Red giant 56200 226042608 2008-07-16T15:56:27Z Freewayguy 4834906 no; we have some few suggestion of Venus will actually survive [[Image:Redgiants.svg|thumb|280px|Comparison between red giants and the Sun. The dashed circular curve indicates the size of the orbit of Mars.|right]] A '''red giant''' is a luminous [[giant star|giant]] [[star]] of low or intermediate mass that is in a late phase of its [[Stellar evolution|evolution]]. The core matter is [[degenerate matter|electron degenerate]] and extremely compressed, so the outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the [[stellar classification|spectral types]] K and M, but also class S stars and most [[carbon star]]s. The most common red giants are the so-called Red Giant Branch stars (RGB stars) whose shells are still [[Proton-proton chain reaction|fusing hydrogen]], while the core is inactive helium. Another case of red giants of interest are the [[Asymptotic Giant Branch]] stars (AGB) that produces carbon by the [[triple-alpha process]] from helium. To the AGB stars belong the carbon stars of type C-N and late C-R. Prominent bright red giants in the night sky include [[Aldebaran]] (Alpha Tauri), and [[Gamma Crucis]] (Gacrux), while the even larger [[Antares]] (Alpha Scorpii) and [[Betelgeuse]] (Alpha Orionis) are [[Red supergiant]]s. <!-- INCORRECT: A ''red giant'' is a [[star]] that has a diminishing [[Proton-proton chain reaction|hydrogen burning]] shell, or has ceased its [[hydrogen]] burning phase, left the [[main sequence]], and ignited later [[Stellar nucleosynthesis|nuclear fusion]] reactions, usually the [[triple-alpha process]] fusing [[helium]] to [[carbon]] and some [[oxygen]]. Since the process of fusing helium to carbon requires higher temperatures and pressures, than burning hydrogen to helium, the density difference between core and surface is much greater than in a hydrogen burner, making the atmosphere [[convective]] and inflated to [[giant star]] size. Red giants are concentrated to the K and M [[stellar classification|stellar classes]], but a small fraction of far developed red giants belong to the [[carbon star]]s, mostly of class C-N and late C-R.--> ==Characteristics and lifespan== [[Image:HR-diag-w-text.svg|300px|left]] Red giants are evolved from [[main sequence]] stars with masses in the range from about 0.5 solar masses to somewhere between 4 and 6 solar masses <ref>{{cite book | coauthors = Jean Audouze, Guy Israël, et al. | title = The Cambridge Atlas of Astronomy | edition = 2 | publisher = [[Cambridge University Press]] | year = 1988 | pages = 255 | isbn = 0-521-36360-8 }}</ref>. Red giants are stars with radii tens to hundreds of times larger than that of the [[Sun]] which have exhausted the supply of [[hydrogen]] in their cores and switched to fusing hydrogen in a shell outside the core. Since the inert [[helium]] core has no source of energy of its own, it contracts and heats up, and its gravity compresses the hydrogen in the layer immediately above it, thus causing it to fuse faster. This in turn causes the star to become more luminous (from 1,000 to 10,000 times brighter) and expand; the degree of expansion outstrips the increase in luminosity, thus causing the [[effective temperature]] to decrease. In stars massive enough to ignite [[helium fusion]], an analogous process occurs when central helium is exhausted and the star switches to fusing helium in a shell, although with the additional complication that in many cases hydrogen fusion will continue in a shell at lesser depth — this puts stars onto the [[asymptotic giant branch]].<ref>{{cite journal | last = Sackmann | first = I.-Juliana | authorlink = | coauthors = Boothroyd, Arnold I.; Kraemer, Kathleen E. | title = Our Sun. III. Present and Future | journal = Astrophysical Journal | volume = 418 | issue = | pages = 457 | publisher = The American Astronomical Society | date = November 1993 | url = | doi = | id = | accessdate = }}</ref><ref>{{cite web | last = Pogge | first = Richard W. | authorlink = | coauthors = | title = Lecture 16: The Evolution of Low-Mass Stars | work = Astronomy 162: Introduction to Stars, Galaxies, & the Universe | publisher = | date = [[2006-01-21]] | url = http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit2/lowmass.html | format = | doi = | accessdate = 2006-12-29 }}</ref> <!-- Should this source [http://www.astro.umd.edu/~miller/teaching/astr606/lecture18.ps] be included? After all, it's a postscript file that majority will not be able to see without installing some software --> The decrease in surface temperature shifts the star's visible light output '''towards''' the red — hence ''red giant'', even though the color usually is orange. Main sequence stars of [[spectral type]]s A through K are believed to become red giants. <ref>[http://www.vendian.org/mncharity/dir3/starcolor/ Color of star ranging blue through orange]</ref> Very low mass stars are thought to be fully convective<ref>{{cite web | last = | first = | authorlink = | coauthors = | title = Main-Sequence Stars | work = Stars | publisher = The Astrophysics Spectator | date = [[2005-02-16]] | url = http://www.astrophysicsspectator.com/topics/stars/MainSequence.html | format = | doi = | accessdate = 2006-12-29 }}</ref> and thus may not accumulate an inert core of helium, and thus may exhaust all of their fuel without ever becoming red giants. <ref>{{cite web | last = Richmond | first = Michael | authorlink = | coauthors = | title = Late stages of evolution for low-mass stars | work = | publisher = | date = | url = http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html | format = | doi = | accessdate = 2006-12-29 }}</ref> Such stars are commonly referred to as [[red dwarfs]]. The predicted lifespan of these stars is much larger than the current age of the universe, and hence there are no actual observations of these stars aging. Very high mass stars instead develop to [[supergiant]] stars that wander back and forth horizontally over the HR diagram, at the right end constituting [[red supergiant]]s. These usually end their life as type II [[supernova]]e. If the star is heavier than 0.4 but less than 2.57 solar masses, the addition of [[helium]] to the core by shell hydrogen fusing will cause a [[helium flash]] — a rapid burst of helium fusing in the core, after which the star will commence a brief period of helium fusing before beginning ''another'' ascent of the red giant branch. Stars more massive than 2.5 solar masses, but less than about 4 to 6, enter the helium fusing phase of their lives much more smoothly. The core helium fusing phase of a star's life is called the [[horizontal branch]] in metal-poor stars, so named because these stars lie on a nearly horizontal line in the [[Hertzsprung-Russell diagram]] of many star clusters. Metal-rich helium-fusing stars do not lie on a horizontal branch, but instead lie in a clump (the [[red clump]]) in the Hertzsprung-Russell diagram.<ref>[http://cfa-www.harvard.edu/~kstanek/RedClump/ Harvard University search for orange-yellow clumps]</ref> Actually, such stars are not big red spheres with sharp limbs (when one is close to it) as displayed on many images. Due to the very low density such stars may not have a sharp [[photosphere]] but a star body which gradually transfers into a '[[corona]]'.<ref>[http://www.keckobservatory.org/article.php?id=87 Measurements of the frequency of starspots on red giant stars]</ref><ref>[http://solarscience.msfc.nasa.gov/feature1.shtml orange sphere of the sun]</ref> [[Image:Sun red giant.svg|thumb|right|200px|When the Sun has exhausted its supply of [[hydrogen fusion|hydrogen to fuse]] it will swell into the Red Giant phase. The size of the current Sun (now in the [[main sequence]]) is here compared to its estimated size during its red giant phase.]] ==The Sun as a red giant== The [[Sun]] is expected to become a red giant approximately 5 billion years from now.<ref>[http://articles.adsabs.harvard.edu//full/1993ApJ...418..457S/0000467.000.html Our Sun. III. Present and Future,by Sackmann, I.-J., Boothroyd, A. I., & Kraemer, K. E., Astrophysical Journal v.418, p.457]</ref> It is calculated that the Sun will become sufficiently large to engulf the current orbits of the [[solar system]]'s [[inner planet]]s, including [[Earth]] and potentially [[Mars]] <ref>{{cite web | title = Red Giants | publisher = HyperPhysics (hosted by the Department of Physics and Astronomy of [[Georgia State University]]) | url = http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redgia.html | accessdate = 2006-12-29 }}</ref><ref>{{cite web | last = Strobel | first = Nick | title = Stages 5-7 | work = Lives and Deaths of Stars | date = [[2004-06-02]] | url = http://www.astronomynotes.com/evolutn/s5.htm | accessdate = 2006-12-29 }}</ref><ref>{{cite web | title = The fading: red giants and white dwarfs | url = http://nrumiano.free.fr/Estars/fading.html | accessdate = 2006-12-29 }}</ref> and its radius will expand a minimum of 200 times,<ref>[http://www.cosmosmagazine.com/news/952/clues-death-our-solar-system Clues of death of our solar system]</ref> The Sun will lose a significant fraction of its mass in the process of becoming a red giant, there is a chance that [[Mars]] and all the outer planets will escape as their resulting orbits will widen. <ref name="astronomy.ohio-state.edu">[http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html The Once & Future Sun<!-- But generated title -->]</ref> [[Mercury (planet)|Mercury]] and most likely [[Venus]] will have been swallowed by sun's outer layer at this time. [[Earth]]'s fate is less clear. Venus and Earth could technically achieve a widening of its orbit and could potentially maintain a sufficiently high angular velocity to keep it from becoming engulfed. In order to do so, its orbit would increase to {{convert|1.3|AU|km| lk=on | abbr=on}} and {{convert|1.7|AU|km| lk=on | abbr=on}}. <ref>[http://www.newscientist.com/article/mg14219191.900.html Sun's fate 1994]</ref> However the results of studies announced in 2008 show that due to [[Roche limit|tidal interaction]] between sun and Earth, Earth would actually fall back into a lower orbit, and get engulfed and incorporated inside the sun before sun reaches its largest size, despite the sun losing about 38% of its mass. <ref>[http://space.newscientist.com/article/dn13369-hope-dims-that-earth-will-survive-suns-death.html?feedId=online-news_rss20 Earth may still vanish before the Sun expands further]</ref> Before this happens, Earth's [[biosphere]] will have long been destroyed by the Sun's steady increase in brightness as its hydrogen supply dwindles and its core contracts, even before the transition to a Red Giant. After just over 1 billion years, the extra solar energy input will cause Earth's oceans to evaporate and the hydrogen from the water to be lost permanently to space, with total loss of water by 3 billion years.<ref>[http://www.harmsy.freeuk.com/sun.html Sun is a powerhouse - Death in our solar system] </ref> Earth's [[atmosphere]] and [[lithosphere]] will become like that of Venus. Over another billion years, most of the atmosphere will get lost in space as well;<ref>{{cite web | last = Pogge | first = Richard W. | title = The Once and Future Sun | work = New Vistas in Astronomy | date = [[1997-06-13]] | url = http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html | accessdate = 2007-01-23 }}</ref>, ultimately leaving Earth as a desiccated, dead planet with a surface of molten rock.<ref>[http://www.msnbc.msn.com/id/23354076/ MSN News Earth has little hope to survive over sun's fate update 2/26/2008]</ref> ==See also== {{wiktionary}} *[[Red clump]] *[[Horizontal branch]] *[[Stellar evolution]] *[[Blue giant]] *[[Red dwarf star]] *[[Supergiant]] *[[Red supergiant]] *[[White dwarf]] ==References== {{reflist|2}} {{Star}} [[Category:Rеd giants| ]] [[Category:Star types]] [[ar:عملاق أحمر]] [[bg:Червен гигант]] [[ca:Gegant vermella]] [[cs:Červený obr]] [[da:Rød kæmpe]] [[de:Roter Riese]] [[el:Ερυθρός γίγαντας]] [[es:Gigante roja]] [[fa:غول قرمز]] [[fr:Géante rouge]] [[gl:Xigante vermella]] [[ko:적색 거성]] [[hr:Crveni div]] [[it:Gigante rossa]] [[he:ענק אדום]] [[lb:Roude Ris]] [[lt:Raudonoji milžinė]] [[hu:Vörös óriás]] [[nl:Rode reus]] [[ja:赤色巨星]] [[no:Rød kjempe (astronomi)]] [[pl:Czerwony olbrzym]] [[pt:Estrela vermelha gigante]] [[ru:Красный гигант]] [[sk:Červený obor]] [[sl:Rdeča orjakinja]] [[sr:Crveni div]] [[fi:Punainen jättiläinen]] [[sv:Röd jätte]] [[vi:Sao khổng lồ đỏ]] [[zh:紅巨星]]