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Hedonism – Wikipedia

Hedonism is a school of thought that argues that pleasure and happiness are the primary or most important intrinsic goods and the aim of human life.[1] A hedonist strives to maximize net pleasure (pleasure minus pain), but when having finally gained that pleasure, happiness remains stationary.

Ethical hedonism is the idea that all people have the right to do everything in their power to achieve the greatest amount of pleasure possible to them. It is also the idea that every person’s pleasure should far surpass their amount of pain. Ethical hedonism is said to have been started by Aristippus of Cyrene, a student of Socrates. He held the idea that pleasure is the highest good.[2]

The name derives from the Greek word for “delight” ( hdonismos from hdon “pleasure”, cognate[according to whom?] with English sweet + suffix – -ismos “ism”). An extremely strong aversion to hedonism is hedonophobia.

In the original Old Babylonian version of the Epic of Gilgamesh, which was written soon after the invention of writing, Siduri gave the following advice “Fill your belly. Day and night make merry. Let days be full of joy. Dance and make music day and night […] These things alone are the concern of men”, which may represent the first recorded advocacy of a hedonistic philosophy.[3]

Scenes of a harper entertaining guests at a feast were common in ancient Egyptian tombs (see Harper’s Songs), and sometimes contained hedonistic elements, calling guests to submit to pleasure because they cannot be sure that they will be rewarded for good with a blissful afterlife. The following is a song attributed to the reign of one of the pharaohs around the time of the 12th dynasty, and the text was used in the eighteenth and nineteenth dynasties.[4][5]

Let thy desire flourish,In order to let thy heart forget the beatifications for thee.Follow thy desire, as long as thou shalt live.Put myrrh upon thy head and clothing of fine linen upon thee,Being anointed with genuine marvels of the gods’ property.Set an increase to thy good things;Let not thy heart flag.Follow thy desire and thy good.Fulfill thy needs upon earth, after the command of thy heart,Until there come for thee that day of mourning.

Democritus seems to be the earliest philosopher on record to have categorically embraced a hedonistic philosophy; he called the supreme goal of life “contentment” or “cheerfulness”, claiming that “joy and sorrow are the distinguishing mark of things beneficial and harmful” (DK 68 B 188).[6]

The Cyrenaics were an ultra-hedonist Greek school of philosophy founded in the 4th century BC, supposedly by Aristippus of Cyrene, although many of the principles of the school are believed to have been formalized by his grandson of the same name, Aristippus the Younger. The school was so called after Cyrene, the birthplace of Aristippus. It was one of the earliest Socratic schools. The Cyrenaics taught that the only intrinsic good is pleasure, which meant not just the absence of pain, but positively enjoyable sensations. Of these, momentary pleasures, especially physical ones, are stronger than those of anticipation or memory. They did, however, recognize the value of social obligation, and that pleasure could be gained from altruism[citation needed]. Theodorus the Atheist was a latter exponent of hedonism who was a disciple of younger Aristippus,[7] while becoming well known for expounding atheism. The school died out within a century, and was replaced by Epicureanism.

The Cyrenaics were known for their skeptical theory of knowledge. They reduced logic to a basic doctrine concerning the criterion of truth.[8] They thought that we can know with certainty our immediate sense-experiences (for instance, that I am having a sweet sensation now) but can know nothing about the nature of the objects that cause these sensations (for instance, that the honey is sweet).[9] They also denied that we can have knowledge of what the experiences of other people are like.[10] All knowledge is immediate sensation. These sensations are motions which are purely subjective, and are painful, indifferent or pleasant, according as they are violent, tranquil or gentle.[9][11] Further they are entirely individual, and can in no way be described as constituting absolute objective knowledge. Feeling, therefore, is the only possible criterion of knowledge and of conduct.[9] Our ways of being affected are alone knowable. Thus the sole aim for everyone should be pleasure.

Cyrenaicism deduces a single, universal aim for all people which is pleasure. Furthermore, all feeling is momentary and homogeneous. It follows that past and future pleasure have no real existence for us, and that among present pleasures there is no distinction of kind.[11] Socrates had spoken of the higher pleasures of the intellect; the Cyrenaics denied the validity of this distinction and said that bodily pleasures, being more simple and more intense, were preferable.[12] Momentary pleasure, preferably of a physical kind, is the only good for humans. However some actions which give immediate pleasure can create more than their equivalent of pain. The wise person should be in control of pleasures rather than be enslaved to them, otherwise pain will result, and this requires judgement to evaluate the different pleasures of life.[13] Regard should be paid to law and custom, because even though these things have no intrinsic value on their own, violating them will lead to unpleasant penalties being imposed by others.[12] Likewise, friendship and justice are useful because of the pleasure they provide.[12] Thus the Cyrenaics believed in the hedonistic value of social obligation and altruistic behaviour.

Epicureanism is a system of philosophy based upon the teachings of Epicurus (c. 341c. 270 BC), founded around 307 BC. Epicurus was an atomic materialist, following in the steps of Democritus and Leucippus. His materialism led him to a general stance against superstition or the idea of divine intervention. Following Aristippusabout whom very little is knownEpicurus believed that the greatest good was to seek modest, sustainable “pleasure” in the form of a state of tranquility and freedom from fear (ataraxia) and absence of bodily pain (aponia) through knowledge of the workings of the world and the limits of our desires. The combination of these two states is supposed to constitute happiness in its highest form. Although Epicureanism is a form of hedonism, insofar as it declares pleasure as the sole intrinsic good, its conception of absence of pain as the greatest pleasure and its advocacy of a simple life make it different from “hedonism” as it is commonly understood.

In the Epicurean view, the highest pleasure (tranquility and freedom from fear) was obtained by knowledge, friendship and living a virtuous and temperate life. He lauded the enjoyment of simple pleasures, by which he meant abstaining from bodily desires, such as sex and appetites, verging on asceticism. He argued that when eating, one should not eat too richly, for it could lead to dissatisfaction later, such as the grim realization that one could not afford such delicacies in the future. Likewise, sex could lead to increased lust and dissatisfaction with the sexual partner. Epicurus did not articulate a broad system of social ethics that has survived but had a unique version of the Golden Rule.

It is impossible to live a pleasant life without living wisely and well and justly (agreeing “neither to harm nor be harmed”),[14] and it is impossible to live wisely and well and justly without living a pleasant life.[15]

Epicureanism was originally a challenge to Platonism, though later it became the main opponent of Stoicism. Epicurus and his followers shunned politics. After the death of Epicurus, his school was headed by Hermarchus; later many Epicurean societies flourished in the Late Hellenistic era and during the Roman era (such as those in Antiochia, Alexandria, Rhodes and Ercolano). The poet Lucretius is its most known Roman proponent. By the end of the Roman Empire, having undergone Christian attack and repression, Epicureanism had all but died out, and would be resurrected in the 17th century by the atomist Pierre Gassendi, who adapted it to the Christian doctrine.

Some writings by Epicurus have survived. Some scholars consider the epic poem On the Nature of Things by Lucretius to present in one unified work the core arguments and theories of Epicureanism. Many of the papyrus scrolls unearthed at the Villa of the Papyri at Herculaneum are Epicurean texts. At least some are thought to have belonged to the Epicurean Philodemus.

Yangism has been described as a form of psychological and ethical egoism. The Yangist philosophers believed in the importance of maintaining self-interest through “keeping one’s nature intact, protecting one’s uniqueness, and not letting the body be tied by other things.” Disagreeing with the Confucian virtues of li (propriety), ren (humaneness), and yi (righteousness) and the Legalist virtue of fa (law), the Yangists saw wei wo, or “everything for myself,” as the only virtue necessary for self-cultivation. Individual pleasure is considered desirable, like in hedonism, but not at the expense of the health of the individual. The Yangists saw individual well-being as the prime purpose of life, and considered anything that hindered that well-being immoral and unnecessary.

The main focus of the Yangists was on the concept of xing, or human nature, a term later incorporated by Mencius into Confucianism. The xing, according to sinologist A. C. Graham, is a person’s “proper course of development” in life. Individuals can only rationally care for their own xing, and should not naively have to support the xing of other people, even if it means opposing the emperor. In this sense, Yangism is a “direct attack” on Confucianism, by implying that the power of the emperor, defended in Confucianism, is baseless and destructive, and that state intervention is morally flawed.

The Confucian philosopher Mencius depicts Yangism as the direct opposite of Mohism, while Mohism promotes the idea of universal love and impartial caring, the Yangists acted only “for themselves,” rejecting the altruism of Mohism. He criticized the Yangists as selfish, ignoring the duty of serving the public and caring only for personal concerns. Mencius saw Confucianism as the “Middle Way” between Mohism and Yangism.

Judaism believes that mankind was created for pleasure, as God placed Adam and Eve in the Garden of EdenEden being the Hebrew word for “pleasure.” In recent years, Rabbi Noah Weinberg articulated five different levels of pleasure; connecting with God is the highest possible pleasure.

Christian doctrine current in some evangelical circles, particularly those of the Reformed tradition.[16] The term was first coined by Reformed Baptist theologian John Piper in his 1986 book Desiring God: My shortest summary of it is: God is most glorified in us when we are most satisfied in him. Or: The chief end of man is to glorify God by enjoying him forever. Does Christian Hedonism make a god out of pleasure? No. It says that we all make a god out of what we take most pleasure in. [16] Piper states his term may describe the theology of Jonathan Edwards, who referred to a future enjoyment of him [God] in heaven.[17] In the 17th century, the atomist Pierre Gassendi adapted Epicureanism to the Christian doctrine.

The concept of hedonism is also found in the Hindu scriptures.[18][19]

Utilitarianism addresses problems with moral motivation neglected by Kantianism by giving a central role to happiness. It is an ethical theory holding that the proper course of action is the one that maximizes the overall good of the society.[20] It is thus one form of consequentialism, meaning that the moral worth of an action is determined by its resulting outcome. The most influential contributors to this theory are considered to be the 18th and 19th-century British philosophers Jeremy Bentham and John Stuart Mill. Conjoining hedonismas a view as to what is good for peopleto utilitarianism has the result that all action should be directed toward achieving the greatest total amount of happiness (see Hedonic calculus). Though consistent in their pursuit of happiness, Bentham and Mill’s versions of hedonism differ. There are two somewhat basic schools of thought on hedonism:[1]

Contemporary proponents of hedonism include Swedish philosopher Torbjrn Tnnsj,[21] Fred Feldman.[22] and Spanish ethic philosopher Esperanza Guisn (published a “Hedonist manifesto” in 1990).[23]

A dedicated contemporary hedonist philosopher and writer on the history of hedonistic thought is the French Michel Onfray. He has written two books directly on the subject (L’invention du plaisir: fragments cyraniques[24] and La puissance d’exister: Manifeste hdoniste).[25] He defines hedonism “as an introspective attitude to life based on taking pleasure yourself and pleasuring others, without harming yourself or anyone else.”[26] Onfray’s philosophical project is to define an ethical hedonism, a joyous utilitarianism, and a generalized aesthetic of sensual materialism that explores how to use the brain’s and the body’s capacities to their fullest extent — while restoring philosophy to a useful role in art, politics, and everyday life and decisions.”[27]

Onfray’s works “have explored the philosophical resonances and components of (and challenges to) science, painting, gastronomy, sex and sensuality, bioethics, wine, and writing. His most ambitious project is his projected six-volume Counter-history of Philosophy,”[27] of which three have been published. For him “In opposition to the ascetic ideal advocated by the dominant school of thought, hedonism suggests identifying the highest good with your own pleasure and that of others; the one must never be indulged at the expense of sacrificing the other. Obtaining this balance my pleasure at the same time as the pleasure of others presumes that we approach the subject from different angles political, ethical, aesthetic, erotic, bioethical, pedagogical, historiographical.”

For this he has “written books on each of these facets of the same world view.”[28] His philosophy aims for “micro-revolutions”, or “revolutions of the individual and small groups of like-minded people who live by his hedonistic, libertarian values.”[29]

The Abolitionist Society is a transhumanist group calling for the abolition of suffering in all sentient life through the use of advanced biotechnology. Their core philosophy is negative utilitarianism. David Pearce is a theorist of this perspective and he believes and promotes the idea that there exists a strong ethical imperative for humans to work towards the abolition of suffering in all sentient life. His book-length internet manifesto The Hedonistic Imperative[30] outlines how technologies such as genetic engineering, nanotechnology, pharmacology, and neurosurgery could potentially converge to eliminate all forms of unpleasant experience among human and non-human animals, replacing suffering with gradients of well-being, a project he refers to as “paradise engineering”.[31] A transhumanist and a vegan,[32] Pearce believes that we (or our future posthuman descendants) have a responsibility not only to avoid cruelty to animals within human society but also to alleviate the suffering of animals in the wild.

In a talk David Pearce gave at the Future of Humanity Institute and at the Charity International ‘Happiness Conference’ he said “Sadly, what won’t abolish suffering, or at least not on its own, is socio-economic reform, or exponential economic growth, or technological progress in the usual sense, or any of the traditional panaceas for solving the world’s ills. Improving the external environment is admirable and important; but such improvement can’t recalibrate our hedonic treadmill above a genetically constrained ceiling. Twin studies confirm there is a [partially] heritable set-point of well-being – or ill-being – around which we all tend to fluctuate over the course of a lifetime. This set-point varies between individuals. [It’s possible to lower an individual’s hedonic set-point by inflicting prolonged uncontrolled stress; but even this re-set is not as easy as it sounds: suicide-rates typically go down in wartime; and six months after a quadriplegia-inducing accident, studies[citation needed] suggest that we are typically neither more nor less unhappy than we were before the catastrophic event.] Unfortunately, attempts to build an ideal society can’t overcome this biological ceiling, whether utopias of the left or right, free-market or socialist, religious or secular, futuristic high-tech or simply cultivating one’s garden. Even if everything that traditional futurists have asked for is delivered – eternal youth, unlimited material wealth, morphological freedom, superintelligence, immersive VR, molecular nanotechnology, etc – there is no evidence that our subjective quality of life would on average significantly surpass the quality of life of our hunter-gatherer ancestors – or a New Guinea tribesman today – in the absence of reward pathway enrichment. This claim is difficult to prove in the absence of sophisticated neuroscanning; but objective indices of psychological distress e.g. suicide rates, bear it out. Unenhanced humans will still be prey to the spectrum of Darwinian emotions, ranging from terrible suffering to petty disappointments and frustrations – sadness, anxiety, jealousy, existential angst. Their biology is part of “what it means to be human”. Subjectively unpleasant states of consciousness exist because they were genetically adaptive. Each of our core emotions had a distinct signalling role in our evolutionary past: they tended to promote behaviours that enhanced the inclusive fitness of our genes in the ancestral environment.”[33]

Russian physicist and philosopher Victor Argonov argues that hedonism is not only a philosophical but also a verifiable scientific hypothesis. In 2014 he suggested “postulates of pleasure principle” confirmation of which would lead to a new scientific discipline, hedodynamics. Hedodynamics would be able to forecast the distant future development of human civilization and even the probable structure and psychology of other rational beings within the universe.[34] In order to build such a theory, science must discover the neural correlate of pleasure – neurophysiological parameter unambiguously corresponding to the feeling of pleasure (hedonic tone).

According to Argonov, posthumans will be able to reprogram their motivations in an arbitrary manner (to get pleasure from any programmed activity).[35] And if pleasure principle postulates are true, then general direction of civilization development is obvious: maximization of integral happiness in posthuman life (product of life span and average happiness). Posthumans will avoid constant pleasure stimulation, because it is incompatible with rational behavior required to prolong life. However, in average, they can become much happier than modern humans.

Many other aspects of posthuman society could be predicted by hedodynamics if the neural correlate of pleasure were discovered. For example, optimal number of individuals, their optimal body size (whether it matters for happiness or not) and the degree of aggression.

Critics of hedonism have objected to its exclusive concentration on pleasure as valuable.

In particular, G. E. Moore offered a thought experiment in criticism of pleasure as the sole bearer of value: he imagined two worldsone of exceeding beauty and the other a heap of filth. Neither of these worlds will be experienced by anyone. The question, then, is if it is better for the beautiful world to exist than the heap of filth. In this Moore implied that states of affairs have value beyond conscious pleasure, which he said spoke against the validity of hedonism.[36]

In Quran, God admonished mankind not to love the worldly pleasures, since it is related with greedy and source of sinful habit. He also threatened those who prefer worldly life rather than hereafter with Hell.

Those who choose the worldly life and its pleasures will be given proper recompense for their deeds in this life and will not suffer any loss. Such people will receive nothing in the next life except Hell fire. Their deeds will be made devoid of all virtue and their efforts will be in vain.

“Hedonism”. Encyclopdia Britannica (11th ed.). 1911.

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Hedonism – Wikipedia

Hedonism | Internet Encyclopedia of Philosophy

The term “hedonism,” from the Greek word (hdon) for pleasure, refers to several related theories about what is good for us, how we should behave, and what motivates us to behave in the way that we do. All hedonistic theories identify pleasure and pain as the only important elements of whatever phenomena they are designed to describe. If hedonistic theories identified pleasure and pain as merely two important elements, instead of the only important elements of what they are describing, then they would not be nearly as unpopular as they all are. However, the claim that pleasure and pain are the only things of ultimate importance is what makes hedonism distinctive and philosophically interesting.

Philosophical hedonists tend to focus on hedonistic theories of value, and especially of well-being (the good life for the one living it). As a theory of value, hedonism states that all and only pleasure is intrinsically valuable and all and only pain is intrinsically not valuable. Hedonists usually define pleasure and pain broadly, such that both physical and mental phenomena are included. Thus, a gentle massage and recalling a fond memory are both considered to cause pleasure and stubbing a toe and hearing about the death of a loved one are both considered to cause pain. With pleasure and pain so defined, hedonism as a theory about what is valuable for us is intuitively appealing. Indeed, its appeal is evidenced by the fact that nearly all historical and contemporary treatments of well-being allocate at least some space for discussion of hedonism. Unfortunately for hedonism, the discussions rarely endorse it and some even deplore its focus on pleasure.

This article begins by clarifying the different types of hedonistic theories and the labels they are often given. Then, hedonisms ancient origins and its subsequent development are reviewed. The majority of this article is concerned with describing the important theoretical divisions within Prudential Hedonism and discussing the major criticisms of these approaches.

When the term “hedonism” is used in modern literature, or by non-philosophers in their everyday talk, its meaning is quite different from the meaning it takes when used in the discussions of philosophers. Non-philosophers tend to think of a hedonist as a person who seeks out pleasure for themselves without any particular regard for their own future well-being or for the well-being of others. According to non-philosophers, then, a stereotypical hedonist is someone who never misses an opportunity to indulge of the pleasures of sex, drugs, and rock n roll, even if the indulgences are likely to lead to relationship problems, health problems, regrets, or sadness for themselves or others. Philosophers commonly refer to this everyday understanding of hedonism as “Folk Hedonism.” Folk Hedonism is a rough combination of Motivational Hedonism, Hedonistic Egoism, and a reckless lack of foresight.

When philosophers discuss hedonism, they are most likely to be referring to hedonism about value, and especially the slightly more specific theory, hedonism about well-being. Hedonism as a theory about value (best referred to as Value Hedonism) holds that all and only pleasure is intrinsically valuable and all and only pain is intrinsically disvaluable. The term “intrinsically” is an important part of the definition and is best understood in contrast to the term “instrumentally.” Something is intrinsically valuable if it is valuable for its own sake. Pleasure is thought to be intrinsically valuable because, even if it did not lead to any other benefit, it would still be good to experience. Money is an example of an instrumental good; its value for us comes from what we can do with it (what we can buy with it). The fact that a copious amount of money has no value if no one ever sells anything reveals that money lacks intrinsic value. Value Hedonism reduces everything of value to pleasure. For example, a Value Hedonist would explain the instrumental value of money by describing how the things we can buy with money, such as food, shelter, and status-signifying goods, bring us pleasure or help us to avoid pain.

Hedonism as a theory about well-being (best referred to as Prudential Hedonism) is more specific than Value Hedonism because it stipulates what the value is for. Prudential Hedonism holds that all and only pleasure intrinsically makes peoples lives go better for them and all and only pain intrinsically makes their lives go worse for them. Some philosophers replace “people” with “animals” or “sentient creatures,” so as to apply Prudential Hedonism more widely. A good example of this comes from Peter Singers work on animals and ethics. Singer questions why some humans can see the intrinsic disvalue in human pain, but do not also accept that it is bad for sentient non-human animals to experience pain.

When Prudential Hedonists claim that happiness is what they value most, they intend happiness to be understood as a preponderance of pleasure over pain. An important distinction between Prudential Hedonism and Folk Hedonism is that Prudential Hedonists usually understand that pursuing pleasure and avoiding pain in the very short-term is not always the best strategy for achieving the best long-term balance of pleasure over pain.

Prudential Hedonism is an integral part of several derivative types of hedonistic theory, all of which have featured prominently in philosophical debates of the past. Since Prudential Hedonism plays this important role, the majority of this article is dedicated to Prudential Hedonism. First, however, the main derivative types of hedonism are briefly discussed.

Motivational Hedonism (more commonly referred to by the less descriptive label, “Psychological Hedonism”) is the theory that the desires to encounter pleasure and to avoid pain guide all of our behavior. Most accounts of Motivational Hedonism include both conscious and unconscious desires for pleasure, but emphasize the latter. Epicurus, William James, Sigmund Freud, Jeremy Bentham, John Stuart Mill, and (on one interpretation) even Charles Darwin have all argued for varieties of Motivational Hedonism. Bentham used the idea to support his theory of Hedonistic Utilitarianism (discussed below). Weak versions of Motivational Hedonism hold that the desires to seek pleasure and avoid pain often or always have some influence on our behavior. Weak versions are generally considered to be uncontroversially true and not especially useful for philosophy.

Philosophers have been more interested in strong accounts of Motivational Hedonism, which hold that all behavior is governed by the desires to encounter pleasure and to avoid pain (and only those desires). Strong accounts of Motivational Hedonism have been used to support some of the normative types of hedonism and to argue against non-hedonistic normative theories. One of the most notable mentions of Motivational Hedonism is Platos Ring of Gyges example in The Republic. Platos Socrates is discussing with Glaucon how men would react if they were to possess a ring that gives its wearer immense powers, including invisibility. Glaucon believes that a strong version of Motivational Hedonism is true, but Socrates does not. Glaucon asserts that, emboldened with the power provided by the Ring of Gyges, everyone would succumb to the inherent and ubiquitous desire to pursue their own ends at the expense of others. Socrates disagrees, arguing that good people would be able to overcome this desire because of their strong love of justice, fostered through philosophising.

Strong accounts of Motivational Hedonism currently garner very little support for similar reasons. Many examples of seemingly-pain-seeking acts performed out of a sense of duty are well-known from the soldier who jumps on a grenade to save his comrades to that time you rescued a trapped dog only to be (predictably) bitten in the process. Introspective evidence also weighs against strong accounts of Motivational Hedonism; many of the decisions we make seem to be based on motives other than seeking pleasure and avoiding pain. Given these reasons, the burden of proof is considered to be squarely on the shoulders of anyone wishing to argue for a strong account of Motivational Hedonism.

Value Hedonism, occasionally with assistance from Motivational Hedonism, has been used to argue for specific theories of right action (theories that explain which actions are morally permissible or impermissible and why). The theory that happiness should be pursued (that pleasure should be pursued and pain should be avoided) is referred to as Normative Hedonism and sometimes Ethical Hedonism. There are two major types of Normative Hedonism, Hedonistic Egoism and Hedonistic Utilitarianism. Both types commonly use happiness (defined as pleasure minus pain) as the sole criterion for determining the moral rightness or wrongness of an action. Important variations within each of these two main types specify either the actual resulting happiness (after the act) or the predicted resulting happiness (before the act) as the moral criterion. Although both major types of Normative Hedonism have been accused of being repugnant, Hedonistic Egoism is considered the most offensive.

Hedonistic Egoism is a hedonistic version of egoism, the theory that we should, morally speaking, do whatever is most in our own interests. Hedonistic Egoism is the theory that we ought, morally speaking, to do whatever makes us happiest that is whatever provides us with the most net pleasure after pain is subtracted. The most repugnant feature of this theory is that one never has to ascribe any value whatsoever to the consequences for anyone other than oneself. For example, a Hedonistic Egoist who did not feel saddened by theft would be morally required to steal, even from needy orphans (if he thought he could get away with it). Would-be defenders of Hedonistic Egoism often point out that performing acts of theft, murder, treachery and the like would not make them happier overall because of the guilt, the fear of being caught, and the chance of being caught and punished. The would-be defenders tend to surrender, however, when it is pointed out that a Hedonistic Egoist is morally obliged by their own theory to pursue an unusual kind of practical education; a brief and possibly painful training period that reduces their moral emotions of sympathy and guilt. Such an education might be achieved by desensitising over-exposure to, and performance of, torture on innocents. If Hedonistic Egoists underwent such an education, their reduced capacity for sympathy and guilt would allow them to take advantage of any opportunities to perform pleasurable, but normally-guilt-inducing, actions, such as stealing from the poor.

Hedonistic Egoism is very unpopular amongst philosophers, not just for this reason, but also because it suffers from all of the objections that apply to Prudential Hedonism.

Hedonistic Utilitarianism is the theory that the right action is the one that produces (or is most likely to produce) the greatest net happiness for all concerned. Hedonistic Utilitarianism is often considered fairer than Hedonistic Egoism because the happiness of everyone involved (everyone who is affected or likely to be affected) is taken into account and given equal weight. Hedonistic Utilitarians, then, tend to advocate not stealing from needy orphans because to do so would usually leave the orphan far less happy and the (probably better-off) thief only slightly happier (assuming he felt no guilt). Despite treating all individuals equally, Hedonistic Utilitarianism is still seen as objectionable by some because it assigns no intrinsic moral value to justice, friendship, truth, or any of the many other goods that are thought by some to be irreducibly valuable. For example, a Hedonistic Utilitarian would be morally obliged to publicly execute an innocent friend of theirs if doing so was the only way to promote the greatest happiness overall. Although unlikely, such a situation might arise if a child was murdered in a small town and the lack of suspects was causing large-scale inter-ethnic violence. Some philosophers argue that executing an innocent friend is immoral precisely because it ignores the intrinsic values of justice, friendship, and possibly truth.

Hedonistic Utilitarianism is rarely endorsed by philosophers, but mainly because of its reliance on Prudential Hedonism as opposed to its utilitarian element. Non-hedonistic versions of utilitarianism are about as popular as the other leading theories of right action, especially when it is the actions of institutions that are being considered.

Perhaps the earliest written record of hedonism comes from the Crvka, an Indian philosophical tradition based on the Barhaspatya sutras. The Crvka persisted for two thousand years (from about 600 B.C.E.). Most notably, the Crvka advocated scepticism and Hedonistic Egoism that the right action is the one that brings the actor the most net pleasure. The Crvka acknowledged that some pain often accompanied, or was later caused by, sensual pleasure, but that pleasure was worth it.

The Cyrenaics, founded by Aristippus (c. 435-356 B.C.E.), were also sceptics and Hedonistic Egoists. Although the paucity of original texts makes it difficult to confidently state all of the justifications for the Cyrenaics positions, their overall stance is clear enough. The Cyrenaics believed pleasure was the ultimate good and everyone should pursue all immediate pleasures for themselves. They considered bodily pleasures better than mental pleasures, presumably because they were more vivid or trustworthy. The Cyrenaics also recommended pursuing immediate pleasures and avoiding immediate pains with scant or no regard for future consequences. Their reasoning for this is even less clear, but is most plausibly linked to their sceptical views perhaps that what we can be most sure of in this uncertain existence is our current bodily pleasures.

Epicurus (c. 341-271 B.C.E.), founder of Epicureanism, developed a Normative Hedonism in stark contrast to that of Aristippus. The Epicureanism of Epicurus is also quite the opposite to the common usage of Epicureanism; while we might like to go on a luxurious “Epicurean” holiday packed with fine dining and moderately excessive wining, Epicurus would warn us that we are only setting ourselves up for future pain. For Epicurus, happiness was the complete absence of bodily and especially mental pains, including fear of the Gods and desires for anything other than the bare necessities of life. Even with only the limited excesses of ancient Greece on offer, Epicurus advised his followers to avoid towns, and especially marketplaces, in order to limit the resulting desires for unnecessary things. Once we experience unnecessary pleasures, such as those from sex and rich food, we will then suffer from painful and hard to satisfy desires for more and better of the same. No matter how wealthy we might be, Epicurus would argue, our desires will eventually outstrip our means and interfere with our ability to live tranquil, happy lives. Epicureanism is generally egoistic, in that it encourages everyone to pursue happiness for themselves. However, Epicureans would be unlikely to commit any of the selfish acts we might expect from other egoists because Epicureans train themselves to desire only the very basics, which gives them very little reason to do anything to interfere with the affairs of others.

With the exception of a brief period discussed below, Hedonism has been generally unpopular ever since its ancient beginnings. Although criticisms of the ancient forms of hedonism were many and varied, one in particular was heavily cited. In Philebus, Platos Socrates and one of his many foils, Protarchus in this instance, are discussing the role of pleasure in the good life. Socrates asks Protarchus to imagine a life without much pleasure but full of the higher cognitive processes, such as knowledge, forethought and consciousness and to compare it with a life that is the opposite. Socrates describes this opposite life as having perfect pleasure but the mental life of an oyster, pointing out that the subject of such a life would not be able to appreciate any of the pleasure within it. The harrowing thought of living the pleasurable but unthinking life of an oyster causes Protarchus to abandon his hedonistic argument. The oyster example is now easily avoided by clarifying that pleasure is best understood as being a conscious experience, so any sensation that we are not consciously aware of cannot be pleasure.

Normative and Motivational Hedonism were both at their most popular during the heyday of Empiricism in the 18th and 19th Centuries. Indeed, this is the only period during which any kind of hedonism could be considered popular at all. During this period, two Hedonistic Utilitarians, Jeremy Bentham (1748-1832) and his protg John Stuart Mill (1806-1873), were particularly influential. Their theories are similar in many ways, but are notably distinct on the nature of pleasure.

Bentham argued for several types of hedonism, including those now referred to as Prudential Hedonism, Hedonistic Utilitarianism, and Motivational Hedonism (although his commitment to strong Motivational Hedonism eventually began to wane). Bentham argued that happiness was the ultimate good and that happiness was pleasure and the absence of pain. He acknowledged the egoistic and hedonistic nature of peoples motivation, but argued that the maximization of collective happiness was the correct criterion for moral behavior. Benthams greatest happiness principle states that actions are immoral if they are not the action that appears to maximise the happiness of all the people likely to be affected; only the action that appears to maximise the happiness of all the people likely to be affected is the morally right action.

Bentham devised the greatest happiness principle to justify the legal reforms he also argued for. He understood that he could not conclusively prove that the principle was the correct criterion for morally right action, but also thought that it should be accepted because it was fair and better than existing criteria for evaluating actions and legislation. Bentham thought that his Hedonic Calculus could be applied to situations to see what should, morally speaking, be done in a situation. The Hedonic Calculus is a method of counting the amount of pleasure and pain that would likely be caused by different actions. The Hedonic Calculus required a methodology for measuring pleasure, which in turn required an understanding of the nature of pleasure and specifically what aspects of pleasure were valuable for us.

Benthams Hedonic Calculus identifies several aspects of pleasure that contribute to its value, including certainty, propinquity, extent, intensity, and duration. The Hedonic Calculus also makes use of two future-pleasure-or-pain-related aspects of actions fecundity and purity. Certainty refers to the likelihood that the pleasure or pain will occur. Propinquity refers to how long away (in terms of time) the pleasure or pain is. Fecundity refers to the likelihood of the pleasure or pain leading to more of the same sensation. Purity refers to the likelihood of the pleasure or pain leading to some of the opposite sensation. Extent refers to the number of people the pleasure or pain is likely to affect. Intensity refers to the felt strength of the pleasure or pain. Duration refers to how long the pleasure or pain are felt for. It should be noted that only intensity and duration have intrinsic value for an individual. Certainty, propinquity, fecundity, and purity are all instrumentally valuable for an individual because they affect the likelihood of an individual feeling future pleasure and pain. Extent is not directly valuable for an individuals well-being because it refers to the likelihood of other people experiencing pleasure or pain.

Benthams inclusion of certainty, propinquity, fecundity, and purity in the Hedonic Calculus helps to differentiate his hedonism from Folk Hedonism. Folk Hedonists rarely consider how likely their actions are to lead to future pleasure or pain, focussing instead on the pursuit of immediate pleasure and the avoidance of immediate pain. So while Folk Hedonists would be unlikely to study for an exam, anyone using Benthams Hedonic Calculus would consider the future happiness benefits to themselves (and possibly others) of passing the exam and then promptly begin studying.

Most importantly for Benthams Hedonic Calculus, the pleasure from different sources is always measured against these criteria in the same way, that is to say that no additional value is afforded to pleasures from particularly moral, clean, or culturally-sophisticated sources. For example, Bentham held that pleasure from the parlor game push-pin was just as valuable for us as pleasure from music and poetry. Since Benthams theory of Prudential Hedonism focuses on the quantity of the pleasure, rather than the source-derived quality of it, it is best described as a type of Quantitative Hedonism.

Benthams indifferent stance on the source of pleasures led to others disparaging his hedonism as the philosophy of swine. Even his student, John Stuart Mill, questioned whether we should believe that a satisfied pig leads a better life than a dissatisfied human or that a satisfied fool leads a better life than a dissatisfied Socrates results that Benthams Quantitative Hedonism seems to endorse.

Like Bentham, Mill endorsed the varieties of hedonism now referred to as Prudential Hedonism, Hedonistic Utilitarianism, and Motivational Hedonism. Mill also thought happiness, defined as pleasure and the avoidance of pain, was the highest good. Where Mills hedonism differs from Benthams is in his understanding of the nature of pleasure. Mill argued that pleasures could vary in quality, being either higher or lower pleasures. Mill employed the distinction between higher and lower pleasures in an attempt to avoid the criticism that his hedonism was just another philosophy of swine. Lower pleasures are those associated with the body, which we share with other animals, such as pleasure from quenching thirst or having sex. Higher pleasures are those associated with the mind, which were thought to be unique to humans, such as pleasure from listening to opera, acting virtuously, and philosophising. Mill justified this distinction by arguing that those who have experienced both types of pleasure realise that higher pleasures are much more valuable. He dismissed challenges to this claim by asserting that those who disagreed lacked either the experience of higher pleasures or the capacity for such experiences. For Mill, higher pleasures were not different from lower pleasures by mere degree; they were different in kind. Since Mills theory of Prudential Hedonism focuses on the quality of the pleasure, rather than the amount of it, it is best described as a type of Qualitative Hedonism.

George Edward Moore (1873-1958) was instrumental in bringing hedonisms brief heyday to an end. Moores criticisms of hedonism in general, and Mills hedonism in particular, were frequently cited as good reasons to reject hedonism even decades after his death. Indeed, since G. E. Moore, hedonism has been viewed by most philosophers as being an initially intuitive and interesting family of theories, but also one that is flawed on closer inspection. Moore was a pluralist about value and argued persuasively against the Value Hedonists central claim that all and only pleasure is the bearer of intrinsic value. Moores most damaging objection against Hedonism was his heap of filth example. Moore himself thought the heap of filth example thoroughly refuted what he saw as the only potentially viable form of Prudential Hedonism that conscious pleasure is the only thing that positively contributes to well-being. Moore used the heap of filth example to argue that Prudential Hedonism is false because pleasure is not the only thing of value.

In the heap of filth example, Moore asks the reader to imagine two worlds, one of which is exceedingly beautiful and the other a disgusting heap of filth. Moore then instructs the reader to imagine that no one would ever experience either world and asks if it is better for the beautiful world to exist than the filthy one. As Moore expected, his contemporaries tended to agree that it would be better if the beautiful world existed. Relying on this agreement, Moore infers that the beautiful world is more valuable than the heap of filth and, therefore, that beauty must be valuable. Moore then concluded that all of the potentially viable theories of Prudential Hedonism (those that value only conscious pleasures) must be false because something, namely beauty, is valuable even when no conscious pleasure can be derived from it.

Moores heap of filth example has rarely been used to object to Prudential Hedonism since the 1970s because it is not directly relevant to Prudential Hedonism (it evaluates worlds and not lives). Moores other objections to Prudential Hedonism also went out of favor around the same time. The demise of these arguments was partly due to mounting objections against them, but mainly because arguments more suited to the task of refuting Prudential Hedonism were developed. These arguments are discussed after the contemporary varieties of hedonism are introduced below.

Several contemporary varieties of hedonism have been defended, although usually by just a handful of philosophers or less at any one time. Other varieties of hedonism are also theoretically available but have received little or no discussion. Contemporary varieties of Prudential Hedonism can be grouped based on how they define pleasure and pain, as is done below. In addition to providing different notions of what pleasure and pain are, contemporary varieties of Prudential Hedonism also disagree about what aspect or aspects of pleasure are valuable for well-being (and the opposite for pain).

The most well-known disagreement about what aspects of pleasure are valuable occurs between Quantitative and Qualitative Hedonists. Quantitative Hedonists argue that how valuable pleasure is for well-being depends on only the amount of pleasure, and so they are only concerned with dimensions of pleasure such as duration and intensity. Quantitative Hedonism is often accused of over-valuing animalistic, simple, and debauched pleasures.

Qualitative Hedonists argue that, in addition to the dimensions related to the amount of pleasure, one or more dimensions of quality can have an impact on how pleasure affects well-being. The quality dimensions might be based on how cognitive or bodily the pleasure is (as it was for Mill), the moral status of the source of the pleasure, or some other non-amount-related dimension. Qualitative Hedonism is criticised by some for smuggling values other than pleasure into well-being by misleadingly labelling them as dimensions of pleasure. How these qualities are chosen for inclusion is also criticised for being arbitrary or ad hoc by some because inclusion of these dimensions of pleasure is often in direct response to objections that Quantitative Hedonism cannot easily deal with. That is to say, the inclusion of these dimensions is often accused of being an exercise in plastering over holes, rather than deducing corollary conclusions from existing theoretical premises. Others have argued that any dimensions of quality can be better explained in terms of dimensions of quantity. For example, they might claim that moral pleasures are no higher in quality than immoral pleasures, but that moral pleasures are instrumentally more valuable because they are likely to lead to more moments of pleasure or less moments of pain in the future.

Hedonists also have differing views about how the value of pleasure compares with the value of pain. This is not a practical disagreement about how best to measure pleasure and pain, but rather a theoretical disagreement about comparative value, such as whether pain is worse for us than an equivalent amount of pleasure is good for us. The default position is that one unit of pleasure (sometimes referred to as a Hedon) is equivalent but opposite in value to one unit of pain (sometimes referred to as a Dolor). Several Hedonistic Utilitarians have argued that reduction of pain should be seen as more important than increasing pleasure, sometimes for the Epicurean reason that pain seems worse for us than an equivalent amount of pleasure is good for us. Imagine that a magical genie offered for you to play a game with him. The game consists of you flipping a fair coin. If the coin lands on heads, then you immediately feel a burst of very intense pleasure and if it lands on tails, then you immediately feel a burst of very intense pain. Is it in your best interests to play the game?

Another area of disagreement between some Hedonists is whether pleasure is entirely internal to a person or if it includes external elements. Internalism about pleasure is the thesis that, whatever pleasure is, it is always and only inside a person. Externalism about pleasure, on the other hand, is the thesis that, pleasure is more than just a state of an individual (that is, that a necessary component of pleasure lies outside of the individual). Externalists about pleasure might, for example, describe pleasure as a function that mediates between our minds and the environment, such that every instance of pleasure has one or more integral environmental components. The vast majority of historic and contemporary versions of Prudential Hedonism consider pleasure to be an internal mental state.

Perhaps the least known disagreement about what aspects of pleasure make it valuable is the debate about whether we have to be conscious of pleasure for it to be valuable. The standard position is that pleasure is a conscious mental state, or at least that any pleasure a person is not conscious of does not intrinsically improve their well-being.

The most common definition of pleasure is that it is a sensation, something that we identify through our senses or that we feel. Psychologists claim that we have at least ten senses, including the familiar, sight, hearing, smell, taste, and touch, but also, movement, balance, and several sub-senses of touch, including heat, cold, pressure, and pain. New senses get added to the list when it is understood that some independent physical process underpins their functioning. The most widely-used examples of pleasurable sensations are the pleasures of eating, drinking, listening to music, and having sex. Use of these examples has done little to help Hedonism avoid its debauched reputation.

It is also commonly recognised that our senses are physical processes that usually involve a mental component, such as the tickling feeling when someone blows gently on the back of your neck. If a sensation is something we identify through our sense organs, however, it is not entirely clear how to account for abstract pleasures. This is because abstract pleasures, such as a feeling of accomplishment for a job well done, do not seem to be experienced through any of the senses in the standard lists. Some Hedonists have attempted to resolve this problem by arguing for the existence of an independent pleasure sense and by defining sensation as something that we feel (regardless of whether it has been mediated by sense organs).

Most Hedonists who describe pleasure as a sensation will be Quantitative Hedonists and will argue that the pleasure from the different senses is the same. Qualitative Hedonists, in comparison, can use the framework of the senses to help differentiate between qualities of pleasure. For example, a Qualitative Hedonist might argue that pleasurable sensations from touch and movement are always lower quality than the others.

Hedonists have also defined pleasure as intrinsically valuable experience, that is to say any experiences that we find intrinsically valuable either are, or include, instances of pleasure. According to this definition, the reason that listening to music and eating a fine meal are both intrinsically pleasurable is because those experiences include an element of pleasure (along with the other elements specific to each activity, such as the experience of the texture of the food and the melody of the music). By itself, this definition enables Hedonists to make an argument that is close to perfectly circular. Defining pleasure as intrinsically valuable experience and well-being as all and only experiences that are intrinsically valuable allows a Hedonist to all but stipulate that Prudential Hedonism is the correct theory of well-being. Where defining pleasure as intrinsically valuable experience is not circular is in its stipulation that only experiences matter for well-being. Some well-known objections to this idea are discussed below.

Another problem with defining pleasure as intrinsically valuable experience is that the definition does not tell us very much about what pleasure is or how it can be identified. For example, knowing that pleasure is intrinsically valuable experience would not help someone to work out if a particular experience was intrinsically or just instrumentally valuable. Hedonists have attempted to respond to this problem by explaining how to find out whether an experience is intrinsically valuable.

One method is to ask yourself if you would like the experience to continue for its own sake (rather than because of what it might lead to). Wanting an experience to continue for its own sake reveals that you find it to be intrinsically valuable. While still making a coherent theory of well-being, defining intrinsically valuable experiences as those you want to perpetuate makes the theory much less hedonistic. The fact that what a person wants is the main criterion for something having intrinsic value, makes this kind of theory more in line with preference satisfaction theories of well-being. The central claim of preference satisfaction theories of well-being is that some variant of getting what one wants, or should want, under certain conditions is the only thing that intrinsically improves ones well-being.

Another method of fleshing out the definition of pleasure as intrinsically valuable experience is to describe how intrinsically valuable experiences feel. This method remains a hedonistic one, but seems to fall back into defining pleasure as a sensation.

It has also been argued that what makes an experience intrinsically valuable is that you like or enjoy it for its own sake. Hedonists arguing for this definition of pleasure usually take pains to position their definition in between the realms of sensation and preference satisfaction. They argue that since we can like or enjoy some experiences without concurrently wanting them or feeling any particular sensation, then liking is distinct from both sensation and preference satisfaction. Liking and enjoyment are also difficult terms to define in more detail, but they are certainly easier to recognise than the rather opaque “intrinsically valuable experience.”

Merely defining pleasure as intrinsically valuable experience and intrinsically valuable experiences as those that we like or enjoy still lacks enough detail to be very useful for contemplating well-being. A potential method for making this theory more useful would be to draw on the cognitive sciences to investigate if there is a specific neurological function for liking or enjoying. Cognitive science has not reached the point where anything definitive can be said about this, but a few neuroscientists have experimental evidence that liking and wanting (at least in regards to food) are neurologically distinct processes in rats and have argued that it should be the same for humans. The same scientists have wondered if the same processes govern all of our liking and wanting, but this question remains unresolved.

Most Hedonists who describe pleasure as intrinsically valuable experience believe that pleasure is internal and conscious. Hedonists who define pleasure in this way may be either Quantitative or Qualitative Hedonists, depending on whether they think that quality is a relevant dimension of how intrinsically valuable we find certain experiences.

One of the most recent developments in modern hedonism is the rise of defining pleasure as a pro-attitude a positive psychological stance toward some object. Any account of Prudential Hedonism that defines pleasure as a pro-attitude is referred to as Attitudinal Hedonism because it is a persons attitude that dictates whether anything has intrinsic value. Positive psychological stances include approving of something, thinking it is good, and being pleased about it. The object of the positive psychological stance could be a physical object, such as a painting one is observing, but it could also be a thought, such as “my country is not at war,” or even a sensation. An example of a pro-attitude towards a sensation could be being pleased about the fact that an ice cream tastes so delicious.

Fred Feldman, the leading proponent of Attitudinal Hedonism, argues that the sensation of pleasure only has instrumental value it only brings about value if you also have a positive psychological stance toward that sensation. In addition to his basic Intrinsic Attitudinal Hedonism, which is a form of Quantitative Hedonism, Feldman has also developed many variants that are types of Qualitative Hedonism. For example, Desert-Adjusted Intrinsic Attitudinal Hedonism, which reduces the intrinsic value a pro-attitude has for our well-being based on the quality of deservedness (that is, on the extent to which the particular object deserves a pro-attitude or not). For example, Desert-Adjusted Intrinsic Attitudinal Hedonism might stipulate that sensations of pleasure arising from adulterous behavior do not deserve approval, and so assign them no value.

Defining pleasure as a pro-attitude, while maintaining that all sensations of pleasure have no intrinsic value, makes Attitudinal Hedonism less obviously hedonistic as the versions that define pleasure as a sensation. Indeed, defining pleasure as a pro-attitude runs the risk of creating a preference satisfaction account of well-being because being pleased about something without feeling any pleasure seems hard to distinguish from having a preference for that thing.

The most common argument against Prudential Hedonism is that pleasure is not the only thing that intrinsically contributes to well-being. Living in reality, finding meaning in life, producing noteworthy achievements, building and maintaining friendships, achieving perfection in certain domains, and living in accordance with religious or moral laws are just some of the other things thought to intrinsically add value to our lives. When presented with these apparently valuable aspects of life, Hedonists usually attempt to explain their apparent value in terms of pleasure. A Hedonist would argue, for example, that friendship is not valuable in and of itself, rather it is valuable to the extent that it brings us pleasure. Furthermore, to answer why we might help a friend even when it harms us, a Hedonist will argue that the prospect of future pleasure from receiving reciprocal favors from our friend, rather than the value of friendship itself, should motivate us to help in this way.

Those who object to Prudential Hedonism on the grounds that pleasure is not the only source of intrinsic value use two main strategies. In the first strategy, objectors make arguments that some specific value cannot be reduced to pleasure. In the second strategy, objectors cite very long lists of apparently intrinsically valuable aspects of life and then challenge hedonists with the prolonged and arduous task of trying to explain how the value of all of them can be explained solely by reference to pleasure and the avoidance of pain. This second strategy gives good reason to be a pluralist about value because the odds seem to be against any monistic theory of value, such as Prudential Hedonism. The first strategy, however, has the ability to show that Prudential Hedonism is false, rather than being just unlikely to be the best theory of well-being.

The most widely cited argument for pleasure not being the only source of intrinsic value is based on Robert Nozicks experience machine thought-experiment. Nozicks experience machine thought-experiment was designed to show that more than just our experiences matter to us because living in reality also matters to us. This argument has proven to be so convincing that nearly every single book on ethics that discusses hedonism rejects it using only this argument or this one and one other.

In the thought experiment, Nozick asks us to imagine that we have the choice of plugging in to a fantastic machine that flawlessly provides an amazing mix of experiences. Importantly, this machine can provide these experiences in a way that, once plugged in to the machine, no one can tell that their experiences are not real. Disregarding considerations about responsibilities to others and the problems that would arise if everyone plugged in, would you plug in to the machine for life? The vast majority of people reject the choice to live a much more pleasurable life in the machine, mostly because they agree with Nozick that living in reality seems to be important for our well-being. Opinions differ on what exactly about living in reality is so much better for us than the additional pleasure of living in the experience machine, but the most common response is that a life that is not lived in reality is pointless or meaningless.

Since this argument has been used so extensively (from the mid 1970s onwards) to dismiss Prudential Hedonism, several attempts have been made to refute it. Most commonly, Hedonists argue that living an experience machine life would be better than living a real life and that most people are simply mistaken to not want to plug in. Some go further and try to explain why so many people choose not to plug in. Such explanations often point out that the most obvious reasons for not wanting to plug in can be explained in terms of expected pleasure and avoidance of pain. For example, it might be argued that we expect to get pleasure from spending time with our real friends and family, but we do not expect to get as much pleasure from the fake friends or family we might have in the experience machine. These kinds of attempts to refute the experience machine objection do little to persuade non-Hedonists that they have made the wrong choice.

A more promising line of defence for the Prudential Hedonists is to provide evidence that there is a particular psychological bias that affects most peoples choice in the experience machine thought experiment. A reversal of Nozicks thought experiment has been argued to reveal just such a bias. Imagine that a credible source tells you that you are actually in an experience machine right now. You have no idea what reality would be like. Given the choice between having your memory of this conversation wiped and going to reality, what would be best for you to choose? Empirical evidence on this choice shows that most people would choose to stay in the experience machine. Comparing this result with how people respond to Nozicks experience machine thought experiment reveals the following: In Nozicks experience machine thought experiment people tend to choose a real and familiar life over a more pleasurable life and in the reversed experience machine thought experiment people tend to choose a familiar life over a real life. Familiarity seems to matter more than reality, undermining the strength of Nozicks original argument. The bias thought to be responsible for this difference is the status quo bias an irrational preference for the familiar or for things to stay as they are.

Regardless of whether Nozicks experience machine thought experiment is as decisive a refutation of Prudential Hedonism as it is often thought to be, the wider argument (that living in reality is valuable for our well-being) is still a problem for Prudential Hedonists. That our actions have real consequences, that our friends are real, and that our experiences are genuine seem to matter for most of us regardless of considerations of pleasure. Unfortunately, we lack a trusted methodology for discerning if these things should matter to us. Perhaps the best method for identifying intrinsically valuable aspects of lives is to compare lives that are equal in pleasure and all other important ways, except that one aspect of one of the lives is increased. Using this methodology, however, seems certain to lead to an artificial pluralist conclusion about what has value. This is because any increase in a potentially valuable aspect of our lives will be viewed as a free bonus. And, most people will choose the life with the free bonus just in case it has intrinsic value, not necessarily because they think it does have intrinsic value.

The main traditional line of criticism against Prudential Hedonism is that not all pleasure is valuable for well-being, or at least that some pleasures are less valuable than others because of non-amount-related factors. Some versions of this criticism are much easier for Prudential Hedonists to deal with than others depending on where the allegedly disvaluable aspect of the pleasure resides. If the disvaluable aspect is experienced with the pleasure itself, then both Qualitative and Quantitative varieties of Prudential Hedonism have sufficient answers to these problems. If, however, the disvaluable aspect of the pleasure is never experienced, then all types of Prudential Hedonism struggle to explain why the allegedly disvaluable aspect is irrelevant.

Examples of the easier criticisms to deal with are that Prudential Hedonism values, or at least overvalues, perverse and base pleasures. These kinds of criticisms tend to have had more sway in the past and doubtless encouraged Mill to develop his Qualitative Hedonism. In response to the charge that Prudential Hedonism mistakenly values pleasure from sadistic torture, sating hunger, copulating, listening to opera, and philosophising all equally, Qualitative Hedonists can simply deny that it does. Since pleasure from sadistic torture will normally be experienced as containing the quality of sadism (just as the pleasure from listening to good opera is experienced as containing the quality of acoustic excellence), the Qualitative Hedonist can plausibly claim to be aware of the difference in quality and allocate less value to perverse or base pleasures accordingly.

Prudential Hedonists need not relinquish the Quantitative aspect of their theory in order to deal with these criticisms, however. Quantitative Hedonists, can simply point out that moral or cultural values are not necessarily relevant to well-being because the investigation of well-being aims to understand what the good life for the one living it is and what intrinsically makes their life go better for them. A Quantitative Hedonist can simply respond that a sadist that gets sadistic pleasure from torturing someone does improve their own well-being (assuming that the sadist never feels any negative emotions or gets into any other trouble as a result). Similarly, a Quantitative Hedonist can argue that if someone genuinely gets a lot of pleasure from porcine company and wallowing in the mud, but finds opera thoroughly dull, then we have good reason to think that having to live in a pig sty would be better for her well-being than forcing her to listen to opera.

Much more problematic for both Quantitative and Qualitative Hedonists, however, are the more modern versions of the criticism that not all pleasure is valuable. The modern versions of this criticism tend to use examples in which the disvaluable aspect of the pleasure is never experienced by the person whose well-being is being evaluated. The best example of these modern criticisms is a thought experiment devised by Shelly Kagan. Kagans deceived businessman thought experiment is widely thought to show that pleasures of a certain kind, namely false pleasures, are worth much less than true pleasures.

Kagan asks us to imagine the life of a very successful businessman who takes great pleasure in being respected by his colleagues, well-liked by his friends, and loved by his wife and children until the day he died. Then Kagan asks us to compare this life with one of equal length and the same amount of pleasure (experienced as coming from exactly the same sources), except that in each case the businessman is mistaken about how those around him really feel. This second (deceived) businessman experiences just as much pleasure from the respect of his colleagues and the love of his family as the first businessman. The only difference is that the second businessman has many false beliefs. Specifically, the deceived businessmans colleagues actually think he is useless, his wife doesnt really love him, and his children are only nice to him so that he will keep giving them money. Given that the deceived businessman never knew of any of these deceptions and his experiences were never negatively impacted by the deceptions indirectly, which life do you think is better?

Nearly everyone thinks that the deceived businessman has a worse life. This is a problem for Prudential Hedonists because the pleasure is quantitatively equal in each life, so they should be equally good for the one living it. Qualitative Hedonism does not seem to be able to avoid this criticism either because the falsity of the pleasures experienced by the deceived businessman is a dimension of the pleasure that he never becomes aware of. Theoretically, an externalist and qualitative version of Attitudinal Hedonism could include the falsity dimension of an instance of pleasure even if the falsity dimension never impacts the consciousness of the person. However, the resulting definition of pleasure bears little resemblance to what we commonly understand pleasure to be and also seems to be ad hoc in its inclusion of the truth dimension but not others. A dedicated Prudential Hedonist of any variety can always stubbornly stick to the claim that the lives of the two businessmen are of equal value, but that will do little to convince the vast majority to take Prudential Hedonism more seriously.

Another major line of criticism used against Prudential Hedonists is that they have yet to come up with a meaningful definition of pleasure that unifies the seemingly disparate array of pleasures while remaining recognisable as pleasure. Some definitions lack sufficient detail to be informative about what pleasure actually is, or why it is valuable, and those that do offer enough detail to be meaningful are faced with two difficult tasks.

The first obstacle for a useful definition of pleasure for hedonism is to unify all of the diverse pleasures in a reasonable way. Phenomenologically, the pleasure from reading a good book is very different to the pleasure from bungee jumping, and both of these pleasures are very different to the pleasure of having sex. This obstacle is unsurpassable for most versions of Quantitative Hedonism because it makes the value gained from different pleasures impossible to compare. Not being able to compare different types of pleasure results in being unable to say if a life is better than another in most even vaguely realistic cases. Furthermore, not being able to compare lives means that Quantitative Hedonism could not be usefully used to guide behavior since it cannot instruct us on which life to aim for.

Attempts to resolve the problem of unifying the different pleasures while remaining within a framework of Quantitative Hedonism, usually involve pointing out something that is constant in all of the disparate pleasures and defining that particular thing as pleasure. When pleasure is defined as a strict sensation, this strategy fails because introspection reveals that no such sensation exists. Pleasure defined as the experience of liking or as a pro-attitude does much better at unifying all of the diverse pleasures. However, defining pleasure in these ways makes the task of filling in the details of the theory a fine balancing act. Liking or pro-attitudes must be described in such a way that they are not solely a sensation or best described as a preference satisfaction theory. And they must perform this balancing act while still describing a scientifically plausible and conceptually coherent account of pleasure. Most attempts to define pleasure as liking or pro-attitudes seem to disagree with either the folk conception of what pleasure is or any of the plausible scientific conceptions of how pleasure functions.

Most varieties of Qualitative Hedonism do better at dealing with the problem of diverse pleasures because they can evaluate different pleasures according to their distinct qualities. Qualitative Hedonists still need a coherent method for comparing the different pleasures with each other in order to be more than just an abstract theory of well-being, however. And, it is difficult to construct such a methodology in a way that avoids counter examples, while still describing a scientifically plausible and conceptually coherent account of pleasure.

The second obstacle is creating a definition of pleasure that retains at least some of the core properties of the common understanding of the term pleasure. As mentioned, many of the potential adjustments to the main definitions of pleasure are useful for avoiding one or more of the many objections against Prudential Hedonism. The problem with this strategy is that the more adjustments that are made, the more apparent it becomes that the definition of pleasure is not recognisable as the pleasure that gave Hedonism its distinctive intuitive plausibility in the first place. When an instance of pleasure is defined simply as when someone feels good, its intrinsic value for well-being is intuitively obvious. However, when the definition of pleasure is stretched, so as to more effectively argue that all valuable experiences are pleasurable, it becomes much less recognisable as the concept of pleasure we use in day-to-day life and its intrinsic value becomes much less intuitive.

The future of hedonism seems bleak. The considerable number and strength of the arguments against Prudential Hedonisms central principle (that pleasure and only pleasure intrinsically contributes positively to well-being and the opposite for pain) seem insurmountable. Hedonists have been creative in their definitions of pleasure so as to avoid these objections, but more often than not find themselves defending a theory that is not particularly hedonistic, realistic or both.

Perhaps the only hope that Hedonists of all types can have for the future is that advances in cognitive science will lead to a better understanding of how pleasure works in the brain and how biases affect our judgements about thought experiments. If our improved understanding in these areas confirms a particular theory about what pleasure is and also provides reasons to doubt some of the widespread judgements about the thought experiments that make the vast majority of philosophers reject hedonism, then hedonism might experience at least a partial revival. The good news for Hedonists is that at least some emerging theories and results from cognitive science do appear to support some aspects of hedonism.

Dan WeijersEmail: danweijers@gmail.comVictoria University of WellingtonNew Zealand

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Caro Hedonista,De momento o nosso site est apenas dsponivel em Ingls.Contudo, a nossa equipa tem sua disposio alguem capaz de lhe responder em Portugus.Por favor no hesite em contactar directamente o nosso especialista, Miguel.

Chers Hdonistes, notre site internet nest disponible pour le moment quen Anglais. Cependant, notre quipe se tient votre disposition pour vous rpondre en Franais. Nhsitez pas contacter directement Maxime notre spcialiste francophone.

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hedonism | Philosophy & Definition | Britannica.com

Hedonism, in ethics, a general term for all theories of conduct in which the criterion is pleasure of one kind or another. The word is derived from the Greek hedone (pleasure), from hedys (sweet or pleasant).

Hedonistic theories of conduct have been held from the earliest times. They have been regularly misrepresented by their critics because of a simple misconception, namely, the assumption that the pleasure upheld by the hedonist is necessarily purely physical in its origins. This assumption is in most cases a complete perversion of the truth. Practically all hedonists recognize the existence of pleasures derived from fame and reputation, from friendship and sympathy, from knowledge and art. Most have urged that physical pleasures are not only ephemeral in themselves but also involve, either as prior conditions or as consequences, such pains as to discount any greater intensity that they may have while they last.

The earliest and most extreme form of hedonism is that of the Cyrenaics as stated by Aristippus, who argued that the goal of a good life should be the sentient pleasure of the moment. Since, as Protagoras maintained, knowledge is solely of momentary sensations, it is useless to try to calculate future pleasures and to balance pains against them. The true art of life is to crowd as much enjoyment as possible into each moment.

No school has been more subject to the misconception noted above than the Epicurean. Epicureanism is completely different from Cyrenaicism. For Epicurus pleasure was indeed the supreme good, but his interpretation of this maxim was profoundly influenced by the Socratic doctrine of prudence and Aristotles conception of the best life. The true hedonist would aim at a life of enduring pleasure, but this would be obtainable only under the guidance of reason. Self-control in the choice and limitation of pleasures with a view to reducing pain to a minimum was indispensable. This view informed the Epicurean maxim Of all this, the beginning, and the greatest good, is prudence. This negative side of Epicureanism developed to such an extent that some members of the school found the ideal life rather in indifference to pain than in positive enjoyment.

In the late 18th century Jeremy Bentham revived hedonism both as a psychological and as a moral theory under the umbrella of utilitarianism. Individuals have no goal other than the greatest pleasure, thus each person ought to pursue the greatest pleasure. It would seem to follow that each person inevitably always does what he or she ought. Bentham sought the solution to this paradox on different occasions in two incompatible directions. Sometimes he says that the act which one does is the act which one thinks will give the most pleasure, whereas the act which one ought to do is the act which really will provide the most pleasure. In short, calculation is salvation, while sin is shortsightedness. Alternatively he suggests that the act which one does is that which will give one the most pleasure, whereas the act one ought to do is that which will give all those affected by it the most pleasure.

The psychological doctrine that a humans only aim is pleasure was effectively attacked by Joseph Butler. He pointed out that each desire has its own specific object and that pleasure comes as a welcome addition or bonus when the desire achieves its object. Hence the paradox that the best way to get pleasure is to forget it and to pursue wholeheartedly other objects. Butler, however, went too far in maintaining that pleasure cannot be pursued as an end. Normally, indeed, when one is hungry or curious or lonely, there is desire to eat, to know, or to have company. These are not desires for pleasure. One can also eat sweets when one is not hungry, for the sake of the pleasure that they give.

Moral hedonism has been attacked since Socrates, though moralists sometimes have gone to the extreme of holding that humans never have a duty to bring about pleasure. It may seem odd to say that a human has a duty to pursue pleasure, but the pleasures of others certainly seem to count among the factors relevant in making a moral decision. One particular criticism which may be added to those usually urged against hedonists is that whereas they claim to simplify ethical problems by introducing a single standard, namely pleasure, in fact they have a double standard. As Bentham said, Nature has placed mankind under the governance of two sovereign masters, pain and pleasure. Hedonists tend to treat pleasure and pain as if they were, like heat and cold, degrees on a single scale, when they are really different in kind.

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hedonism | Philosophy & Definition | Britannica.com

Clothing Optional Resorts, Negril, Jamaica | Hedonism II

Select Departure City Albany, Ny [ALB] Albuquerque, Nm [ABQ] Allentown, Pa [ABE] Amarillo, Tx [AMA] Anchorage, Ak [ANC] Appleton, Mn [AQP] Arcata, Ca [ACV] Asheville, Nc [AVL] Aspen, Co [ASE] Atlanta, Ga [ATL] Atlantic City, Nj [ACY] Austin, Tx [AUS] Baltimore, Md [BWI] Bangor, Me [BGR] Beaumont, Tx [BPT] Bethel, Ak [BET] Billings, Mt [BIL] Binghamton, Ny [BGM] Birmingham, Al [BHM] Bismarck, Nd [BIS] Bloomington, Il [BMI] Boise, Id [BOI] Boston, Ma [BOS] Brownsville, Tx [BRO] Brunswick, Ga [BQK] Buffalo, Ny [BUF] Burbank, Ca [BUR] Burlington, Vt [BTV] Calgary [YYC] Cedar Rapids, Ia [CID] Charleston, Sc [CHS] Charleston, Wv [CRW] Charlotte, Nc [CLT] Charlottesville, Va [CHO] Chicago (Midway), Il [MDW] Chicago (O’Hare), Il [ORD] Cincinnati, Oh [CVG] Cleveland, Oh [CLE] College Station, Tx [CLL] Colorado Springs, Co [COS] Columbia, Mo [COU] Columbia, Sc [CAE] Columbus, Oh [CMH] Cordova, Ak [CDV] Corpus Christi, Tx [CRP] Dallas Love Field, Tx [DAL] Dallas/Fort Worth, Tx [DFW] Dayton, Oh [DAY] Denver, Co [DEN] Des Moines, Ia [DSM] Detroit, Mi [DTW] Duluth, Mn [DLH] Durango, Co [DRO] Edmonton Intntl [YEG] Eastern Iowa, Ia [CID] El Paso, Tx [ELP] Erie, Pa [ERI] Eugene, Or [EUG] Eureka, Ca [EKA] Fairbanks, Ak [FAI] Fargo, Nd [FAR] Flint, Mi [FNT] Fresno, Ca [FAT] Ft. Lauderdale, Fl [FLL] Ft. Myers, Fl [RSW] Ft. Walton/Okaloosa [VPS] Ft. Wayne, In [FWA] Gainesville, Fl [GNV] Grand Forks, Nd [GFK] Grand Rapids, Mi [GRR] Great Falls, Mt [GTF] Green Bay, Wi [GRB] Greensboro, Nc [GSO] Greenville, Sc [GSP] Gulfport, Ms [GPT] Halifax Intntl [YHZ] Harlingen [HRL] Harrisburg, Pa [MDT] Hartford, Ct [BDL] Helena, Mt [HLN] Hilo, Hi [ITO] Hilton Head, Sc [HHH] Honolulu, Hi [HNL] Houston Hobby, Tx [HOU] Houston Busch, Tx [IAH] Huntington, Wv [HTS] Huntsville Intl, Al [HSV] Idaho Falls, Id [IDA] Indianapolis, In [IND] Islip, Ny [ISP] Ithaca, Ny [ITH] Jackson Hole, Wy [JAC] Jackson Int’L, Ms [JAN] Jacksonville, Fl [JAX] Juneau, Ak [JNU] Kahului, Hi [OGG] Kansas City, Mo [MCI] Kapalua, Hi [JHM] Kauai, Hi [LIH] Key West, Fl [EYW] Knoxville, Tn [TYS] Kona, Hi [KOA] Lanai, Hi [LNY] Lansing, Mi [LAN] Las Vegas, Nv [LAS] Lexington, Ky [LEX] Lincoln, Ne [LNK] Little Rock, Ar [LIT] Long Beach, Ca [LGB] Los Angeles, Ca [LAX] Louisville, Ky [SDF] Lubbock, Tx [LBB] Lynchburg, Va [LYH] Montreal Mirabel [YMX] Montreal Trudeau [YUL] Madison, Wi [MSN] Manchester, Nh [MHT] Maui, Hi [OGG] Mcallen, Tx [MFE] Medford, Or [MFR] Melbourne, Fl [MLB] Memphis, Tn [MEM] Miami, Fl [MIA] Midland/Odessa, Tx [MAF] Milwaukee, Wi [MKE] Minneapolis/St. Paul [MSP] Missoula, Mt [MSO] Mobile Regional, Al [MOB] Molokai, Hi [MKK] Monterey, Ca [MRY] Montgomery, Al [MGM] Myrtle Beach, Sc [MYR] Naples, Fl [APF] Nashville, Tn [BNA] New Braunfels, Tx [BAZ] New Orleans, La [MSY] New York Kennedy, Ny [JFK] New York Laguardia [LGA] Newark, Nj [EWR] Norfolk, Va [ORF] Ottawa Mcdonald [YOW] Oakland, Ca [OAK] Oklahoma City, Ok [OKC] Omaha, Ne [OMA] Ontario, Ca [ONT] Orange County, Ca [SNA] Orlando, Fl [MCO] Palm Springs, Ca [PSP] Panama City, Fl [PFN] Pensacola, Fl [PNS] Peoria, Il [PIA] Philadelphia, Pa [PHL] Phoenix, Az [PHX] Pittsburgh, Pa [PIT] Port Angeles, Wa [CLM] Portland Intl, Or [PDX] Portland, Me [PWM] Providence, Ri [PVD] Quebec Intntl [YQB] Raleigh/Durham, Nc [RDU] Rapid City, Sd [RAP] Redmond, Or [RDM] Reno, Nv [RNO] Richmond, Va [RIC] Roanoke, Va [ROA] Rochester, Ny [ROC] Rockford, Il [RFD] Sacramento, Ca [SMF] Saginaw, Mi [MBS] Salem, Or [SLE] Salt Lake City, Ut [SLC] San Antonio, Tx [SAT] San Diego, Ca [SAN] San Francisco, Ca [SFO] San Jose, Ca [SJC] Santa Barbara, Ca [SBA] Santa Rosa, Ca [STS] Sarasota/Bradenton [SRQ] Savannah, Ga [SAV] Seattle/Tacoma, Wa [SEA] Shreveport, La [SHV] Sioux City, Ia [SUX] Sioux Falls, Sd [FSD] Spokane, Wa [GEG] Springfield, Il [SPI] Springfield, Mo [SGF] St. Louis, Mo [STL] St. Petersburg, Fl [PIE] Syracuse, Ny [SYR] Toronto Pearson [YYZ] Tallahassee, Fl [TLH] Tampa, Fl [TPA] Traverse City, Mi [TVC] Tucson, Az [TUS] Tulsa, Ok [TUL] Vancouver Intntl [YVR] Victoria Intntl [YYJ] Winnipeg Intntl [YWG] Washington Natl, Dc [DCA] Washington/Dulles, Dc [IAD] Wenatchee, Wa [EAT] West Palm Beach, Fl [PBI] White Plains, Ny [HPN] Wichita, Ks [ICT] Wilkes-Barre/Scranton [AVP]

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Clothing Optional Resorts, Negril, Jamaica | Hedonism II

Institute for Astronomy

May 3, 2018: University of Hawaii Astronomer John Tonry Elected to National Academy of Sciences

University of Hawaii at Mnoa astronomer John Tonry has been named as one of the National Academy of Sciences’ 84 newly chosen members. Tonry, who has been with the UH Mnoa Institute for Astronomy since 1996, joins an elite group of fewer than 2,400 exceptional scientists worldwide. NAS members are recognized for their distinguished and continuing achievements in original research.

Press Release

Today, NASA launched the Transiting Exoplanet Survey Satellite (TESS), its newest telescope to search for planets beyond our Solar System, and astronomers from the University of Hawaii Institute for Astronomy and Maunakea telescopes will be a part of the adventure.

Press Release

Paul Coleman, an astronomer at the University of Hawaii Institute for Astronomy, passed away at his home on January 16th, 2018. Paul was the first Native Hawaiian with a doctorate in astrophysics. In his 15 years with the IfA, Paul played a key role in our education and public outreach efforts, and advocated tirelessly for astronomy in Hawaii.

Obituary

The University of Hawaii ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope on Mauna Loa captured images on February 8, 2018 of the Tesla Roadster launched into space as part of SpaceX’s Falcon Heavy test.

Press Release

Extremely distant galaxies are usually too faint to be seen, even by the largest telescopes. But nature has a solution – gravitational lensing, predicted by Albert Einstein and observed many times by astronomers. Now, an international team of astronomers led by Harald Ebeling from the University of Hawaii has discovered one of the most extreme instances of magnification by gravitational lensing.

Press Release

University of Hawaii Institute for Astronomy (IfA) Director Gnther Hasinger will be leaving UH to be the next Director of Science at the European Space Agency (ESA), Europe’s equivalent to NASA.

UH will name an interim Director for the IfA and begin a search for a new Director.

UH Press Release

A team of astronomers from Maryland, Hawaii, Israel, and France has produced the most detailed map ever of the orbits of galaxies in our extended local neighborhood, showing the past motions of almost 1400 galaxies within 100 million light years of the Milky Way.

Press Release

Since astronomers first measured the size of an extrasolar planet seventeen years ago, they have struggled to answer the question: how did the largest planets get to be so large? Thanks to the recent discovery of twin planets by a University of Hawaii Institute for Astronomy team led by graduate student Samuel Grunblatt, we are getting closer to an answer.

Press Release

In October, astronomers at the University of Hawaii’s Institute for Astronomy (IfA) made a stunning discovery with the Pan-STARRS1 telescope – the first interstellar object seen passing through our Solar System. Now, an international team lead by Karen Meech (ifA) has made detailed measurements of the visitor’s properties. “This thing is very strange,” said Karen Meech.

Press Release

A small, recently discovered asteroid – or perhaps a comet – appears to have originated from outside the solar system, coming from somewhere else in our galaxy. If so, it would be the first “interstellar object” to be observed and confirmed by astronomers. This unusual object – for now designated A/2017 U1 – was discovered Oct. 19 by the University of Hawaii’s Pan-STARRS 1 telescope on Haleakala during the course of its nightly search for Near-Earth Objects for NASA.

Press Release

UH astronomers and their international collaborators announced the discovery and study of the first binary neutron star merger detected in gravitational waves in articles published today in Science, Nature, and the Astrophysical Journal. The study of this event shows that at least some of the elements heavier than iron were originally created in binary neutron star mergers like this one.

Press Release

The brightest members of the Pleiades cluster form a spectacular group of naked-eye stars that have played a central role in cultures around the world for millennia. Now, an international team of astronomers, including Daniel Huber from the University of Hawaii Institute for Astronomy, used the Kepler Space Telescope to perform the most detailed study to date of their variability – with some interesting new discoveries.

Press Release

The cosmic web – the distribution of matter on the largest scales in the universe – has usually been defined through the distribution of galaxies. Now, a new study by a team of astronomers from France, Israel, and Hawaii demonstrates a novel approach. Instead of using galaxy positions, they mapped the motions of thousands of galaxies. Because galaxies are pulled toward gravitational attractors and move away from empty regions, these motions allowed the team to locate the denser matter in clusters and filaments and the absence of matter in regions called voids.

Press Release

The University of Hawaii’s 2.2 meter (88-inch) telescope on Maunakea will soon be producing images nearly as sharp as those from the Hubble Space Telescope, thanks to a new instrument using the latest image sharpening technologies. Astronomer Christoph Baranec, at the University of Hawaii’s Institute for Astronomy (IfA), has been awarded a nearly $1 million grant from the National Science Foundation to build an autonomous adaptive optics system called Robo-AO-2 for the UH telescope.

Press Release

The UH Institute for Astronomy celebrates its 50th Anniversary with a special three-day meeting in Honolulu from June 28-30, 2017. Everyone with a history or relationship with the IfA is invited to attend, including former and present graduate students, postdocs, staff and faculty. See below for two free public events that are also part of the celebration.

Event Information

June 27th, 7:30PM, UH Manoa Orvis Auditorium: Perpetual Motion: Galileo and His RevolutionsSarah Pillow, soprano & Mary-Anne Ballard, viola da gamba, with guests Daniel Swenberg, lute and theorbo; author Dava Sobel & Marc Wagnon, video artistA moving and compelling account of a remarkable moment in the history of science, human thought and music, Perpetual Motion ties together the groundbreaking repertoire of Galileo’s day, narration by acclaimed best-selling science writer Dava Sobel, and images of Earth and the cosmos. The UH Bookstore will have copies of Dava’s books for sale, and she will be signing them!

Free Tickets (required) via Ticketbud

June 28th, 7:30PM, UH Manoa Orvis Auditorium: Dava Sobel talks on “The Glass Universe”The acclaimed author of Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time (Walker, 1995), Dava Sobel will be speaking about her new book, The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars (Viking, 2016), which tells the story of the women who worked at the Harvard College Observatory from the late 1800s through the mid-1900s. The UH Bookstore will have copies of Dava’s books for sale, and she will be signing them!

Free Tickets (required) via Ticketbud

Since the mid-1990s, when the first planet around another sun-like star was discovered, astronomers have been amassing what is now a large collection of exoplanets – nearly 3,500 have been confirmed so far. In a new study, whose lead author is an IfA graduate student, researchers have classified these planets in much the same way that biologists identify new animal species and have learned that the majority of exoplanets found to date fall into two distinct size groups: rocky Earth-like planets and larger mini-Neptunes. The team used data from NASA’s Kepler mission and the W. M. Keck Observatory.

Press Release

An international team of astronomers, including IfA graduate student Jason Chu and Astronomer David Sanders, has used the Herschel Space Observatory to take far-infrared images of the 200 most infrared-luminous galaxies in the Local Universe.

Press Release

A team of astronomers, lead by IfA graduate Trent Dupuy and IfA astronomy Michael Liu, have shown what separates real stars from the wannabes. Not in Hollywood, but out in the universe. They found that an object must weigh at least 70 Jupiters in order to start hydrogen fusion. If it weighs less, the star does not ignite and becomes a brown dwarf instead.

Press Release

Starting the week of May 1, the University of Hawaii 88-inch telescope (UH88) will undergo much needed repair and maintenance. The renovation will include fresh paint and repaired siding on the exterior, roof repair and weather sealing of the dome, improved lightning protection, as well as safety upgrades.

Press Release

A good showing today for the IfA at the UH Awards Ceremony. Faculty members Christoph Baranec and Jeff Kuhn received the Board of Regents’ Medal for Excellence in Research awards for excellence in research, while graduate student Will Best received the award for Student Excellence in Research (Doctoral Level).

More Information

Join us at our Manoa Headquarters on April 23rd, from 11am-4pm, for a day of family-friendly activities and talks!

More Information

The Daniel K. Inouye Solar Telescope (DKIST), currently under construction on Haleakala, Maui, is expected to start observing the Sun in 2020. When it does, it will rely on two complex infrared instruments being built by the University of Hawaii Institute for Astronomy (IfA). Their goal is to measure the Sun’s weak magnetic field.The first of these to be completed is called the Cryogenic Near-Infrared Spectropolarimeter (CryoNIRSP). In a major milestone, it took its first look at the Sun from the laboratories at the IfA’s Advanced Technology Research Center on Maui.

Press Release

The IfA mourns the loss of our long-time faculty member and professor emeritus Toby Owen. Tobias (Toby) C. Owen, passed away on March 4, 2017, in Sacramento, California, where he had been living after retiring from the IfA in 2012.

Obituary, by Alan Tokunaga

In a groundbreaking study published in Nature Astronomy, a team of researchers, including Brent Tully from the University of Hawaii Institute for Astronomy, reports the discovery of a previously unknown, nearly empty region in our extragalactic neighborhood. Largely devoid of galaxies, this void exerts a repelling force, pushing our Local Group of galaxies through space.

Press Release

IfA Astronomer Nick Kaiser has been awarded the Gold Medal in Astronomy by the Royal Astronomical Society (RAS). The Medal’s past recipients include Albert Einstein, Edwin Hubble, Arthur Eddington and Stephen Hawking. Dr. Kaiser is receiving the award for his extensive theoretical and observational work on cosmology, including how matter – both dark and visible – is distributed on the largest scales.

Press Release

The Pan-STARRS project at the University of Hawaii Institute for Astronomy is publicly releasing the world’s largest digital sky survey today, via the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

Press Release

At first glance, Ceres, the largest body in the main asteroid belt, may not look icy. Images from NASA’s Dawn spacecraft have revealed a dark, heavily cratered world whose brightest area is made of highly reflective salts — not ice. But newly published studies from Dawn scientists, including University of Hawaii astronomer Norbert Schrghofer, show two distinct lines of evidence for ice at or near the surface of the dwarf planet. These findings, which verify predictions made by scientists formerly at UH, are being presented at the 2016 American Geophysical Union meeting in San Francisco, California.

Press Release

Astronomers from the University of Hawaii Institute for Astronomy (IfA), Brazil, and Stanford University may have solved a long-standing solar mystery.Two decades ago, scientists discovered that the outer five percent of the Sun spins more slowly than the rest of its interior. Now, in a new study to be published in the journal Physical Review Letters, IfA Maui scientists Ian Cunnyngham, Jeff Kuhn, and Isabelle Scholl, together with Marcelo Emilio (Brazil) and Rock Bush (Stanford), describe the physical mechanism responsible for slowing the Sun’s outer layers.

Press Release

“A Magnificent Celestial Show in 2017: The August 21 Total Solar Eclipse in North America ” with IfA astronomer Shadia Habbal, 7:30 p.m., UH Mnoa Art Building Auditorium (room 132). Free Admission (Campus Parking $6). Poster

One of nature’s most spectacular celestial sights is the magnificent solar corona, visible only during a total solar eclipse. On August 21, 2017, the moon’s shadow will sweep across the entire United States from Oregon to South Carolina over a span of approximately 90 minutes. Everyone in the 48 contiguous states and Alaska will witness at least a partial solar eclipse. Those directly under the moon’s 60 mile-wide shadow will have 2 minutes of totality – one of life’s most awesome experiences. Learn why people become eclipse chasers, traveling the world to enjoy their beauty – and do some science.

The annual IfA Maui Open House will be held Friday, Oct. 7, from 6 to 8 p.m. at the Maikalani building in Pukalani, Maui. Free Admission. Flier

Comet 332P/Ikeya-Murakami survived for 4.5 billion years in the frigid Kuiper Belt, a vast reservoir of icy bodies on the outskirts of our solar system. But within the last few million years, the unlucky comet was gravitationally kicked to the inner solar system by the outer planets – and this new home, closer to the sun, has doomed the comet. The Hubble Space Telescope caught the latest cloud of debris ejected by Comet 332P. The images, taken over three days in January 2016, represent one of the sharpest, most detailed observations of a comet breaking apart. The doomed comet may disintegrate in only 150 years.

Press Release

A team of astronomers known as the Kepler Habitable Zone Working Group, including University of Hawai’i Institute for Astronomy astronomer Nader Haghighipour, has identified which of the more than 4,000 exoplanets discovered by the NASA Kepler mission are most likely to be similar to our rocky home.

Press Release

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Institute for Astronomy

National Optical Astronomy Observatory

Image credits: T. Eglin/NOAO/AURA/NSF; P. Marenfeld/NOAO/AURA/NSF; NOAO/AURA/NSF

The April 2018 NOAO Newsletter is online and ready to download. It contains sections on Science Highlights, Community Science & Data, System Observing, and NOAO Operations & Staff.

On the Cover Clockwise from upper right: a) Nicholas Mayall at the guiding eyepiece at the prime focus during commissioning of the KPNO 4-meter telescope that now bears his name; b) the Mayall 4-meter under moonlight during the MzLS imaging survey, which was completed in February 2018; c) the Mayalls primary mirror being lifted out of its cell during an early stage of the work to prepare the telescope for installation of the Dark Energy Spectroscopic Instru- ment (DESI). By mid-June the entire top end of the telescope will have been removed, and installation of the new prime focus system for DESI will begin. Initial commissioning with the new DESI hardware should begin in November 2018.

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National Optical Astronomy Observatory

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 where amateurs still play an active role, especially in the discovery and observation of transient phenomena. 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 was the first to propose 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 discovered by the Persian astronomer Azophi and first described in his Book of Fixed Stars.[25] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and the Chinese astronomers in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[26][27] It is also believed that the ruins at Great Zimbabwe and Timbuktu[28] may have housed an astronomical observatory.[29] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.[30][31][32][33]

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

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

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

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

National Optical Astronomy Observatory

Image Credit: R. Hahn; Inset: NASA/JPL-Caltech

While most supernovae studied to date brighten and fade over a period of weeks, a handful are known to evolve much quicker. KSN2015K reached its maximum brightness in 2 days and faded to half that in only 7 days. To explain the rapid evolution, the discovery team, which includes NOAO astronomers Alfredo Zenteno and Chris Smith, has argued that the star bumped into itself in the explosion!

Read more in Notre Dame Press Release.

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National Optical Astronomy Observatory

National Optical Astronomy Observatory

Reidar Hahn, Fermilab

In a drama taking place in the Small Magellanic Cloud, a close companion galaxy to the Milky Way, a rare star is speeding out of the galaxy at 300,000 miles per hour. The first runaway yellow supergiant star ever discovered, and only the second evolved runaway star found in another galaxy, the star was once a member of a binary star system and was ejected at high speed when its companion star exploded as a supernova. The discovery was made using the CTIO 4-m Blanco telescope.

Read more in Lowell Observatory press release.

DECam Community Science Workshop 2018: Science Highlights, Coming Opportunities, LSST Synergies.May 21-22, 2018Tucson, Arizona

New Chapter Begins for Kitt Peak Telescope: Over the next 15 months, the telescope will prepare for the installation of the Dark Energy Spectroscopic Instrument.

Join the international star-hunt campaign, Globe at Night, April 6-15. Use a smart phone to submit sky brightness in real time! Visit globeatnight.org.

New Stellar Streams Confirm Melting Pot History of the Galaxy: View & Share the Trailer Video. Read more in NOAO Press Release 18-01.

Rivers in the Sky: Some of the new stellar streams discovered by the Dark Energy Survey have been named by Chilean children for bodies of water close to home (Lea en espaol).

Peering back in time, to when the Universe was only 5% of its current age, astronomers have spotted the most distant supermassive black hole discovered yet. Read more in NOAO Press Release 17-07.

Interstellar interloper 1I/2017 U1, recently spotted streaking through the Solar System, provides an up close look at an object from another planetary system.

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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 April 17

Explanation: Except for the ringsof Saturn, the RingNebula (M57) is probably the most famous celestial band.Its classic appearance isunderstood to be due to our own perspective, though.The recent mapping of theexpanding nebula’s3-D structure, based in part onthis clear Hubble image,indicates thatthe nebula is a relatively dense, donut-like ring wrappedaround the middle of a (American) football-shaped cloud of glowing gas.The view from planet Earth looks down the long axis of the football,face-on to the ring.Of course, in this well-studied example of aplanetary nebula, the glowing materialdoes not come from planets.Instead, the gaseous shroud represents outer layers expelledfrom the dying,oncesun-like star, now a tiny pinprick of lightseen at the nebula’s center.Intense ultraviolet light from the hot central starionizes atoms in the gas.The Ring Nebula is about one light-yearacross and 2,000 light-years away.

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

Bad Astronomy – : Bad Astronomy

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

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

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

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

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

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

Thanks.

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

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

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

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

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

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

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

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

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

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

Related posts:

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

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

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

[Click to cascadienate.]

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

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

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

Image credit: NASA

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

Related Posts:

That such a place exists Time lapse: Crater Lake Incredible surreal volcanic riverscapes Looking down on the snow of Kilimanjaro

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.

Related Posts:

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

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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.

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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.

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Bad Astronomy – : Bad Astronomy

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 where amateurs still play an active role, especially in the discovery and observation of transient phenomena. 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 was the first to propose 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 discovered by the Persian astronomer Azophi and first described in his Book of Fixed Stars.[25] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and the Chinese astronomers in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[26][27] It is also believed that the ruins at Great Zimbabwe and Timbuktu[28] may have housed an astronomical observatory.[29] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.[30][31][32][33]

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

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

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

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

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 March 27

Explanation: What that bright red spot between the Lagoon and Trifid Nebulas? Mars.This gorgeouscolor deep-sky photograph captured the red planet passing between the twonotable nebulas — cataloged by the 18th century cosmic registrar CharlesMessier as M8 and M20.M20 (upper right of center),the Trifid Nebula, presents a striking contrast inred/blue colors and dark dust lanes.Across the bottom right is the expansive, alluring red glow of M8,the Lagoon Nebula.Both nebulae are a few thousand light-years distant.By comparison, temporarily situated between them both, is the dominant “local” celestial beaconMars.Taken last week, the red planetwas only about 10 light-minutes away.

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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

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 where amateurs still play an active role, especially in the discovery and observation of transient phenomena. 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 was the first to propose 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 discovered by the Persian astronomer Azophi and first described in his Book of Fixed Stars.[25] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and the Chinese astronomers in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[26][27] It is also believed that the ruins at Great Zimbabwe and Timbuktu[28] may have housed an astronomical observatory.[29] Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.[30][31][32][33]

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

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

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

The English astronomer John Flamsteed catalogued over 3000 stars.[37] Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.[38] The distance to a star was first 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 March 23

Explanation: Normally faint and elusive, the Jellyfish Nebula is caught inthis alluring telescopic image.Centered in the scene it’s anchored right and left by two bright stars,MuandEtaGeminorum, at the foot of thecelestialtwin.The Jellyfish Nebula is the brighter arcingridge of emission with dangling tentacles.In fact, the cosmic jellyfish is part of bubble-shapedsupernova remnant IC 443, the expandingdebris cloud from amassivestar that exploded.Light from the explosion first reached planet Earth over 30,000 yearsago.Like its cousin inastrophysical waters theCrab Nebulasupernova remnant, the Jellyfish Nebula isknownto harbor a neutron star, the remnant of the collapsed stellar core.An emission nebula cataloged asSharpless 249fills the field at the upper left.The Jellyfish Nebula is about 5,000 light-years away.At that distance, this image would be about 300 light-years across.

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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