Editing Star (section)

===Post–main sequence===
{{Main|Subgiant|Red giant|Horizontal branch|Red clump|Asymptotic giant branch}}
[[File:Betelgeuse captured by ALMA.jpg|thumb|upright=0.8|[[Betelgeuse]] as seen by [[Atacama Large Millimeter Array|ALMA]]. This is the first time that ALMA has observed the surface of a star and resulted in the highest-resolution image of Betelgeuse available.]]

As stars of at least {{Solar mass|0.4}}<ref name="late stages">{{cite web
 | last= Richmond | first= Michael
 | url= http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html
 | title= Late stages of evolution for low-mass stars
 | publisher= Rochester Institute of Technology
 | access-date= 2006-08-04}}</ref> exhaust the supply of hydrogen at their core, they start to fuse hydrogen in a shell surrounding the helium core. The outer layers of the star expand and cool greatly as they transition into a [[red giant]]. In some cases, they will fuse heavier [[chemical element|elements]] at the core or in shells around the core. As the stars expand, they throw part of their mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.<ref>{{cite web
 | url= http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_intro.html
 | archive-url= https://web.archive.org/web/20080210154901/http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_intro.html
 | archive-date= 2008-02-10
 | title= Stellar Evolution & Death
 | publisher= NASA Observatorium
 | url-status= dead
 | access-date= 2006-06-08}}</ref> In about 5&nbsp;billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly {{convert|1|AU|e6km|lk=off|abbr=off}}, 250 times its present size, and lose 30% of its current mass.<ref name="sun_future">
{{cite journal |last1= Sackmann |first1= I. J. |last2= Boothroyd |first2= A. I. |last3= Kraemer |first3= K. E. |title= Our Sun. III. Present and Future |page= 457 |journal= Astrophysical Journal |date= 1993 |volume= 418 |bibcode= 1993ApJ...418..457S |doi= 10.1086/173407|doi-access= free }}
</ref><ref name="sun_future_schroder">
{{cite journal |last1=Schröder |first1=K.-P. |last2=Smith |first2=Robert Connon |date=2008 |title=Distant future of the Sun and Earth revisited |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=386 |issue=1 |pages=155–163 |arxiv=0801.4031 |bibcode=2008MNRAS.386..155S |doi=10.1111/j.1365-2966.2008.13022.x |s2cid=10073988 |doi-access=free}} See also {{cite news |last=Palmer |first=Jason |date=2008-02-22 |title=Hope dims that Earth will survive Sun's death |url=https://www.newscientist.com/article/dn13369 |access-date=2008-03-24 |work=NewScientist.com news service}}
</ref>

As the hydrogen-burning shell produces more helium, the core increases in mass and temperature. In a red giant of up to {{Solar mass|2.25}}, the mass of the helium core becomes degenerate prior to [[helium fusion]]. Finally, when the temperature increases sufficiently, core helium fusion begins explosively in what is called a [[helium flash]], and the star rapidly shrinks in radius, increases its surface temperature, and moves to the [[horizontal branch]] of the HR diagram. For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the [[red clump]], slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.<ref name="iben">{{cite journal
 | last= Iben | first= Icko Jr.
 | title=Single and binary star evolution
 | journal=Astrophysical Journal Supplement Series
 | date=1991 | volume=76 | pages=55–114
 | bibcode=1991ApJS...76...55I
 | doi=10.1086/191565| doi-access=free
 }}</ref>

After a star has fused the helium of its core, it begins fusing helium along a shell surrounding the hot carbon core. The star then follows an evolutionary path called the [[asymptotic giant branch]] (AGB) that parallels the other described red-giant phase, but with a higher luminosity. The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate. During the AGB phase, stars undergo [[thermal pulse]]s due to instabilities in the core of the star. In these thermal pulses, the luminosity of the star [[variable star|varies]] and matter is ejected from the star's atmosphere, ultimately forming a planetary nebula. As much as 50 to 70% of a star's mass can be ejected in this [[stellar mass loss|mass loss]] process. Because energy transport in an AGB star is primarily by [[convection]], this ejected material is enriched with the fusion products [[dredge-up|dredged up]] from the core. Therefore, the planetary nebula is enriched with elements like carbon and oxygen. Ultimately, the planetary nebula disperses, enriching the general interstellar medium.<ref name='carroll_ostlie_ch13'>{{cite book |last1=Carroll |first1=Bradley W. |last2=Ostlie |first2=Dale A. |title=An Introduction to Modern Astrophysics |location=Cambridge, United Kingdom |publisher=Cambridge University Press |isbn=978-1108422161 |edition=2nd |chapter=Chapter 13|date=7 September 2017 }}</ref> Therefore, future generations of stars are made of the "star stuff" from past stars.<ref>{{cite episode | series= Cosmos: A Personal Voyage | title= The Lives of the Stars | last=Sagan | first=Carl | author-link = Carl Sagan | year=1980 | url=https://libquotes.com/carl-sagan/quote/lbe8f9i | series-link=Cosmos: A Personal Voyage}}</ref>

====Massive stars====
{{Main|Supergiant star|Hypergiant|Wolf–Rayet star}}
[[File:Layers of an evolved star.png|thumb|upright=1.6|Onion-like layers at the core of a massive, evolved star just before core collapses]]
During their helium-burning phase, a star of more than 9 solar masses expands to form first a [[blue supergiant]] and then a [[red supergiant]]. Particularly massive stars (exceeding 40 solar masses, like [[Alnilam]], the central blue supergiant of [[Orion's Belt]])<ref>{{Cite journal |last1=Puebla |first1=Raul E. |last2=Hillier |first2=D. John |last3=Zsargó |first3=Janos |last4=Cohen |first4=David H. |last5=Leutenegger |first5=Maurice A. |date=2016-03-01 |title=X-ray, UV and optical analysis of supergiants: &epsilon; Ori |journal=Monthly Notices of the Royal Astronomical Society |volume=456 |issue=3 |pages=2907–2936 |doi=10.1093/mnras/stv2783 |doi-access=free |issn=0035-8711|arxiv=1511.09365 }}</ref> do not become red supergiants due to high mass loss.<ref>{{Cite journal |last1=Vanbeveren |first1=D. |last2=De Loore |first2=C. |last3=Van Rensbergen |first3=W. |date=1998-12-01 |title=Massive stars |url=https://doi.org/10.1007/s001590050015 |journal=The Astronomy and Astrophysics Review |language=en |volume=9 |issue=1 |pages=63–152 |doi=10.1007/s001590050015 |bibcode=1998A&ARv...9...63V |issn=1432-0754}}</ref> These may instead evolve to a [[Wolf–Rayet star]], characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss, or from stripping of the outer layers.<ref name="ContiLoore2012">{{cite book |last1=Conti |first1=P. S. |url=https://books.google.com/books?id=aZbnCAAAQBAJ |title=Mass Loss and Evolution of O-Type Stars |last2=de Loore |first2=C. |date=2012 |publisher=Springer Science & Business Media |isbn=978-94-009-9452-2}}</ref>

When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse [[carbon]] (see [[Carbon-burning process]]). This process continues, with the successive stages being fueled by [[neon]] (see [[neon-burning process]]), [[oxygen]] (see [[oxygen-burning process]]), and [[silicon]] (see [[silicon-burning process]]). Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.<ref>
{{cite web |url= https://www.e-education.psu.edu/astro801/content/l6_p5.html |title=The Evolution of Massive Stars and Type II Supernovae |publisher=Penn Stats College of Science |access-date= 2016-01-05}}
</ref>

The final stage occurs when a massive star begins producing iron. Since iron nuclei are more [[binding energy|tightly bound]] than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.<ref name="sneden">{{Cite journal |title= Astronomy: The age of the Universe |journal= Nature |date= 2001-02-08  |pages= 673–675 |volume= 409 |issue= 6821 |doi= 10.1038/35055646 |pmid= 11217843 |first= Christopher |last= Sneden|s2cid= 4316598 |doi-access= free }}</ref>

Some massive stars, particularly [[luminous blue variable]]s, are very unstable to the extent that they violently shed their mass into space in events known as [[supernova impostor]]s, becoming significantly brighter in the process. [[Eta Carinae]] is known for having undergone a supernova impostor event, the Great Eruption, in the 19th century.

====Collapse====
As a star's core shrinks, the intensity of radiation from that surface increases, creating such [[radiation pressure]] on the outer shell of gas that it will push those layers away, forming a planetary nebula. If what remains after the outer atmosphere has been shed is less than roughly {{Solar mass|1.4}}, it shrinks to a relatively tiny object about the size of Earth, known as a [[white dwarf]]. White dwarfs lack the mass for further gravitational compression to take place.<ref>
{{cite journal |last1=Liebert |first1=James |title=White dwarf stars |journal=Annual Review of Astronomy and Astrophysics |date=1980 |volume=18 |issue=2 |pages=363–398 |bibcode=1980ARA&A..18..363L |doi= 10.1146/annurev.aa.18.090180.002051}}
</ref> The [[electron-degenerate matter]] inside a white dwarf is no longer a plasma. Eventually, white dwarfs fade into [[black dwarf]]s over a very long period of time.<ref>{{cite news |last1=Mann |first1=Adam |title=This is the way the universe ends: not with a whimper, but a bang |url=https://www.science.org/content/article/way-universe-ends-not-whimper-bang |work=Science {{!}} AAAS |date=2020-08-11 |language=en}}</ref>

[[File:Crab Nebula.jpg|thumb|The [[Crab Nebula]], remnants of a supernova that was first observed around 1050 AD]]

In massive stars, fusion continues until the iron core has grown so large (more than {{Solar mass|1.4}}) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, [[neutrino]]s, and gamma rays in a burst of [[electron capture]] and [[inverse beta decay]]. The [[shock wave|shockwave]] formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become 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 seemingly existed before.<ref name="supernova">
{{cite web
 | date=2006-04-06
 | url=http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html
 | title=Introduction to Supernova Remnants
 | publisher=Goddard Space Flight Center
 | access-date=2006-07-16}}
</ref>

A supernova explosion blows away the star's outer layers, leaving a [[supernova remnant|remnant]] such as the Crab Nebula.<ref name="supernova" /> The core is compressed into a [[neutron star]], which sometimes manifests itself as a [[pulsar]] or [[X-ray burster]]. In the case of the largest stars, the remnant is a black hole greater than {{Solar mass|4}}.<ref>{{cite journal |last1=Fryer |first1=C. L. |title=Black-hole formation from stellar collapse |journal=Classical and Quantum Gravity |date=2003 |volume=20 |issue=10 |pages=S73–S80 |doi= 10.1088/0264-9381/20/10/309 |bibcode=2003CQGra..20S..73F|s2cid=122297043 |url=https://zenodo.org/record/1235744 }}</ref> In a neutron star the matter is in a state known as [[neutron-degenerate matter]], with a more exotic form of degenerate matter, [[QCD matter]], possibly present in the core.<ref>{{cite journal |bibcode=2019NuPhA.982...36V |title=Neutron stars and stellar mergers as a laboratory for dense QCD matter |last1=Vuorinen |first1=Aleksi |journal=Nuclear Physics A |year=2019 |volume=982 |page=36 |doi=10.1016/j.nuclphysa.2018.10.011 |arxiv=1807.04480 |s2cid=56422826 }}</ref>

The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.<ref name="supernova" />

====Binary stars====
[[Binary stars]]' evolution may significantly differ from that of single stars of the same mass. For example, when any star expands to become a red giant, it may overflow its [[Roche lobe]], the surrounding region where material is gravitationally bound to it; if stars in a binary system are close enough, some of that material may overflow to the other star, yielding phenomena including [[contact binaries]], [[common envelope|common-envelope]] binaries, [[Cataclysmic variable star|cataclysmic variables]], [[blue stragglers]],<ref>{{cite journal |last1=Leiner |first1=Emily M. |last2=Geller |first2=Aaron |title=A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics |journal=The Astrophysical Journal |date=2021-01-01 |volume=908 |issue=2 |page=229 |doi=10.3847/1538-4357/abd7e9 |arxiv=2101.11047 |bibcode=2021ApJ...908..229L |s2cid=231718656 |doi-access=free }}</ref> and [[type Ia supernova]]e. Mass transfer leads to cases such as the [[Algol paradox]], where the most-evolved star in a system is the least massive.<ref>{{cite journal |last1=Brogaard |first1=K. |last2=Christiansen |first2=S. M. |last3=Grundahl |first3=F. |last4=Miglio |first4=A. |last5=Izzard |first5=R. G. |last6=Tauris |first6=T. M. |last7=Sandquist |first7=E. L. |last8=VandenBerg |first8=D. A. |last9=Jessen-Hansen |first9=J. |last10=Arentoft |first10=T. |last11=Bruntt |first11=H. |last12=Frandsen |first12=S. |last13=Orosz |first13=J. A. |last14=Feiden |first14=G. A. |last15=Mathieu |first15=R. |date=2018-12-21 |title=The blue straggler V106 in NGC 6791: a prototype progenitor of old single giants masquerading as young |journal=Monthly Notices of the Royal Astronomical Society |volume=481 |issue=4 |pages=5062–5072 |arxiv=1809.00705 |bibcode=2018MNRAS.481.5062B |doi=10.1093/mnras/sty2504 |doi-access=free |last16=Geller |first16=A. |last17=Shetrone |first17=M. |last18=Ryde |first18=N. |last19=Stello |first19=D. |last20=Platais |first20=I. |last21=Meibom |first21=S.}}</ref>

The evolution of binary star and higher-order [[star system]]s is intensely researched since so many stars have been found to be members of binary systems. Around half of Sun-like stars, and an even higher proportion of more massive stars, form in multiple systems, and this may greatly influence such phenomena as novae and supernovae, the formation of certain types of star, and the enrichment of space with nucleosynthesis products.<ref name="beccari2019">{{cite book |last1=Beccari |first1=Giacomo |url=https://books.google.com/books?id=Um2MDwAAQBAJ |title=The Impact of Binary Stars on Stellar Evolution |last2=Boffin |first2=Henri M. J. |date=2019 |publisher=Cambridge University Press |isbn=978-1-108-42858-3}}</ref>

The influence of binary star evolution on the formation of evolved massive stars such as [[luminous blue variable]]s, Wolf–Rayet stars, and the progenitors of certain classes of [[core collapse supernova]] is still disputed. Single massive stars may be unable to expel their outer layers fast enough to form the types and numbers of evolved stars that are observed, or to produce progenitors that would explode as the supernovae that are observed. Mass transfer through gravitational stripping in binary systems is seen by some astronomers as the solution to that problem.<ref>{{cite journal |bibcode=2017ApJ...840...10Y |title=Type Ib and IIb Supernova Progenitors in Interacting Binary Systems |last1=Yoon |first1=Sung-Chul |last2=Dessart |first2=Luc |last3=Clocchiatti |first3=Alejandro |journal=The Astrophysical Journal |year=2017 |volume=840 |issue=1 |page=10 |doi=10.3847/1538-4357/aa6afe |arxiv=1701.02089 |s2cid=119058919 |doi-access=free }}</ref><ref>{{cite journal |bibcode=2016MNRAS.459.1505M |title=Helium stars: Towards an understanding of Wolf-Rayet evolution |last1=McClelland |first1=L. A. S. |last2=Eldridge |first2=J. J. |journal=Monthly Notices of the Royal Astronomical Society |year=2016 |volume=459 |issue=2 |page=1505 |doi=10.1093/mnras/stw618 |doi-access=free |arxiv=1602.06358 |s2cid=119105982 }}</ref><ref>{{cite journal |bibcode=2020A&A...634A..79S |title=Why binary interaction does not necessarily dominate the formation of Wolf-Rayet stars at low metallicity |last1=Shenar |first1=T. |last2=Gilkis |first2=A. |last3=Vink |first3=J. S. |last4=Sana |first4=H. |last5=Sander |first5=A. A. C. |journal=Astronomy and Astrophysics |year=2020 |volume=634 |pages=A79 |doi=10.1051/0004-6361/201936948 |arxiv=2001.04476 |s2cid=210472736 }}</ref>