Editing Star (section)

==Formation and evolution==
{{Main|Stellar evolution}}
{{Multiple image|perrow = 1|total_width = 350
 | direction         = vertical

 <!--image 1-->
 | image1            = star life cycles red dwarf en.svg
 | alt1              =
 | caption1          = Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics
 <!--image 2-->
 | image2            = A Comparison of Star Sizes.jpg
 | alt2              =
 | caption2          = Size comparison (radius and mass) of a [[red dwarf]], the Sun, a supermassive [[blue supergiant]], and a [[red giant]].
}}

Stars condense from regions of [[interstellar medium|space]] of higher matter density, yet those regions are less dense than within a [[vacuum chamber]]. These regions—known as ''[[molecular cloud]]s''—consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements. One example of such a star-forming region is the [[Orion Nebula]].<ref>
{{cite journal
 | last=Woodward | first=P. R.
 | title=Theoretical models of star formation
 | journal=Annual Review of Astronomy and Astrophysics
 | date=1978 | volume=16
 | issue=1 | pages=555–584 | doi= 10.1146/annurev.aa.16.090178.003011
 | bibcode=1978ARA&A..16..555W
}}
</ref> Most stars form in groups of dozens to hundreds of thousands of stars.<ref>
{{cite journal
 | last1=Lada | first1=C. J.
 | last2=Lada | first2= E. A.
 | title= Embedded Clusters in Molecular Clouds
 | journal=Annual Review of Astronomy and Astrophysics
 | date=2003 | volume= 41
 | issue=1 | pages= 57–115 | doi=10.1146/annurev.astro.41.011802.094844
 | bibcode=2003ARA&A..41...57L
|arxiv = astro-ph/0301540 | s2cid=16752089
 }}
</ref> [[O-type star|Massive stars]] in these groups may powerfully illuminate those clouds, [[ion]]izing the hydrogen, and creating [[H II region]]s. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.<ref>{{cite journal |doi=10.1088/0004-637X/729/2/133 |title=Star Formation Efficiencies and Lifetimes of Giant Molecular Clouds in the Milky Way |year=2011 |last1=Murray |first1=Norman |journal=The Astrophysical Journal |volume=729 |issue=2 |page=133 |arxiv=1007.3270 |bibcode=2011ApJ...729..133M |s2cid=118627665 }}</ref>
All stars spend the majority of their existence as ''[[main sequence]] stars'', fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment. Accordingly, astronomers often group stars by their mass:<ref>
{{cite book
 | first=Sun | last=Kwok | date=2000 | pages=103–104
 | title=The origin and evolution of planetary nebulae
 | volume=33 | series=Cambridge astrophysics series
 | publisher=Cambridge University Press | isbn=978-0-521-62313-1}}
</ref>
* ''Very low mass stars'', with masses below {{Solar mass|0.5}}, are fully convective and distribute [[helium]] evenly throughout the whole star while on the main sequence. Therefore, they never undergo shell burning and never become [[red giant]]s. After exhausting their hydrogen they become [[white dwarf#Stars with very low mass|helium white dwarfs]] and slowly cool.<ref name=adams/> As the lifetime of {{solar mass|0.5}} stars is longer than the [[age of the universe]], no such star has yet reached the white dwarf stage.
* ''Low mass stars'' (including the Sun), with a mass between {{solar mass|0.5}} and ~{{Solar mass|2.25}} depending on composition, do become red giants as their core hydrogen is depleted and they begin to burn helium in core in a [[helium flash]]; they develop a degenerate carbon-oxygen core later on the [[asymptotic giant branch]]; they finally blow off their outer shell as a [[planetary nebula]] and leave behind their core in the form of a white dwarf.<ref name=Kolb2014>{{cite book
 | title=Astrobiology, An Evolutionary Approach
 | pages=21–25 | isbn=978-1466584617
 | publisher=Taylor & Francis | date= 2014
 | editor-first=Vera M. | editor-last=Kolb
 | url=https://books.google.com/books?id=75hBBAAAQBAJ&pg=PA23
}}</ref><ref name=Bisnovatyi-Kogan2013>{{cite book
 | title=Stellar Physics: Stellar Evolution and Stability
 | first= G. S. | last=Bisnovatyi-Kogan | date=2013
 | translator1-first=A. Y. | translator1-last=Blinov
 | translator2-first=M. | translator2-last=Romanova
 | pages=108–125 | isbn=978-3662226391
 | publisher=Springer Berlin Heidelberg
 | url=https://books.google.com/books?id=AkjtCAAAQBAJ&pg=PA108
}}</ref>
* ''Intermediate-mass stars'', between ~{{solar mass|2.25}} and ~{{solar mass|8}}, pass through evolutionary stages similar to low mass stars, but after a relatively short period on the [[red-giant branch]] they ignite helium without a flash and spend an extended period in the [[red clump]] before forming a degenerate carbon-oxygen core.<ref name=Kolb2014/><ref name=Bisnovatyi-Kogan2013/>
* ''Massive stars'' generally have a minimum mass of ~{{Solar mass|8}}.<ref>{{cite journal
 | title=The Metallicity Dependence of the Minimum Mass for Core-collapse Supernovae
 | last1=Ibeling | first1=Duligur | last2=Heger | first2=Alexander
 | journal=The Astrophysical Journal Letters
 | volume=765 | issue=2 | id=L43 | pages=4
 | date=March 2013 | doi=10.1088/2041-8205/765/2/L43
 | arxiv=1301.5783 | bibcode=2013ApJ...765L..43I
| s2cid=118474569 }}</ref> After exhausting the hydrogen at the core these stars become [[supergiant]]s and go on to [[nuclear fusion|fuse]] elements heavier than helium. Many end their lives when their cores collapse and they explode as supernovae.<ref name=Kolb2014/><ref>{{cite book
 | last1=Thielemann | first1=F. -K. | last2=Hirschi | first2=R.
 | last3=Liebendörfer | first3=M. | last4=Diehl | first4=R.
 | chapter=Massive Stars and their Supernovae
 | display-authors=1 | title=Astronomy with Radioactivities
 | editor1-first=Roland | editor1-last=Diehl
 | editor2-first=Dieter H. | editor2-last=Hartmann
 | editor3-first=Nikos | editor3-last=Prantzos
 | display-editors=1 | series=Lecture Notes in Physics
 | volume=812 | publication-place=Berlin
 | publisher=Springer | date=2011 | pages=153–232
 | doi=10.1007/978-3-642-12698-7_4 | isbn=978-3-642-12697-0
 | arxiv=1008.2144 | bibcode=2011LNP...812..153T | s2cid=119254840 }}</ref>

===Star formation===
{{Main|Star formation}}
{{Multiple image|perrow = 1|total_width = 240
 | direction         = vertical

 <!--image 1-->
 | image1            = Witness the Birth of a Star.jpg
 | alt1              =
 | caption1          = Artist's conception of the birth of a star within a dense [[molecular cloud]]
 <!--image 2-->
 | image2            = W40_star-forming_region.jpg
 | alt2              =
 | caption2          = A cluster of approximately 500 young stars lies within the nearby [[Westerhout 40|W40]] stellar nursery.
}}

The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density—often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the [[Interacting galaxy|collision of galaxies]] (as in a [[starburst galaxy]]).<ref>
{{cite journal
 | last1=Elmegreen | first1=B. G.
 | last2=Lada | first2= C. J.
 | title= Sequential formation of subgroups in OB associations
 | journal= Astrophysical Journal, Part 1
 | date=1977 | volume= 214
 | pages= 725–741 | doi=10.1086/155302
 | bibcode=1977ApJ...214..725E
}}
</ref><ref>
{{cite journal
 | last1=Getman | first1=K. V.
 | display-authors=1
 | last2=Feigelson | first2= E. D.
 | last3=Sicilia-Aguilar| first3=A.
 | last4=Broos | first4=P. S.
 | last5=Kuhn | first5=M. A.
 | last6=Garmire | first6=G. P.
 | title= The Elephant Trunk Nebula and the Trumpler 37 cluster: contribution of triggered star formation to the total population of an H II region
 | journal= Monthly Notices of the Royal Astronomical Society
 | date=2012 | volume= 426 | issue=4
 | pages= 2917–2943 | doi=10.1111/j.1365-2966.2012.21879.x
 | doi-access=free
 | bibcode=2012MNRAS.426.2917G
|arxiv = 1208.1471 | s2cid=49528100
 }}
</ref> When a region reaches a sufficient density of matter to satisfy the criteria for [[Jeans instability]], it begins to collapse under its own gravitational force.<ref>
{{cite book
 | first=Michael David | last=Smith | date=2004
 | title=The Origin of Stars | url=https://archive.org/details/originstars00smit | url-access=limited | publisher=Imperial College Press
 | isbn=978-1-86094-501-4 | pages=[https://archive.org/details/originstars00smit/page/n70 57]–68}}
</ref>

As the cloud collapses, individual conglomerations of dense dust and gas form "[[Bok globule]]s". As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of [[hydrostatic equilibrium]], a [[protostar]] forms at the core.<ref>
{{cite web
 | last= Seligman | first= Courtney
 | url= http://courtneyseligman.com/text/stars/starevol2.htm
 | archive-url= https://web.archive.org/web/20080623190408/http://courtneyseligman.com/text/stars/starevol2.htm
 | archive-date= 2008-06-23
 | title= Slow Contraction of Protostellar Cloud | work= Self-published
 | url-status= usurped | access-date= 2006-09-05}}
</ref> These [[pre-main-sequence star]]s are often surrounded by a [[protoplanetary disk]] and powered mainly by the conversion of gravitational energy. The period of gravitational contraction lasts about 10&nbsp;million years for a star like the sun, up to 100&nbsp;million years for a red dwarf.<ref name="Hanslmeier2010">{{cite book |author=Hanslmeier |first=Arnold |url=https://books.google.com/books?id=Mj5tSld5tjMC&pg=PA163 |title=Water in the Universe |date=2010 |publisher=Springer Science & Business Media |isbn=978-90-481-9984-6 |pages=163}}</ref>

Early stars of less than {{Solar mass|2}} are called [[T Tauri star]]s, while those with greater mass are [[Herbig Ae/Be star]]s. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the [[angular momentum]] of the collapsing star and result in small patches of nebulosity known as [[Herbig–Haro object]]s.<ref>
{{cite conference |last1=Bally |first1=J. |last2=Morse |first2=J. |last3=Reipurth |first3=B. |date=1996 |editor1-last=Benvenuti |editor1-first=Piero |editor2-last=Macchetto |editor2-first=F. D. |editor3-last=Schreier |editor3-first=Ethan J. |title=The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks |conference=Space Science Institute Workshop, Paris, France, December 4–8, 1995 |publisher=Space Telescope Science Institute |page=491 |bibcode=1996swhs.conf..491B |book-title=Science with the Hubble Space Telescope – II. Proceedings of a workshop held in Paris, France, December 4–8, 1995}}
</ref><ref name=smith04>
{{cite book
 | first=Michael David | last=Smith
 | title=The origin of stars | url=https://archive.org/details/originstars00smit | url-access=limited | page=[https://archive.org/details/originstars00smit/page/n189 176] | date=2004
 | isbn=978-1-86094-501-4
 | publisher=Imperial College Press
}}
</ref>
These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.<ref>
{{cite news
 | first=Tom | last=Megeath | date=2010-05-11
 | title=Herschel finds a hole in space
 | url=http://www.esa.int/esaCP/SEMFEAKPO8G_index_0.html
 | publisher=ESA | access-date=2010-05-17}}
</ref>

Early in their development, T Tauri stars follow the [[Hayashi track]]—they contract and decrease in luminosity while remaining at roughly the same temperature. Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the [[Henyey track]].<ref name="Darling2004">{{cite book |author=Darling |first=David |url=https://books.google.com/books?id=L5zuAAAAMAAJ |title=The Universal Book of Astronomy: From the Andromeda Galaxy to the Zone of Avoidance |publisher=Wiley |year=2004 |isbn=978-0-471-26569-6 |page=229}}</ref>

Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.<ref>
{{cite journal
 | last1=Duquennoy | first1=A.
 | last2=Mayor | first2= M.
 | title= Multiplicity among solar-type stars in the solar neighbourhood. II – Distribution of the orbital elements in an unbiased sample
 | journal= Astronomy & Astrophysics
 | date=1991 | volume= 248 | issue=2
 | pages= 485–524
 | bibcode=1991A&A...248..485D
}}
</ref> A gas cloud must lose its angular momentum in order to collapse and form a star. The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters. These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound. This produces the separation of binaries into their two observed populations distributions.<ref name="Padmanabhan2000">{{cite book |author=Padmanabhan |first=T. |url=https://books.google.com/books?id=TOjwtYYb63cC&pg=PA557 |title=Theoretical Astrophysics: Volume 2, Stars and Stellar Systems |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-56631-5 |pages=557}}</ref>

===Main sequence===
{{Main|Main sequence}}

Stars spend about 90% of their lifetimes fusing hydrogen into helium in high-temperature-and-pressure reactions in their cores. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.<ref>
{{cite journal | display-authors=1
 | last1=Mengel | first1=J. G. | last2=Demarque | first2=P.
 | last3=Sweigart | first3=A. V. | last4=Gross | first4=P. G.
 | title=Stellar evolution from the zero-age main sequence
 | journal=Astrophysical Journal Supplement Series
 | date=1979 | volume=40 | pages=733–791
 | bibcode=1979ApJS...40..733M | doi= 10.1086/190603
}}</ref>
The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6&nbsp;billion ({{val|4.6|e=9}}) years ago.<ref name=sun_future />

Every star generates a [[stellar wind]] of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses {{val|e=−14|u=Solar mass}} every year,<ref>
{{cite journal | display-authors=1
 | last1=Wood | first1=B. E. | last2=Müller | first2=H.-R.
 | last3=Zank | first3=G. P. | last4=Linsky | first4=J. L.
 | title=Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity
 | journal=The Astrophysical Journal | date=2002
 | volume=574 | issue=1 | pages=412–425
| doi= 10.1086/340797 | bibcode=2002ApJ...574..412W
|arxiv= astro-ph/0203437| s2cid=1500425 }}
</ref> or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose {{val|e=−7}} to {{val|e=−5|u=Solar mass}} each year, significantly affecting their evolution.<ref>
{{cite journal
 | last1=de Loore | first1=C. | last2=de Greve | first2=J. P.
 | last3=Lamers | first3=H. J. G. L. M.
 | title=Evolution of massive stars with mass loss by stellar wind
 | journal=Astronomy and Astrophysics | date=1977 | volume=61
 | issue=2 | pages=251–259
 | bibcode=1977A&A....61..251D}}
</ref> Stars that begin with more than {{Solar mass|50}} can lose over half their total mass while on the main sequence.<ref>{{cite web |title=The evolution of stars between 50 and 100 times the mass of the Sun |url=http://certificate.ulo.ucl.ac.uk/modules/year_one/ROG/stellar_evolution/conWebDoc.727.html |url-status=dead |archive-url=https://web.archive.org/web/20151118161020/http://certificate.ulo.ucl.ac.uk/modules/year_one/ROG/stellar_evolution/conWebDoc.727.html |archive-date=2015-11-18 |access-date=2015-11-17 |publisher=Royal Greenwich Observatory |publication-place=England}}</ref>

[[File:H-R diagram -edited-3.gif|thumb|upright=1.6|An example of a [[Hertzsprung–Russell diagram]] for a set of stars that includes the Sun (center) (see [[#Classification|Classification]])]]

The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10&nbsp;billion ({{val|e=10}}) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than {{Solar mass|0.25}}, called [[red dwarf]]s, are able to fuse nearly all of their mass while stars of about {{Solar mass|1}} can only fuse about 10% of their mass. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion ({{val|10|e=12}}) years; the most extreme of {{Solar mass|0.08}} will last for about 12&nbsp;trillion years. Red dwarfs become [[blue dwarf (red-dwarf stage)|hotter and more luminous]] as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.<ref name="adams">{{cite conference |last1=Adams |first1=Fred C. |last2=Laughlin |first2=Gregory |last3=Graves |first3=Genevieve J. M. |title=Red Dwarfs and the End of the Main Sequence |url=http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf |conference= |publisher=Revista Mexicana de Astronomía y Astrofísica |pages=46–49 |bibcode=2004RMxAC..22...46A |archive-url=https://web.archive.org/web/20190711072446/http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf |archive-date=11 July 2019 |access-date=2008-06-24 |book-title=Gravitational Collapse: From Massive Stars to Planets |url-status=dead}}</ref> Since the lifespan of such stars is greater than the current age of the universe (13.8&nbsp;billion years), no stars under about {{Solar mass|0.85}}<ref name="saomainseq">
{{cite encyclopedia |title=Main Sequence Lifetime |url=http://astronomy.swin.edu.au/cosmos/M/Main+Sequence+Lifetime |encyclopedia=Swinburne Astronomy Online Encyclopedia of Astronomy |publisher=Swinburne University of Technology}}
</ref> are expected to have moved off the main sequence.

Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical [[concentration]] of these elements in a star, its [[metallicity]]. A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,<ref>
{{cite journal
 | display-authors=1
 | last1=Pizzolato | first1=N. | last2=Ventura | first2=P.
 | last3=D'Antona | first3=F. | last4=Maggio | first4=A.
 | last5=Micela | first5=G. | last6=Sciortino | first6=S.
 | title=Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests
 | journal=Astronomy & Astrophysics
 | date=2001 | volume=373
 | issue=2 | pages=597–607
 | doi=10.1051/0004-6361:20010626
| bibcode=2001A&A...373..597P| doi-access=free }}
</ref> which affects the strength of its stellar wind.<ref>
{{cite web
 | date= 2004-06-18
 | url= http://www.star.ucl.ac.uk/groups/hotstar/research_massloss.html
 | archive-url= https://web.archive.org/web/20041122143115/http://www.star.ucl.ac.uk/groups/hotstar/research_massloss.html
 | archive-date= 2004-11-22
 | title= Mass loss and Evolution | publisher= UCL Astrophysics Group
 | access-date= 2006-08-26}}
</ref> Older, [[stellar population|population II]] stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their [[stellar atmosphere|atmospheres]].<ref name="Astrophysics1984">{{cite book|author=Rutherford Appleton Laboratory. Workshop on Astronomy and Astrophysics|title=Gas in the Interstellar Medium: Rutherford Appleton Laboratory Workshop on Astronomy and Astrophysics : 21–23 May, 1983, The Cosener's House, Abingdon|url=https://books.google.com/books?id=e37vAAAAMAAJ|year=1984|publisher=Science and Engineering Research Council, Rutherford Appleton Laboratory}}</ref>

===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>