View source for Astronomy:Red supergiant star

{{short description|Stars with a supergiant luminosity class}}
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'''Red supergiants''' ('''RSGs''') are [[Astronomy:Star|star]]s with a [[Astronomy:Supergiant|supergiant]] luminosity class ([[Astronomy:Stellar classification#Yerkes spectral classification|Yerkes class]] '''I''') of spectral type K or M.<ref name="LamersCassinelli1999">{{cite book|author1=Henny J. G. L. M. Lamers|author2=Joseph P. Cassinelli|title=Introduction to Stellar Winds|url=https://books.google.com/books?id=MJuDvniBNokC&pg=PA53|access-date=31 August 2012|date=17 June 1999|publisher=Cambridge University Press|isbn=978-0-521-59565-0|pages=53–}}</ref> They are the [[Astronomy:List of largest stars|largest stars]] in the universe in terms of [[Physics:Volume|volume]], although they are not the most [[Physics:Mass|mass]]ive or [[Astronomy:Luminosity#Stellar luminosity|luminous]]. [[Astronomy:Betelgeuse|Betelgeuse]] and [[Astronomy:Antares|Antares]] are the brightest and best known red supergiants (RSGs), indeed the only first [[Astronomy:Magnitude|magnitude]] red supergiant stars.

==Classification==
Stars are classified as supergiants on the basis of their spectral luminosity class.  This system uses certain diagnostic spectral lines to estimate the [[Physics:Surface gravity|surface gravity]] of a star, hence determining its size relative to its mass.  Larger stars are more luminous at a given temperature and can now be grouped into bands of differing luminosity.<ref name=geisler>{{cite journal|bibcode=1984PASP...96..723G|title=Luminosity classification with the Washington system|journal=Publications of the Astronomical Society of the Pacific|volume=96|pages=723|last1=Geisler|first1=D.|year=1984|doi=10.1086/131411|doi-access=free}}</ref>

The luminosity differences between stars are most apparent at low temperatures, where giant stars are much brighter than main-sequence stars.  Supergiants have the lowest surface gravities and hence are the largest and brightest at a particular temperature.

The ''Yerkes'' or ''Morgan-Keenan'' (MK) classification system<ref name=mk>{{cite journal|bibcode=1973ARA&A..11...29M|title=Spectral Classification|journal=Annual Review of Astronomy and Astrophysics|volume=11|pages=29–50|last1=Morgan|first1=W. W.|last2=Keenan|first2=P. C.|year=1973|doi=10.1146/annurev.aa.11.090173.000333}}</ref> is almost universal.  It groups stars into five main luminosity groups designated by [[Roman numerals|roman numerals]]:
* I [[Astronomy:Supergiant|supergiant]];
* II [[Astronomy:Bright giant|bright giant]];
* III giant;
* IV [[Astronomy:Subgiant|subgiant]];
* V dwarf ([[Astronomy:Main sequence|main sequence]]).

Specific to supergiants, the luminosity class is further divided into normal supergiants of class Ib and brightest supergiants of class Ia.  The intermediate class Iab is also used.  Exceptionally bright, low surface gravity, stars with strong indications of mass loss may be designated by luminosity class 0 (zero) although this is rarely seen.<ref>{{cite journal|bibcode=1992A&A...263..123P|title=Photometry of yellow semiregular variables - HR 8752 (= V509 Cassiopeiae)|journal=Astronomy and Astrophysics|volume=263|pages=123|last1=Percy|first1=J. R.|last2=Zsoldos|first2=E.|year=1992}}</ref>  More often the designation Ia-0 will be used,<ref>{{cite journal|bibcode=1992A&A...259..600A|title=A photometric study of the G0-4 Ia(+) hypergiant HD 96918 (V382 Carinae)|journal=Astronomy and Astrophysics|volume=259|pages=600|last1=Achmad|first1=L.|last2=Lamers|first2=H. J. G. L. M.|last3=Nieuwenhuijzen|first3=H.|last4=Van Genderen|first4=A. M.|year=1992}}</ref> and more commonly still Ia<sup>+</sup>.<ref>{{cite journal|doi=10.1007/s001590050009|title=The yellow hypergiants|journal=Astronomy and Astrophysics Review|volume=8|issue=3|pages=145–180|year=1998|last1=De Jager|first1=Cornelis|bibcode=1998A&ARv...8..145D}}</ref>  These [[Astronomy:Hypergiant|hypergiant]] spectral classifications are very rarely applied to red supergiants, although the term red hypergiant is sometimes used for the most extended and unstable red supergiants like [[Astronomy:VY Canis Majoris|VY Canis Majoris]] and [[Astronomy:NML Cygni|NML Cygni]].<ref>{{cite journal|doi=10.1088/0004-637X/744/1/23|title=DISTANCE AND KINEMATICS OF THE RED HYPERGIANT VY CMa: VERY LONG BASELINE ARRAY AND VERY LARGE ARRAY ASTROMETRY|journal=The Astrophysical Journal|volume=744|issue=1|pages=23|year=2012|last1=Zhang|first1=B.|last2=Reid|first2=M. J.|last3=Menten|first3=K. M.|last4=Zheng|first4=X. W.|bibcode=2012ApJ...744...23Z|doi-access=free}}</ref><ref>{{cite journal|bibcode=2012A&A...544A..42Z|title=The distance and size of the red hypergiant NML Cygni from VLBA and VLA astrometry|journal=Astronomy & Astrophysics|volume=544|pages=A42|last1=Zhang|first1=B.|last2=Reid|first2=M. J.|last3=Menten|first3=K. M.|last4=Zheng|first4=X. W.|last5=Brunthaler|first5=A.|year=2012|doi=10.1051/0004-6361/201219587|arxiv = 1207.1850 |s2cid=55509287}}</ref>

The "red" part of "red supergiant" refers to the cool temperature. Red supergiants are the coolest supergiants, M-type, and at least some K-type stars although there is no precise cutoff. K-type supergiants are uncommon compared to M-type because they are a short-lived transition stage and somewhat unstable. The K-type stars, especially early or hotter K types, are sometimes described as orange supergiants (e.g. [[Astronomy:Zeta Cephei|Zeta Cephei]]), or even as yellow (e.g. [[Astronomy:Yellow hypergiant|yellow hypergiant]] [[Astronomy:HR 5171|HR 5171]] Aa).<ref name=levesque/>

==Properties==
{| class="wikitable" style="float: right; margin-left: 0.5em;"
|+ Temperature scale for red supergiants<ref name=levesque/>
! [[Astronomy:Stellar classification|Spectral<br/>type]]
! [[Astronomy:Effective temperature|Temperature]]<br/>([[Kelvin|K]])
|-
| K1-1.5 || style="text-align: right" | 4,100
|-
| K2-3 || style="text-align: right" | 4,015
|-
| K5-M0 || style="text-align: right" | 3,840
|-
| M0 || style="text-align: right" | 3,790
|-
| M1 || style="text-align: right" | 3,745
|-
| M1.5 || style="text-align: right" | 3,710
|-
| M2 || style="text-align: right" | 3,660
|-
| M2.5 || style="text-align: right" | 3,615
|-
| M3 || style="text-align: right" | 3,605
|-
| M3.5 || style="text-align: right" | 3,550
|-
| M4-4.5 || style="text-align: right" | 3,535
|-
| M5 || style="text-align: right" | 3,450
|}
Red supergiants are cool and large.  They have spectral types of K and M, hence surface temperatures below 4,100&nbsp;[[Kelvin|K]].<ref name=levesque>{{cite journal|doi=10.1086/430901|title=The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought|journal=The Astrophysical Journal|volume=628|issue=2|pages=973–985|year=2005|last1=Levesque|first1=Emily M.|last2=Massey|first2=Philip|last3=Olsen|first3=K. A. G.|last4=Plez|first4=Bertrand|last5=Josselin|first5=Eric|last6=Maeder|first6=Andre|last7=Meynet|first7=Georges|bibcode=2005ApJ...628..973L|arxiv = astro-ph/0504337 |s2cid=15109583}}</ref> They are typically several hundred to over a thousand times the radius of the [[Astronomy:Sun|Sun]],<ref name=levesque/> although size is not the primary factor in a star being designated as a supergiant. A bright cool giant star can easily be larger than a hotter supergiant. For example, [[Astronomy:Alpha Herculis|Alpha Herculis]] is classified as a giant star with a radius of {{solar radius|between 264 to 303}} while [[Astronomy:Epsilon Pegasi|Epsilon Pegasi]] is a K2 supergiant of only {{solar radius|185}}.

Although red supergiants are much cooler than the Sun, they are so much larger than they are highly luminous, typically {{solar luminosity|tens or hundreds of thousands}}.<ref name=levesque/> There is an upper limit to the radius of a red supergiant at around {{solar radius|1,500}}.<ref name=levesque/> Stars above this radius would be too unstable and simply do not form.

Red supergiants have masses between about {{solar mass|10}} and {{solar mass|40}}. Main-sequence stars more massive than about {{solar mass|40}} do not expand and cool to become red supergiants. Red supergiants at the upper end of the possible mass and luminosity range are the largest known. Their low surface gravities and high luminosities cause extreme mass loss, millions of times higher than the Sun, producing observable nebulae surrounding the star.<ref name=smith>{{cite journal|doi=10.1086/318748|title=The Asymmetric Nebula Surrounding the Extreme Red Supergiant Vy Canis Majoris|journal=The Astronomical Journal|volume=121|issue=2|pages=1111–1125|year=2001|last1=Smith|first1=Nathan|last2=Humphreys|first2=Roberta M.|last3=Davidson|first3=Kris|last4=Gehrz|first4=Robert D.|last5=Schuster|first5=M. T.|last6=Krautter|first6=Joachim|bibcode=2001AJ....121.1111S|doi-access=free}}</ref> By the end of their lives red supergiants may have lost a substantial fraction of their initial mass. The more massive supergiants lose mass much more rapidly and all red supergiants appear to reach a similar mass of the order of {{solar mass|10}} by the time their cores collapse. The exact value depends on the initial chemical makeup of the star and its rotation rate.<ref name=ekstrom/>

Most red supergiants show some degree of visual [[Astronomy:Variable star|variability]], but only rarely with a well-defined period or amplitude. Therefore, they are usually classified as [[Astronomy:Irregular variable|irregular]] or [[Astronomy:Semiregular variable star|semiregular]] variables. They even have their own sub-classes, SRC and LC for slow semi-regular and [[Astronomy:Slow irregular variable|slow irregular]] supergiant variables respectively. Variations are typically slow and of small amplitude, but amplitudes up to four magnitudes are known.<ref name=kiss>{{cite journal|doi=10.1111/j.1365-2966.2006.10973.x|title=Variability in red supergiant stars: Pulsations, long secondary periods and convection noise|journal=Monthly Notices of the Royal Astronomical Society|volume=372|issue=4|pages=1721–1734|year=2006|last1=Kiss|first1=L. L.|last2=Szabo|first2=G. M.|last3=Bedding|first3=T. R.|bibcode=2006MNRAS.372.1721K|arxiv = astro-ph/0608438 |s2cid=5203133}}</ref>

Statistical analysis of many known variable red supergiants shows a number of likely causes for variation: just a few stars show large amplitudes and strong noise indicating variability at many frequencies, thought to indicate powerful [[Astronomy:Stellar wind|stellar wind]]s that occur towards the end of the life of a red supergiant; more common are simultaneous radial mode variations over a few hundred days and probably non-radial mode variations over a few thousand days; only a few stars appear to be truly irregular, with small amplitudes, likely due to photospheric granulation. Red supergiant photospheres contain a relatively small number of very large convection cells compared to stars like the [[Astronomy:Sun|Sun]]. This causes variations in surface brightness that can lead to visible [[Astronomy:Variable star|brightness variations]] as the star rotates.<ref name=Schwarzschild>
{{cite journal
| volume = 195
| pages = 137–144
| last1 = Schwarzschild
| first1 = Martin
| title = On the scale of photospheric convection in red giants and supergiants.
| journal = Astrophysical Journal
| date = 1975
|bibcode = 1975ApJ...195..137S
| doi = 10.1086/153313
}}</ref>

The spectra of red supergiants are similar to other cool stars, dominated by a forest of absorption lines of metals and molecular bands. Some of these features are used to determine the luminosity class, for example certain near-infrared [[Chemistry:Cyanogen|cyanogen]] band strengths and the Ca II triplet.<ref name=white>{{cite journal|bibcode=1978ApJ...222..209W|title=Photoelectric two-dimensional spectral classification of M supergiants|journal=Astrophysical Journal|volume=222|pages=209|last1=White|first1=N. M.|last2=Wing|first2=R. F.|year=1978|doi=10.1086/156136}}</ref>

[[Astronomy:Astrophysical maser|Maser]] emission is common from the circumstellar material around red supergiants. Most commonly this arises from H<sub>2</sub>O and SiO, but hydroxyl (OH) emission also occurs from narrow regions.<ref name=masers>{{cite journal|doi=10.1088/0004-637X/760/1/65|title=Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters|journal=The Astrophysical Journal|volume=760|issue=1|pages=65|year=2012|last1=Fok|first1=Thomas K. T.|last2=Nakashima|first2=Jun-Ichi|last3=Yung|first3=Bosco H. K.|last4=Hsia|first4=Chih-Hao|last5=Deguchi|first5=Shuji|bibcode=2012ApJ...760...65F|arxiv = 1209.6427 |s2cid=53393926}}</ref> In addition to high resolution mapping of the circumstellar material around red supergiants,<ref>{{cite journal|doi=10.1046/j.1365-8711.1999.02606.x|title=Maser mapping of small-scale structure in the circumstellar envelope of S Persei|journal=Monthly Notices of the Royal Astronomical Society|volume=306|issue=4|pages=954–974|year=1999|last1=Richards|first1=A. M. S.|last2=Yates|first2=J. A.|last3=Cohen|first3=R. J.|bibcode=1999MNRAS.306..954R|doi-access=free}}</ref> VLBI or [[Astronomy:Very Long Baseline Array|VLBA]] observations of masers can be used to derive accurate parallaxes and distances to their sources.<ref>{{cite journal|doi=10.1088/0004-637X/774/2/107|title=Distance and Proper Motion Measurement of the Red Supergiant, Pz Cas, in Very Long Baseline Interferometry H2O Maser Astrometry|journal=The Astrophysical Journal|volume=774|issue=2|pages=107|year=2013|last1=Kusuno|first1=K.|last2=Asaki|first2=Y.|last3=Imai|first3=H.|last4=Oyama|first4=T.|bibcode=2013ApJ...774..107K|arxiv = 1308.3580 |s2cid=118867155}}</ref> Currently this has been applied mainly to individual objects, but it may become useful for analysis of galactic structure and discovery of otherwise obscured red supergiant stars.<ref>{{cite journal|bibcode=2012A&A...541A..36V|title=SiO maser emission from red supergiants across the Galaxy . I. Targets in massive star clusters|journal=Astronomy & Astrophysics|volume=541|pages=A36|last1=Verheyen|first1=L.|last2=Messineo|first2=M.|last3=Menten|first3=K. M.|year=2012|doi=10.1051/0004-6361/201118265|arxiv = 1203.4727 |s2cid=55630819}}</ref>

Surface abundances of red supergiants are dominated by hydrogen even though hydrogen at the core has been completely consumed. In the latest stages of mass loss, before a star explodes, surface helium may become enriched to levels comparable with hydrogen. In theoretical extreme mass loss models, sufficient hydrogen may be lost that helium becomes the most abundant element at the surface.  When pre-red supergiant stars leave the main sequence, oxygen is more abundant than carbon at the surface, and nitrogen is less abundant than either, reflecting abundances from the formation of the star. Carbon and oxygen are quickly depleted and nitrogen enhanced as a result of the dredge-up of CNO-processed material from the fusion layers.<ref name=georgy>{{cite journal|doi=10.1051/0004-6361/201118372|title=Yellow supergiants as supernova progenitors: An indication of strong mass loss for red supergiants?|journal=Astronomy & Astrophysics|volume=538|pages=L8|year=2012|last1=Georgy|first1=C.|bibcode=2012A&A...538L...8G|arxiv = 1111.7003 |s2cid=55001976}}</ref>

Red supergiants are observed to rotate slowly or very slowly. Models indicate that even rapidly rotating main-sequence stars should be braked by their mass loss so that red supergiants hardly rotate at all. Those red supergiants such as [[Astronomy:Betelgeuse|Betelgeuse]] that do have modest rates of rotation may have acquired it after reaching the red supergiant stage, perhaps through binary interaction. The cores of red supergiants are still rotating and the differential rotation rate can be very large.<ref name=meynet/>

{{clear}}

==Definition==
[[File:Betelgeuse pulsating UV (HST).jpg|250px|left|thumb|Betelgeuse pulsating and showing spectral line profile changes (HST UV images)]]
Supergiant luminosity classes are easy to determine and apply to large numbers of stars, but they group a number of very different types of star into a single category.  An evolutionary definition restricts the term supergiant to those massive stars which start core helium fusion without developing a degenerate helium core and without undergoing a helium flash.  They will universally go on to burn heavier elements and undergo core-collapse resulting in a [[Astronomy:Supernova|supernova]].<ref>{{cite journal|bibcode=2005A&A...438..273V|title=An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars|journal=Astronomy and Astrophysics|volume=438|issue=1|pages=273–289|last1=Van Loon|first1=J. Th.|last2=Cioni|first2=M.-R. L.|last3=Zijlstra|first3=A. A.|last4=Loup|first4=C.|year=2005|doi=10.1051/0004-6361:20042555|arxiv = astro-ph/0504379 |s2cid=16724272}}</ref>

Less massive stars may develop a supergiant spectral luminosity class at relatively low luminosity, around {{solar luminosity|1,000}}, when they are on the [[Astronomy:Asymptotic giant branch|asymptotic giant branch]] (AGB) undergoing helium shell burning.  Researchers now prefer to categorize these as AGB stars distinct from supergiants because they are less massive, have different chemical compositions at the surface, undergo different types of pulsation and variability, and will evolve in a different way, usually producing a planetary nebula and white dwarf.<ref>{{cite journal|bibcode=2009A&A...506.1277G|title=Luminosities and mass-loss rates of SMC and LMC AGB stars and red supergiants|journal=Astronomy and Astrophysics|volume=506|issue=3|pages=1277–1296|last1=Groenewegen|first1=M. A. T.|last2=Sloan|first2=G. C.|last3=Soszyński|first3=I.|last4=Petersen|first4=E. A.|year=2009|doi=10.1051/0004-6361/200912678|arxiv = 0908.3087 |s2cid=14560155}}</ref>  Most AGB stars will not become supernovae although there is interest in a class of ''super-AGB stars'', those almost massive enough to undergo full carbon fusion, which may produce peculiar supernovae although without ever developing an iron core.<ref name=sagb/>  One notable group of low mass high luminosity stars are the [[Astronomy:RV Tauri variable|RV Tauri variable]]s, AGB or post-AGB stars lying on the [[Astronomy:Instability strip|instability strip]] and showing distinctive semi-regular variations.

==Evolution==
[[File:PESSTO Snaps Supernova in Messier 74.jpg|250px|right|thumb|A red supergiant ends its life as a type II supernova (bottom left) in a spiral arm of [[Astronomy:Messier 74|M74]]<ref>{{cite journal|bibcode= 2014MNRAS.439L..56F|doi=10.1093/mnrasl/slt179|title=On the progenitor of the Type IIP SN 2013ej in M74|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=439|pages=L56–L60|year=2013|last1=Fraser|first1=M.|last2=Maund|first2=J. R.|last3=Smartt|first3=S. J.|last4=Kotak|first4=R.|last5=Lawrence|first5=A.|last6=Bruce|first6=A.|last7=Valenti|first7=S.|last8=Yuan|first8=F.|last9=Benetti|first9=S.|last10=Chen|first10=T.-W.|last11=Gal-Yam|first11=A.|last12=Inserra|first12=C.|last13=Young|first13=D. R.|arxiv = 1309.4268 |s2cid=53415703}}</ref>]]
Red supergiants develop from main-sequence stars with masses between about {{solar mass|8}} and {{solar mass|30}}.  Higher-mass stars never cool sufficiently to become red supergiants.  Lower-mass stars develop a degenerate helium core during a red giant phase, undergo a helium flash before fusing helium on the [[Astronomy:Horizontal branch|horizontal branch]], evolve along the AGB while burning helium in a shell around a degenerate carbon-oxygen core, then rapidly lose their outer layers to become a white dwarf with a planetary nebula.<ref name=ekstrom>{{cite journal|bibcode=2012A&A...537A.146E|title=Grids of stellar models with rotation. I. Models from 0.8 to 120 M⊙ at solar metallicity (Z = 0.014)|journal=Astronomy & Astrophysics|volume=537|pages=A146|last1=Ekström|first1=S.|last2=Georgy|first2=C.|last3=Eggenberger|first3=P.|last4=Meynet|first4=G.|last5=Mowlavi|first5=N.|last6=Wyttenbach|first6=A.|last7=Granada|first7=A.|last8=Decressin|first8=T.|last9=Hirschi|first9=R.|last10=Frischknecht|first10=U.|last11=Charbonnel|first11=C.|last12=Maeder|first12=A.|year=2012|doi=10.1051/0004-6361/201117751|arxiv = 1110.5049 |s2cid = 85458919}}</ref>  AGB stars may develop spectra with a supergiant luminosity class as they expand to extreme dimensions relative to their small mass, and they may reach luminosities tens of thousands times the sun's.  Intermediate "super-AGB" stars, around {{solar mass|9}}, can undergo [[Chemistry:Carbon|carbon]] fusion and may produce an electron capture supernova through the collapse of an [[Chemistry:Oxygen|oxygen]]-[[Chemistry:Neon|neon]] core.<ref name=sagb>{{cite journal|doi=10.1086/520872|title=The Supernova Channel of Super‐AGB Stars|journal=The Astrophysical Journal|volume=675|issue=1|pages=614–625|year=2008|last1=Poelarends|first1=A. J. T.|last2=Herwig|first2=F.|last3=Langer|first3=N.|last4=Heger|first4=A.|bibcode=2008ApJ...675..614P|arxiv = 0705.4643 |s2cid=18334243}}</ref>

Main-sequence stars, burning hydrogen in their cores, with masses between {{solar mass|10 and 30}} will have temperatures between about 25,000K and 32,000K and spectral types of early B, possibly very late O.  They are already very luminous stars of {{solar luminosity|10,000-100,000}} due to rapid [[Astronomy:CNO cycle|CNO cycle]] fusion of hydrogen and they have fully convective cores.  In contrast to the Sun, the outer layers of these hot main-sequence stars are not convective.<ref name=ekstrom/>

These pre-red supergiant main-sequence stars exhaust the hydrogen in their cores after 5-20 million years.  They then start to burn a shell of hydrogen around the now-predominantly helium core, and this causes them to expand and cool into supergiants.  Their luminosity increases by a factor of about three.  The surface abundance of helium is now up to 40% but there is little enrichment of heavier elements.<ref name=ekstrom/>

The supergiants continue to cool and most will rapidly pass through the Cepheid instability strip, although the most massive will spend a brief period as [[Astronomy:Yellow hypergiant|yellow hypergiant]]s.  They will reach late K or M class and become a red supergiant.  Helium fusion in the core begins smoothly either while the star is expanding or once it is already a red supergiant, but this produces little immediate change at the surface.  Red supergiants develop deep convection zones reaching from the surface over halfway to the core and these cause strong enrichment of [[Chemistry:Nitrogen|nitrogen]] at the surface, with some enrichment of heavier elements.<ref name=heger>{{cite journal|doi=10.1086/308158|title=Presupernova Evolution of Rotating Massive Stars. I. Numerical Method and Evolution of the Internal Stellar Structure|journal=The Astrophysical Journal|volume=528|issue=1|pages=368–396|year=2000|last1=Heger|first1=A.|last2=Langer|first2=N.|last3=Woosley|first3=S. E.|bibcode=2000ApJ...528..368H|arxiv = astro-ph/9904132 |s2cid=3369610}}</ref>

Some red supergiants undergo [[Astronomy:Blue loop|blue loop]]s where they temporarily increase in temperature before returning to the red supergiant state.  This depends on the mass, rate of rotation, and chemical makeup of the star. While many red supergiants will not experience a blue loop, some can have several.  Temperatures can reach 10,000K at the peak of the blue loop.  The exact reasons for blue loops vary in different stars, but they are always related to the helium core increasing as a proportion of the mass of the star and forcing higher mass-loss rates from the outer layers.<ref name=meynet>{{cite journal|doi=10.1051/0004-6361/201424671|title=Impact of mass-loss on the evolution and pre-supernova properties of red supergiants|journal=Astronomy & Astrophysics|volume=575|pages=A60|year=2015|last1=Meynet|first1=G.|last2=Chomienne|first2=V.|last3=Ekström|first3=S.|last4=Georgy|first4=C.|last5=Granada|first5=A.|last6=Groh|first6=J.|last7=Maeder|first7=A.|last8=Eggenberger|first8=P.|last9=Levesque|first9=E.|last10=Massey|first10=P.|bibcode=2015A&A...575A..60M|arxiv = 1410.8721 |s2cid=38736311}}</ref>

All red supergiants will exhaust the helium in their cores within one or two million years and then start to burn carbon.  This continues with fusion of heavier elements until an iron core builds up, which then inevitably collapses to produce a supernova.  The time from the onset of carbon fusion until the core collapse is no more than a few thousand years.  In most cases, core-collapse occurs while the star is still a red supergiant, the large remaining hydrogen-rich atmosphere is ejected, and this produces a [[Astronomy:Type II supernova|type II supernova]] spectrum.  The [[Physics:Opacity (optics)|opacity]] of this ejected hydrogen decreases as it cools and this causes an extended delay to the drop in brightness after the initial supernova peak, the characteristic of a Type II-P supernova.<ref name=ekstrom/><ref name=heger/>

The most luminous red supergiants, at near solar [[Astronomy:Metallicity|metallicity]], are expected to lose most of their outer layers before their cores collapse, hence they evolve back to yellow hypergiants and luminous blue variables.  Such stars can explode as type II-L supernovae, still with hydrogen in their spectra but not with sufficient hydrogen to cause an extended brightness plateau in their light curves.  Stars with even less hydrogen remaining may produce the uncommon type IIb supernova, where there is so little hydrogen remaining that the hydrogen lines in the initial type II spectrum fade to the appearance of a Type Ib supernova.<ref name=woosley>{{cite journal|doi=10.1103/RevModPhys.74.1015|title=The evolution and explosion of massive stars|journal=Reviews of Modern Physics|volume=74|issue=4|pages=1015–1071|year=2002|last1=Woosley|first1=S. E.|last2=Heger|first2=A.|last3=Weaver|first3=T. A.|bibcode=2002RvMP...74.1015W|s2cid=55932331|url=https://semanticscholar.org/paper/1d84953a98c38541343f61d3b8d77d2b72e9d22a}}</ref>

The observed progenitors of type II-P supernovae all have temperatures between 3,500K and 4,400K and luminosities between {{solar luminosity|10,000}} and {{solar luminosity|300,000}}.  This matches the expected parameters of lower mass red supergiants.  A small number of progenitors of type II-L and type IIb supernovae have been observed, all having luminosities around {{solar luminosity|100,000}} and somewhat higher temperatures up to 6,000K.  These are a good match for slightly higher mass red supergiants with high mass-loss rates.  There are no known supernova progenitors corresponding to the most luminous red supergiants, and it is expected that these evolve to Wolf Rayet stars before exploding.<ref name=meynet/>

==Clusters==
[[File:Ssc2006-03a.jpg|left|250px|thumb|RSGC1, the first of several massive clusters found to contain multiple red supergiants.]]
Red supergiants are necessarily no more than about 25 million years old and such massive stars are expected to form only in relatively large [[Astronomy:Star cluster|clusters of stars]], so they are expected to be found mostly near prominent clusters.  However they are fairly short-lived compared to other phases in the life of a star and only form from relatively uncommon massive stars, so there will generally only be small numbers of red supergiants in each cluster at any one time. The massive [[Astronomy:Hodge 301|Hodge 301]] cluster in the [[Astronomy:Tarantula Nebula|Tarantula Nebula]] contains three.<ref name=slesnick>{{cite journal|bibcode=2002ApJ...576..880S|title=The Star Formation History and Mass Function of the Double Cluster h and χ Persei|journal=The Astrophysical Journal|volume=576|issue=2|pages=880–893|last1=Slesnick|first1=Catherine L.|last2=Hillenbrand|first2=Lynne A.|last3=Massey|first3=Philip|year=2002|doi=10.1086/341865|arxiv = astro-ph/0205130 |s2cid=11463246}}</ref>  Until the 21st century the largest number of red supergiants known in a single cluster was five in [[Astronomy:NGC 7419|NGC 7419]].<ref name=caron>{{cite journal|doi=10.1086/377314|title=The Lack of Blue Supergiants in NGC 7419, a Red Supergiant-rich Galactic Open Cluster with Rapidly Rotating Stars|journal=The Astronomical Journal|volume=126|issue=3|pages=1415–1422|year=2003|last1=Caron|first1=Genevive|last2=Moffat|first2=Anthony F. J.|last3=St-Louis|first3=Nicole|last4=Wade|first4=Gregg A.|last5=Lester|first5=John B.|bibcode=2003AJ....126.1415C|doi-access=free}}</ref>  Most red supergiants are found singly, for example [[Astronomy:Betelgeuse|Betelgeuse]] in the [[Organization:Orion OB1 Association|Orion OB1 Association]] and [[Astronomy:Antares|Antares]] in the Scorpius-Centaurus Association.

Since 2006, a series of massive clusters have been identified near the base of the Crux-Scutum Arm of the galaxy, each containing multiple red supergiants.  [[Astronomy:RSGC1|RSGC1]] contains at least 12 red supergiants, RSGC2 (also known as [[Astronomy:Stephenson 2|Stephenson 2]]) contains at least 26 ([[Astronomy:Stephenson 2-18|Stephenson 2-18]], one of the stars, is possibly the [[Astronomy:List of largest stars|largest star known]]), [[Astronomy:RSGC3|RSGC3]] contains at least 8, and RSGC4 (also known as [[Astronomy:Alicante 8|Alicante 8]]) also contains at least 8.  A total of 80 confirmed red supergiants have been identified within a small area of the sky in the direction of these clusters.  These four clusters appear to be part of a massive burst of star formation 10-20 million years ago at the near end of the bar at the centre of the galaxy.<ref>{{cite journal|bibcode=  2012A&A...547A..15N|doi= 10.1051/0004-6361/201219540|title= Red supergiants around the obscured open cluster Stephenson&nbsp;2|journal= Astronomy & Astrophysics|volume= 547|pages= A15|year= 2012|last1= Negueruela|first1= I.|last2= Marco|first2= A.|last3= González-Fernández|first3= C.|last4= Jiménez-Esteban|first4= F.|last5= Clark|first5= J. S.|last6= Garcia|first6= M.|last7= Solano|first7= E.|arxiv = 1208.3282 |s2cid= 53662263}}</ref>  Similar massive clusters have been found near the far end of the galactic bar, but not such large numbers of red supergiants.<ref name=davies>{{cite journal|bibcode=2012MNRAS.419.1860D|title=A newly discovered young massive star cluster at the far end of the Galactic Bar|journal=Monthly Notices of the Royal Astronomical Society|volume=419|issue=3|pages=1860–1870|last1=Davies|first1=Ben|last2=de la Fuente|first2=Diego|last3=Najarro|first3=Francisco|last4=Hinton|first4=Jim A.|last5=Trombley|first5=Christine|last6=Figer|first6=Donald F.|last7=Puga|first7=Elena|year=2012|doi=10.1111/j.1365-2966.2011.19840.x|arxiv = 1111.2630 |s2cid=59405479}}</ref>

==Examples==
[[File:Orion Head to Toe.jpg|200px|right|thumb|The [[Astronomy:Orion (constellation)|Orion]] region showing the red supergiant Betelgeuse]]
Red supergiants are rare stars, but they are visible at great distance and are often variable so there are a number of well-known naked-eye examples:
* [[Astronomy:Psi1 Aurigae|Psi1 Aurigae]]
* CE Tauri
* [[Astronomy:Antares|Antares]]
* [[Astronomy:Epsilon Pegasi|Epsilon Pegasi]]
* [[Astronomy:Betelgeuse|Betelgeuse]]
* [[Astronomy:Lambda Velorum|Lambda Velorum]]
* [[Astronomy:Mu Cephei|Mu Cephei]]

Other examples have become known on account of their enormous size, more than {{solar radius|1,000}}:
* [[Astronomy:Stephenson 2-18|Stephenson 2-18]]
* [[Astronomy:VY Canis Majoris|VY Canis Majoris]]
* [[Astronomy:UY Scuti|UY Scuti]]
* [[Astronomy:WOH G64|WOH G64]]
* [[Astronomy:Westerlund 1-26|Westerlund 1-26]]
* [[Astronomy:S Persei|S Persei]]
* [[Astronomy:NML Cygni|NML Cygni]]

A survey expected to capture virtually all Magellanic Cloud red supergiants<ref>{{Cite journal | last1 = Levesque | first1 = E. M. | last2 = Massey | first2 = P. | last3 = Olsen | first3 = K. A. G. | last4 = Plez | first4 = B. | last5 = Meynet | first5 = G. | last6 = Maeder | first6 = A. | doi = 10.1086/504417 | title = The Effective Temperatures and Physical Properties of Magellanic Cloud Red Supergiants: The Effects of Metallicity | journal = The Astrophysical Journal | volume = 645 | issue = 2 | pages = 1102–1117 | year = 2006 |arxiv = astro-ph/0603596 |bibcode = 2006ApJ...645.1102L | s2cid = 5150686 }}</ref> detected around a dozen M class stars M<sub>v</sub>−7 and brighter, around a quarter of a million times more luminous than the Sun, and from about 1,000 times the radius of the Sun upwards.

== References ==
{{reflist|30em}}

== External links ==
*[http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redsup.html Red Supergiant Stars]

{{Star}}
{{supernovae}}

[[Category:M-type supergiants| ]]
[[Category:Star types]]
[[Category:Stellar phenomena]]

{{Sourceattribution|Red supergiant star|1}}