky & Telescope magazine with a circulation of 80,023.[3]

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Astronomy (magazine) – Wikipedia

Welcome | Department of Astronomy

The Leitner Family Observatory and Planetariumis open to the public for the following weekly events:

Two showings on Tuesday nights, 7pm and 8pm:Faster Than Light: the Dream of Interstellar FlightObserving afterwards (at dark, ~9p), weather permitting.

Admission to Tuesday public shows is $5/person. (Free for children under 13 and Yale students).To find out more about public events, please see the LFOP website.

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Welcome | Department of Astronomy

Astronomy | Reference.com

Q: How Do Space Craters Get Their Names?

A: The International Astronomical Union (IAU), an organization of astronomers, names the craters on planets and moons in the solar system by giving each planet a creative theme. For example, the moons craters are usually named for deceased explorers, scientists and scholars, while large craters on Venus are named for famous women in various professional fields.

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Astronomy | Reference.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

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

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

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.

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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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DC Comics pins Krypton to the star map My Nerdist episode is online! Nerd TV Great Tysons ghost! Neil Tyson and I talk time travel

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

Related Posts:

Lunar boulder hits a hole in one! Excavating a long-dead lunar fire fountain A lunar crater is graben the spotlight Peaking into lunar craters

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.

Related Posts:

Uwingu: how *you* can directly fund science Saving space science do you Uwingu? Helping save the planetary space program Barnstorming the final frontier

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.

Related Posts:

ALPHA CENTAURI HAS A PLANET! Kepler confirms first planet found in the habitable zone of a Sun-like star! A nearby star may have more planets than we do Exoplanet in a triple star system, smack dab in the habitable zone Super-Earth exoplanet likely to be a waterworld

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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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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DC Comics pins Krypton to the star map My Nerdist episode is online! Nerd TV Great Tysons ghost! Neil Tyson and I talk time travel

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

Related Posts:

Lunar boulder hits a hole in one! Excavating a long-dead lunar fire fountain A lunar crater is graben the spotlight Peaking into lunar craters

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.

Related Posts:

Uwingu: how *you* can directly fund science Saving space science do you Uwingu? Helping save the planetary space program Barnstorming the final frontier

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.

Related Posts:

ALPHA CENTAURI HAS A PLANET! Kepler confirms first planet found in the habitable zone of a Sun-like star! A nearby star may have more planets than we do Exoplanet in a triple star system, smack dab in the habitable zone Super-Earth exoplanet likely to be a waterworld

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.

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Astronomy Picture of the Day


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