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STAR

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{{otheruses}} Image:Pleiades large.jpg Pleiades (star cluster) thumb|right|300px|The [[Pleiades (star cluster)|Pleiades star cluster.html" title="Meaning of Pleiades.html" title="Meaning of thumb|right|300px|The [[Pleiades (star cluster)|Pleiades">thumb|right|300px|The [[Pleiades (star cluster)|Pleiades star cluster">Pleiades.html" title="Meaning of thumb|right|300px|The [[Pleiades (star cluster)|Pleiades">thumb|right|300px|The [[Pleiades (star cluster)|Pleiades star cluster A '''star''' is a massive body of Plasma (physics) plasma in outer space that is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense blackbody radiation heat. Scientifically, stars are defined as Gravity#Self-gravitating_system self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Stellar astronomy is the study of stars. A Hertzsprung-Russell diagram shows the pattern of the temperature of stars against their absolute magnitude.

Star formation and evolution
{{main|stellar evolution}} Star formation occurs in molecular clouds, large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or the collision of two galaxy galaxies (as in a starburst galaxy). High mass stars powerfully illuminate the clouds from which they formed. One example of such a nebula is the Orion Nebula. Stars spend about 90% of their lifetime nuclear fusion fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no black dwarfs exist yet. As most stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume both Mercury (planet) Mercury and Venus (planet) Venus. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process. Since iron nuclei are more binding energy tightly bound than any heavier nuclei, they cannot be fused to release energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by Nuclear fission fission. In old, very massive stars, a large core of inert iron will accumulate in the center of the star. An average-size star will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into black dwarfs over very long stretches of time. Image:Crab.nebula.arp.750pix.jpg Crab Nebula.html" title="Meaning of thumb thumb|200px|right|The [[Crab Nebula, remnants of a supernova which occurred around 1050 AD..html" title="Meaning of 200px|right|The [[Crab Nebula">thumb|200px|right|The [[Crab Nebula, remnants of a supernova which occurred around 1050 AD.">200px|right|The [[Crab Nebula">thumb|200px|right|The [[Crab Nebula, remnants of a supernova which occurred around 1050 AD. In larger stars, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of electron capture inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before. Eventually, most of the matter in a star is blown away by the explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars, a black hole. The blown-off outer layers of dying stars include heavy chemical element elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the solar wind stellar wind of large stars play an important part in shaping the interstellar medium.

Appearance and distribution of stars
All stars except the Sun appear to the human eye as shining points in the night sky that Scintillation (astrophysics) twinkle because of the effect of the Earth's atmosphere. Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide sunlight daylight. Stars are not spread uniformly across the universe, but are typically grouped into galaxy galaxies. A typical galaxy contains hundreds of billions of stars. The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Astronomers estimate that there are at least 70 sextillion (7Ã—10²²) stars in the known universe [http://news.bbc.co.uk/2/hi/science/nature/3085885.stm]. That is 70 000 000 000 000 000 000 000, or 230 billion times as many as the 300 billion in our own Milky Way. The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos. Small (''dwarf'') stars such as the Sun generally have essentially featureless disks with only small sunspots starspots. Larger (''giant'') stars have much bigger, much more obvious starspots, and also exhibit strong stellar limb-darkening (the brightness decreases towards the edge of the stellar disk).

Age and size of stars
Image:The sun1.jpg Sun.html" title="Meaning of thumb thumb|200px|right|The [[Sun is the nearest star to Earth..html" title="Meaning of 200px|right|The [[Sun">thumb|200px|right|The [[Sun is the nearest star to Earth.">200px|right|The [[Sun">thumb|200px|right|The [[Sun is the nearest star to Earth. Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. (See Big Bang Big Bang theory and stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion (constellation) Orion constellation, which have a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometers. However, these have a much lower density than the Sun. One of the most massive stars known is Eta Carinae, with 100–150 times as much mass as the Sun. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the Galactic Center core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive stars is long extinct, however, and currently only theoretical. With a mass only 93 times that of Jupiter (planet) Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.

Star classification
There are different stellar classification classifications of stars according to their stellar spectroscopy spectra ranging from type '''O''', which are very hot, to '''M''', which are so cool that molecules may form in their atmospheres. The main classifications can be easily remembered using the mnemonic "'''O'''h, '''B'''e '''A''' '''F'''ine '''G'''irl, '''K'''iss '''M'''e" (variant: change "girl" to "guy"), invented by Annie Jump Cannon. There are many List of mnemonics for star classification other mnemonics for star classification. A variety of rare spectral types have special classifications. The most common of these are types '''L''' and '''T''', which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 subclassifications numbered (hottest to coldest) from '''0''' to '''9'''. This system matches closely with temperature, but breaks down at the extreme hottest end; class '''O0''' and '''O1''' stars may not exist. In addition, stars may be classified by their "luminosity effects", which correspond to their spatial size. These range from '''0''' (hypergiants) through '''III''' (giant star giants) to '''V''' (main sequence dwarfs) and '''VII''' (white dwarfs). Most stars fall into the main sequence which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and spectral type. Our Sun is a '''G2V''' (yellow dwarf), being of intermediate temperature and ordinary size. The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star), and most characteristics of other stars are usually given in solar units. :solar mass: ''M''_Sun = 1.9891Ã—10³⁰ kilogram kg :solar luminosity: ''L''_Sun = 3.827Ã—10²⁶ Watt W.

Naming of stars
Most stars are identified only by catalogue numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are neither recognized by the scientific community nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named. See star designations for more information on how stars are named. For a list of traditional names, see the list of stars by constellation.

Energy production
The energy produced by stars solar radiation radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind). The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically significant. The apparent brightness of a star is measurement measured by its apparent magnitude.

Nuclear fusion reaction pathways
A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis). Stars begin as a cloud of mostly hydrogen with about 23â€“28% helium and a few percent heavier elements. In the Sun, with a 10⁷ K core, hydrogen fuses to form helium in the proton-proton chain: :4protium ¹H → 2deuterium ²H + 2positron e⁺ + 2neutrino ν_e (4.0 Melectronvolt eV + 1.0 MeV) :2¹H + 2²H → 2Helium-3 ³He + 2gamma ray γ (5.5 MeV) :2³He → Helium-4 ⁴He + 2¹H (12.9 MeV) These reactions result in the overall reaction: :4¹H → ⁴He + 2e⁺ + 2γ + 2ν_e (26.7 MeV) In more massive stars, helium is produced in a cycle of reactions catalyst catalyzed by carbon, the CNO cycle carbon-nitrogen-oxygen cycle. In stars with cores at 10⁸ K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process: :⁴He + ⁴He + 92 keV → ^8*Be :⁴He + ^8*Be + 67 keV → ^12*C :^12*C → Carbon-12 ¹²C + γ + 7.4 MeV For an overall reaction of: :3⁴He → ¹²C + γ + 7.2 MeV

Star mythology
As well as certain constellations and the Sun itself, stars as a whole have their own mythology. They were thought to be the souls of the dead or gods and goddesses. In the Greco-Roman pantheon_(gods) pantheon, some "stars", later identified as planets, represented various important deities, from which the names of the planets Mercury (planet) Mercury, Venus (planet) Venus, Mars (planet) Mars, Jupiter (planet) Jupiter and Saturn (planet) Saturn were taken. (Uranus, Neptune and Pluto were also Roman mythology Roman gods, but none of these three planets were known to the Romans due to their low brightness. Their names were assigned by later astronomers.)

References
* Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press * John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press.

See also
* Black hole * Blue straggler * Constellation Overview of star constellations * Nursery rhyme ''Twinkle twinkle little star'' * sidereal clock * Star count * Star clocks * List of stars Stars with articles in * Stellar navigation * Stellar evolution * Timeline of stellar astronomy * Variable star

Related lists
* Lists of stars * List of brightest stars List of brightest stars (apparent & absolute magnitude) * List of heaviest stars List of heaviest stars (by solar mass) * List of largest stars List of largest stars (by diameter) * List of mnemonics for star classification * List of nearest bright stars * List of nearest stars * List of stars by constellation * List of stars with confirmed extrasolar planets

External links

- Images of starspots on the surface of Betelgeuse
- Find out what is known about any given star by entering its name or position
- There are more single stars than previously thought
- Top stars picked in alien search
- Star Classification
- Star Worldbook Category:Stars Category:Astronomical objects bg:Ð—Ð²ÐµÐ·Ð´Ð° bs:Zvijezde ca:Estel cs:HvÄ›zda cy:Seren da:Stjerne de:Stern {{Link FA|de}} et:TÃ¤ht (astronoomia) es:Estrella{{Link FA|es}} eo:Stelo fa:Ø³ØªØ§Ø±Ù‡ fr:Ã‰toile ko:í•ì„± hr:Zvijezda io:Stelo id:Bintang ia:Stella is:Stjarna it:Stella he:×›×•×›×‘ ku:StÃªr kn:à²¨à²•à³?à²·à²¤à³?à²° la:Stella lb:StÃ¤r lv:Zvaigzne hu:Csillag (Ã©gitest) mk:Ð…Ð²ÐµÐ·Ð´Ð° ms:Bintang nah:Sitialin nl:Ster ja:æ?’æ˜Ÿ no:Stjerne pl:Gwiazda pt:Estrela ro:Stea {{Link FA|ro}} ru:Ð—Ð²ÐµÐ·Ð´Ð° scn:Stidda simple:Star sk:Hviezda sl:Zvezda sr:Ð—Ð²ÐµÐ·Ð´Ð° fi:TÃ¤hti sv:StjÃ¤rna tr:YÄ±ldÄ±z th:à¸”à¸²à¸§à¸¤à¸?à¸©à¹Œ zh:æ?’æ˜Ÿ '''STAR''' is an acronym for: * Satellite Television Asian Region, or STAR TV * STAR Labs Scientific and Technological Advanced Research Labs, a fictional research organization in the DC Comics universe. * Single-Handed Trans-Atlantic Race, also known as the ''OSTAR'' * Situation, Task, Action, Result, an interview technique * Shock Trauma and Rescue, or Shock Trauma Air Rescue, a synonym for air ambulance * Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astonomy club. * Solenoidal Tracker At RHIC, one of the four detectors at the RHIC * Source tree adaptive routing protocol, a method of organizing wireless mesh networks * Special Tactics and Rescue (Singapore) Special Tactics and Rescue of the Singapore Police Force * Special Tactics and Response, a synonym of SWAT * Special Tasks and Rescue, a synonym of SWAT * Standard Terminal Arrival Route, the route that planes follow under instrument flight rules when they approach an airport * Standardized Testing and Reporting, a measure of school performance in California * Star (STrategies + ARchitecture), an architecture office in the Netherlands * STAR (software) STAR Math, STAR Reading, or STAR Early Literacy--periodic assessments of mathematical, reading, and early literacy abilities, respectively, created by Renaissance Learning, Inc. * STAR Repair, the Best Buy Repair and Service Database, based on Clarify eFrontOffice * S.T.A.R.S.; Special Tactics And Rescue Service (or Squad), a fictional task force in the ''Resident Evil'' video game franchise * Students Taking Action with Recognition, in STAR Events. A Program in FCCLA. * Supplementary Tests of Achievement in Reading published by the New Zealand Council for Educational Research * STAR (interbank network) STAR, an interbank network {{4LA}}

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