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{{Short description|Large self-illuminated object in space}}
{{About|the astronomical object|other uses|Star (disambiguation)|and|Stars (disambiguation)}}
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 | image1            = The Sun in white light.jpg
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 | caption1          = The [[Sun]], a [[G-type main-sequence star]], the closest to Earth

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 | image2            = Starsinthesky.jpg
 | alt2              =
 | caption2          = A [[Star formation#Stellar nurseries|star-forming]] region in the [[Large Magellanic Cloud]]
}}

A '''star''' is a luminous [[spheroid]] of [[plasma (physics)|plasma]] held together by [[Self-gravitation|self-gravity]].<ref>{{cite web | title=What is a star? | first=Maria | last=Temming | date=July 15, 2014 | publisher=AAS Sky Publishing, LLC | url=https://skyandtelescope.org/astronomy-resources/what-is-a-star/ | access-date=2024-04-22 }}</ref> The [[List of nearest stars and brown dwarfs|nearest star]] to Earth is the [[Sun]]. Many other stars are visible to the naked eye at [[night sky|night]]; their immense distances from Earth make them appear as [[fixed stars|fixed]] points of light. The most prominent stars have been categorised into [[constellation]]s and [[asterism (astronomy)|asterisms]], and many of the brightest stars have [[proper name]]s. [[Astronomer]]s have assembled [[star catalogue]]s that identify the known stars and provide standardized [[stellar designation]]s. The [[observable universe]] contains an estimated {{val|e=22}} to {{val|e=24}} stars. Only about 4,000 of these stars are visible to the naked eye—all within the [[Milky Way]] [[galaxy]].<ref>{{cite book | title=Galileo and 400 Years of Telescopic Astronomy | first1=Peter | last1=Grego | first2=David | last2=Mannion |year= 2010 | isbn=978-1441955920 | publisher=Springer New York | url=https://books.google.com/books?id=nsi7R3NTcmEC&pg=PA88 }}</ref>

A star's life [[star formation|begins]] with the [[gravitational collapse]] of a gaseous [[nebula]] of material largely comprising [[hydrogen]], helium, and traces of heavier elements. Its [[stellar mass|total mass]] mainly determines its [[stellar evolution|evolution]] and eventual fate. A star shines for [[main sequence|most of its active life]] due to the [[thermonuclear fusion]] of hydrogen into [[helium]] in its core. This process releases energy that traverses the star's interior and [[radiation|radiates]] into [[outer space]]. At the end of a star's lifetime, fusion ceases and its core becomes a [[stellar remnant]]: a [[white dwarf]], a [[neutron star]], or—if it is sufficiently massive—a [[black hole]].

[[Stellar nucleosynthesis]] in stars or their remnants creates almost all naturally occurring [[chemical element]]s heavier than [[lithium]]. [[Stellar mass loss]] or [[supernova]] explosions return chemically enriched material to the [[interstellar medium]]. These elements are then recycled into new stars. Astronomers can determine stellar properties—including mass, age, [[metallicity]] (chemical composition), [[variable star|variability]], [[distance (astronomy)|distance]], and motion through [[space]]—by carrying out observations of a star's [[apparent magnitude|apparent brightness]], [[astronomical spectroscopy|spectrum]], and [[astrometry|changes in its position in the sky]] over time.

Stars can form orbital systems with other [[astronomical object]]s, as in [[planetary system]]s and [[star system]]s with [[binary star|two]] or [[star system#Multiple star systems|more]] stars. When two such stars orbit closely, their gravitational interaction can significantly impact their evolution. Stars can form part of a much larger gravitationally bound structure, such as a [[star cluster]] or a galaxy.

== Etymology ==
The English word ''star'' ultimately derives from the [[Proto-Indo-European]] root {{lang|ine-x-proto|h₂stḗr}}, also meaning 'star'{{snd}}which is further analyzable as {{lang|ine-x-proto|h₂eh₁s-}} 'to burn' (also the source of the word ''ash'') plus {{lang|ine-x-proto|-tēr}} (the [[agentive suffix]]). Its [[cognate]]s in other languages include  Latin {{Lang|la|stella}}, Greek {{tlit|el|aster}}, and German {{Lang|de|Stern}};<ref>{{multiref | {{cite encyclopedia | last=Kunth | first=Daniel | last2=Terlevich | first2=Elena | title=StarWords | entry=Stars | publisher=Springer |series=Popular Astronomy | year=2024 | isbn=978-3-031-49023-1 | doi=10.1007/978-3-031-49024-8_3 |page=19}} | {{cite encyclopedia | last=Mallory | first=J. P. | last2=Adams | first2=Douglas Q. | encyclopedia=Encyclopedia of Indo-European Culture | publisher=Taylor & Francis | date=1997 | isbn=978-1-884964-98-5 |entry=Star |page=543 |ol=402075M }} }}</ref> further cognates in English include ''[[asterisk]]'', ''[[asteroid]]'', ''[[wikt:astral|astral]]'', ''[[constellation]]'', and ''[[Esther]]''.<ref>{{cite dictionary |last=Harper |first=Douglas |date=2001–2022 |entry=*ster- (2) |url=https://www.etymonline.com/word/*ster-#etymonline_v_52592 |access-date=28 February 2022 |dictionary=Online Etymology Dictionary}}</ref>

==Observation history==
{{seealso|Stars in astrology}}
[[File:Dibuix de Leo.png|thumb|A 1690 depiction of the constellation of [[Leo (constellation)|Leo]] the lion by [[Johannes Hevelius]].<ref>{{cite book
 | first=Johannis | last=Hevelius | date=1690
 | title=Firmamentum Sobiescianum, sive Uranographia
 | location=Gdansk}}</ref>]]

Historically, stars have been important to [[civilization]]s throughout the world. They have been part of religious practices, [[divination]] rituals, [[myth]]ology, used for [[celestial navigation]] and orientation, to mark the passage of seasons, and to define calendars.

Early astronomers recognized a difference between "[[fixed stars]]", whose position on the [[celestial sphere]] does not change, and "wandering stars" ([[planet]]s), which move noticeably relative to the fixed stars over days or weeks.<ref>{{cite web |date=n.d. |title=Ancient Greek Astronomy and Cosmology |url=https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/ancient-greek-astronomy-and-cosmology |access-date=28 February 2022 |website=Digital Collections |publisher=The [[Library of Congress]]}}</ref> Many ancient astronomers believed that the stars were permanently affixed to a [[heavenly sphere]] and that they were immutable. By convention, astronomers grouped prominent stars into [[asterism (astronomy)|asterisms]] and [[constellation]]s and used them to track the motions of the planets and the inferred position of the Sun.<ref name="forbes">{{cite book
 | last1=Forbes | first1=George | title=History of Astronomy
 | publisher=Watts & Co. | location=London | date=1909
 | url=http://www.gutenberg.org/ebooks/8172
 | isbn=((978-1-153-62774-0))}}</ref> The motion of the Sun against the background stars (and the horizon) was used to create [[solar calendar|calendars]], which could be used to regulate agricultural practices.<ref>{{cite web |last=Tøndering |first=Claus |date=2008 |title=Other Ancient Calendars |url=http://webexhibits.org/calendars/calendar-ancient.html |access-date=28 February 2022 |website=Calendars Through The Ages |publisher=Webexhibits}}</ref> The [[Gregorian calendar]], currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun.

The oldest accurately dated [[star chart]] was the result of ancient [[Egyptian astronomy]] in 1534&nbsp;BC.<ref>{{cite journal
 | last=von Spaeth | first=Ove
 | title=Dating the Oldest Egyptian Star Map
 | journal=[[Centaurus (journal)|Centaurus]]
 | date=2000 | volume=42 | issue=3 | pages=159–179
 | url=http://www.moses-egypt.net/star-map/senmut1-mapdate_en.asp
 | access-date=2007-10-21 | doi=10.1034/j.1600-0498.2000.420301.x|bibcode= 2000Cent...42..159V}}</ref> The [[Babylonian star catalogues|earliest known star catalogues]] were compiled by the ancient [[Babylonian astronomy|Babylonian astronomers]] of [[Mesopotamia]] in the late 2nd millennium BC, during the [[Kassites|Kassite Period]] ({{Circa|1531 BC|1155 BC}}).<ref name="north 1995 30 31">{{cite book
 | last=North | first=John | date=1995
 | title=The Norton History of Astronomy and Cosmology
 | url=https://archive.org/details/nortonhistoryofa0000nort | url-access=registration | location=New York and London | pages=[https://archive.org/details/nortonhistoryofa0000nort/page/30 30–31]
 | publisher=W.W. Norton & Company | isbn=978-0-393-03656-5}}</ref>
[[File:Свет от деревни - panoramio.jpg|alt=Alternative text|thumb|Stars in the night sky]]
The first [[star catalogue]] in [[Ancient Greek astronomy|Greek astronomy]] was created by [[Aristillus]] in approximately 300&nbsp;BC, with the help of [[Timocharis]].<ref>{{cite book
 | last=Murdin | first=P. |date=2000
 | chapter=Aristillus (c. 200 BC)
 | doi=10.1888/0333750888/3440
 | title=Encyclopedia of Astronomy and Astrophysics
 | bibcode=2000eaa..bookE3440.
 | isbn=978-0-333-75088-9}}</ref> The star catalog of [[Hipparchus]] (2nd century BC) included 1,020 stars, and was used to assemble [[Ptolemy]]'s star catalogue.<ref>{{cite book
 | first=Gerd | last=Grasshoff | date=1990
 | title=The history of Ptolemy's star catalogue
 | publisher=Springer | pages=1–5 | isbn=978-0-387-97181-0}}</ref> Hipparchus is known for the discovery of the first recorded ''[[nova]]'' (new star).<ref>{{cite web |last=Pinotsis |first=Antonios D. |date=2008 |title=Astronomy in Ancient Rhodes |url=http://conferences.phys.uoa.gr/jets2008/historical.html |url-status=dead |archive-url=https://web.archive.org/web/20210907070144/http://conferences.phys.uoa.gr/jets2008/historical.html |archive-date=September 7, 2021 |access-date=February 28, 2022 |website=Protostellar Jets In Context |publisher=[[University of Athens]], Greece}}</ref> Many of the constellations and star names in use today derive from Greek astronomy.

Despite the apparent immutability of the heavens, [[Chinese astronomy|Chinese astronomers]] were aware that new stars could appear.<ref name="clark">{{cite conference |last1=Clark |first1=D. H. |last2=Stephenson |first2=F. R. |date=1981-06-29 |title=The Historical Supernovae |conference=Advanced Study Institute |location=Cambridge, UK |publisher=Dordrecht, D. Reidel Publishing Company |pages=355–370 |bibcode=1982ASIC...90..355C |book-title=Supernovae: A survey of current research; Proceedings of the Advanced Study Institute}}</ref> In 185&nbsp;AD, they were the first to observe and write about a [[supernova]], now known as [[SN 185]].<ref>{{cite journal |last1=Zhao |first1=Fu-Yuan |last2=Strom |first2=R. G. |last3=Jiang |first3=Shi-Yang |date=2006 |title=The Guest Star of AD185 must have been a Supernova |journal=Chinese Journal of Astronomy and Astrophysics |volume=6 |issue=5 |pages=635 |bibcode=2006ChJAA...6..635Z |doi=10.1088/1009-9271/6/5/17 |doi-access=free }}</ref> The brightest stellar event in recorded history was the [[SN 1006]] supernova, which was observed in 1006 and written about by the Egyptian astronomer [[Ali ibn Ridwan]] and several Chinese astronomers.<ref>{{cite web |last1=Isbell |first1=Douglas |last2=Benoit |first2=Phil |date=5 March 2003 |title=Astronomers Peg Brightness of History's Brightest Star |url=http://www.noao.edu/outreach/press/pr03/pr0304.html |url-status=dead |archive-url=https://web.archive.org/web/20030402121341/http://www.noao.edu/outreach/press/pr03/pr0304.html |archive-date=2 April 2003 |access-date=28 February 2022 |website=NOIRLab |publisher=National Optical Astronomy Observatory}}</ref> The [[SN 1054]] supernova, which gave birth to the [[Crab Nebula]], was also observed by Chinese and Islamic astronomers.<ref name="SN1054">{{cite web
 | last1=Frommert | first1=Hartmut | last2=Kronberg | first2=Christine
 | date=2006-08-30 | work=SEDS
 | publisher=University of Arizona
 | title=Supernova 1054 – Creation of the Crab Nebula
 | url=http://messier.seds.org/more/m001_sn.html
}}</ref><ref name="PASP1942">{{cite journal
 | last=Duyvendak | first=J. J. L.
 | title=Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054 A.D. Part I. The Ancient Oriental Chronicles
 | journal=Publications of the Astronomical Society of the Pacific | volume=54 | issue=318 | pages=91–94
 |date=April 1942 | bibcode=1942PASP...54...91D
 | doi=10.1086/125409| doi-access=free}}<br />
{{Cite journal
 | last1=Mayall | first1=N. U. | last2=Oort |first2=Jan Hendrik
 | title=Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054 A.D. Part II. The Astronomical Aspects
 | journal=Publications of the Astronomical Society of the Pacific | volume=54 | issue=318 | pages=95–104 |date=April 1942 | bibcode=1942PASP...54...95M
 | doi=10.1086/125410
| doi-access=free }}</ref><ref>{{cite journal | display-authors=1
 | last1=Brecher | first1=K. | last2=Fesen | first2=R. A.
 | last3=Maran | first3=S. P. | last4=Brandt | first4=J. C. | date=1983
 | title=Ancient records and the Crab Nebula supernova
 | journal=The Observatory | volume=103 | pages=106–113
 | bibcode=1983Obs...103..106B}}</ref>

[[Astronomy in the medieval Islamic world|Medieval Islamic astronomers]] gave [[list of Arabic star names|Arabic names to many stars]] that are still used today and they invented numerous [[astronomy in medieval Islam#Instruments|astronomical instruments]] that could compute the positions of the stars. They built the first large [[observatory]] research institutes, mainly to produce ''[[Zij]]'' star catalogues.<ref>{{cite journal
 |last=Kennedy |first=Edward S. |date=1962
 |title=Review: ''The Observatory in Islam and Its Place in the General History of the Observatory'' by Aydin Sayili
 |journal=[[Isis (journal)|Isis]] |volume=53
 |issue=2 |pages=237–239 |doi=10.1086/349558}}</ref> Among these, the ''[[Book of Fixed Stars]]'' (964) was written by the [[Persian people|Persian]] astronomer [[Abd al-Rahman al-Sufi]], who observed a number of stars, [[star cluster]]s (including the [[Omicron Velorum]] and [[Brocchi's Cluster]]s) and [[galaxy|galaxies]] (including the [[Andromeda Galaxy]]).<ref name=Jones>{{cite book
 | title=Messier's nebulae and star clusters
 | url=https://books.google.com/books?id=IuhLR35I9QUC
 | first=Kenneth Glyn | last=Jones
 | publisher=[[Cambridge University Press]]
 | date=1991 | isbn=978-0-521-37079-0 | page=1}}</ref> According to A. Zahoor, in the 11th century, the Persian [[polymath]] scholar [[Abu Rayhan Biruni]] described the [[Milky Way]] galaxy as a multitude of fragments having the properties of [[nebula|nebulous]] stars, and gave the [[latitude]]s of various stars during a [[lunar eclipse]] in 1019.<ref>{{cite web
 | last=Zahoor | first=A. | date=1997
 | url=http://www.unhas.ac.id/~rhiza/saintis/biruni.html
 | archive-url=https://web.archive.org/web/20080626074150/http://www.unhas.ac.id/~rhiza/saintis/biruni.html
 | archive-date=2008-06-26
 | title=Al-Biruni | publisher=Hasanuddin University
 | access-date=2007-10-21}}</ref>

According to Josep Puig, the [[Al-Andalus|Andalusian]] astronomer [[Ibn Bajjah]] proposed that the Milky Way was made up of many stars that almost touched one another and appeared to be a continuous image due to the effect of [[refraction]] from sublunary material, citing his observation of the [[conjunction (astronomy and astrology)|conjunction]] of Jupiter and Mars on 500 [[Islamic calendar|AH]] (1106/1107&nbsp;AD) as evidence.<ref name=Montada>{{cite encyclopedia
 | first=Josep Puig | last=Montada
 | title=Ibn Bajja | encyclopedia=[[Stanford Encyclopedia of Philosophy]]
 | url= http://plato.stanford.edu/entries/ibn-bajja
 | date=2007-09-28 | access-date=2008-07-11}}</ref> <!--
[[File:Andromedaurania.jpg|thumb|[[Andromeda (constellation)|Andromeda]] as depicted in ''Urania's Mirror'', set of [[constellation]] cards published in London c. 1825]] -->
Early European astronomers such as [[Tycho Brahe]] identified new stars in the [[night sky]] (later termed ''novae''), suggesting that the heavens were not immutable. In 1584, [[Giordano Bruno]] suggested that the stars were like the Sun, and may have [[extrasolar planet|other planets]], possibly even Earth-like, in orbit around them,<ref name="he history">{{cite web
 | last=Drake | first=Stephen A. | date=2006-08-17
 | url=http://heasarc.gsfc.nasa.gov/docs/heasarc/headates/heahistory.html
 | title=A Brief History of High-Energy (X-ray & Gamma-Ray) Astronomy
 | publisher=NASA HEASARC | access-date=2006-08-24
}}</ref> an idea that had been suggested earlier by the ancient [[Greek philosophy|Greek philosophers]], [[Democritus]] and [[Epicurus]],<ref>{{cite web | first1=Peter | last1=Greskovic | first2=Peter | last2=Rudy | date=2006-07-24 | url=http://www.eso.org/public/outreach/eduoff/cas/cas2004/casreports-2004/rep-228/ | title=Exoplanets | publisher=ESO | access-date=2012-06-15 | archive-date=10 October 2008 | archive-url=https://web.archive.org/web/20081010140635/http://www.eso.org/public/outreach/eduoff/cas/cas2004/casreports-2004/rep-228/ | url-status=dead }}</ref> and by medieval [[Cosmology in medieval Islam|Islamic cosmologists]]<ref>{{cite journal
 | title=The impact of the Qur'anic conception of astronomical phenomena on Islamic civilization
 | first=I. A. | last=Ahmad | journal=Vistas in Astronomy
 | volume=39 | issue=4 | date=1995
 | pages=395–403 [402]
 | doi=10.1016/0083-6656(95)00033-X |bibcode= 1995VA.....39..395A}}</ref> such as [[Fakhr al-Din al-Razi]].<ref name="Setia">{{cite journal|title= Fakhr Al-Din Al-Razi on Physics and the Nature of the Physical World: A Preliminary Survey|first= Adi|last= Setia|journal= Islam & Science|volume= 2|date= 2004|issue= 2|url= http://www.cis-ca.org/jol/vol2-no2/adi.pdf|access-date= 26 May 2018|archive-date= 9 January 2020|archive-url= https://web.archive.org/web/20200109010911/http://www.cis-ca.org/jol/vol2-no2/adi.pdf|url-status= dead}}</ref> By the following century, the idea of the stars being the same as the Sun was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the Solar System, [[Isaac Newton]] suggested that the stars were equally distributed in every direction, an idea prompted by the theologian [[Richard Bentley]].<ref>{{cite journal
 | last=Hoskin | first=Michael | date=1998
 | url=http://www.stsci.edu/stsci/meetings/lisa3/hoskinm.html
 | title=The Value of Archives in Writing the History of Astronomy
 | journal=Library and Information Services in Astronomy III | volume=153 | pages=207 | access-date=2006-08-24| bibcode=1998ASPC..153..207H }}</ref>

The Italian astronomer [[Geminiano Montanari]] recorded observing variations in luminosity of the star [[Algol]] in 1667. [[Edmond Halley]] published the first measurements of the [[proper motion]] of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient [[Ancient Greece|Greek]] astronomers Ptolemy and Hipparchus.<ref name="he history" />

[[William Herschel]] was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he established a series of gauges in 600 directions and counted the stars observed along each line of sight. From this, he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way [[Galactic Center|core]]. His son [[John Herschel]] repeated this study in the southern hemisphere and found a corresponding increase in the same direction.<ref>{{cite journal
 | last=Proctor | first=Richard A.
 | title=Are any of the nebulæ star-systems? | journal=Nature
 | date=1870 | pages=331–333
 | url=http://digicoll.library.wisc.edu/cgi-bin/HistSciTech/HistSciTech-idx?type=div&did=HISTSCITECH.0012.0052.0005&isize=M
 | issue=13
 | doi=10.1038/001331a0 | volume=1 |bibcode= 1870Natur...1..331P| doi-access=free}}</ref> In addition to his other accomplishments, William Herschel is noted for his discovery that some stars do not merely lie along the same line of sight, but are physical companions that form binary star systems.<ref name="Magill1992">{{cite book |author=Magill |first=Frank Northen |url=https://books.google.com/books?id=W33WAAAAMAAJ |title=Magill's Survey of Science: A-Cherenkov detectors |publisher=Salem Press |year=1992 |isbn=978-0-89356-619-7 |page=219}}</ref>

The science of [[astronomical spectroscopy|stellar spectroscopy]] was pioneered by [[Joseph von Fraunhofer]] and [[Angelo Secchi]]. By comparing the spectra of stars such as [[Sirius]] to the Sun, they found differences in the strength and number of their [[spectral line|absorption lines]]—the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies. In 1865, Secchi began classifying stars into [[stellar classification|spectral types]].<ref>{{cite web
 | last=MacDonnell | first=Joseph
 | url=http://www.faculty.fairfield.edu/jmac/sj/scientists/secchi.htm
 | archive-url=https://web.archive.org/web/20110721210124/http://www.faculty.fairfield.edu/jmac/sj/scientists/secchi.htm
 | archive-date=2011-07-21
 | title=Angelo Secchi, S.J. (1818–1878) the Father of Astrophysics
 | publisher=[[Fairfield University]]
 | access-date=2006-10-02}}</ref> The modern version of the stellar classification scheme was developed by [[Annie Jump Cannon|Annie J. Cannon]] during the early 1900s.<ref name="HubenyMihalas2014">{{cite book |last1=Hubeny |first1=Ivan |url=https://books.google.com/books?id=TmuYDwAAQBAJ&pg=PA23 |title=Theory of Stellar Atmospheres: An Introduction to Astrophysical Non-equilibrium Quantitative Spectroscopic Analysis |last2=Mihalas |first2=Dimitri |date=2014 |publisher=Princeton University Press |isbn=978-0-691-16329-1 |page=23}}</ref>

The first direct measurement of the distance to a star ([[61 Cygni]] at 11.4 [[light-years]]) was made in 1838 by [[Friedrich Bessel]] using the [[parallax]] technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.<ref name="he history" /> Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion. [[Edward Charles Pickering|Edward Pickering]] discovered the first [[spectroscopic binary]] in 1899 when he observed the periodic splitting of the spectral lines of the star [[Mizar (star)|Mizar]] in a 104-day period. Detailed observations of many binary star systems were collected by astronomers such as [[Friedrich Georg Wilhelm von Struve]] and [[Sherburne Wesley Burnham|S. W. Burnham]], allowing the masses of stars to be determined from computation of [[orbital elements]]. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.<ref>{{cite book
 | first=Robert G. | last=Aitken | title=The Binary Stars | page=66
 | publisher=Dover Publications Inc. | location=New York
 | date=1964 | isbn=((978-0-486-61102-0))}}</ref>

The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. [[Karl Schwarzschild]] discovered that the color of a star and, hence, its temperature, could be determined by comparing the [[visual magnitude]] against the [[photographic magnitude]]. The development of the [[Photoelectric effect|photoelectric]] [[photometer]] allowed precise measurements of magnitude at multiple wavelength intervals. In 1921 [[Albert A. Michelson]] made the first measurements of a stellar diameter using an [[interferometer]] on the [[100-inch Hooker telescope|Hooker telescope]] at [[Mount Wilson Observatory]].<ref>{{cite journal
 | last1=Michelson | first1=A. A. | last2=Pease | first2=F. G.
 | title=Measurement of the diameter of Alpha Orionis with the interferometer
 | journal=Astrophysical Journal | date=1921 | volume=53 | issue=5 | pages=249–259
 | bibcode=1921ApJ....53..249M | doi= 10.1086/142603 | pmid=16586823 | pmc=1084808 | s2cid=21969744 }}</ref>

Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. In 1913, the [[Hertzsprung-Russell diagram]] was developed, propelling the astrophysical study of stars. Successful [[Stellar model|models]] were developed to explain the interiors of stars and stellar evolution. [[Cecilia Payne-Gaposchkin]] first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.<ref>{{cite web
 |url        = http://cwp.library.ucla.edu/Phase2/Payne-Gaposchkin,_Cecilia_Helena@861234567.html
 |archive-url = https://web.archive.org/web/20050318221903/http://cwp.library.ucla.edu/Phase2/Payne-Gaposchkin,_Cecilia_Helena@861234567.html
 |url-status=dead
 |archive-date = 2005-03-18
 |title      = " Payne-Gaposchkin, Cecilia Helena." CWP
 |publisher  = [[University of California]]
 |access-date = 2013-02-21
}}</ref> The spectra of stars were further understood through advances in [[quantum mechanics|quantum physics]]. This allowed the chemical composition of the stellar atmosphere to be determined.<ref name="new cosmos">{{cite book
 | last1=Unsöld | first1=Albrecht | title=The New Cosmos
 | publisher=Springer | location=New York
 | date=2001 | edition=5th | pages=180–185, 215–216
 | isbn=978-3-540-67877-9}}</ref>

[[File:Milky Way IR Spitzer.jpg|thumb|[[Spitzer Space Telescope]] infrared image showing a multitude of stars in the [[Milky Way]] galaxy]]
With the exception of rare events such as supernovae and [[supernova impostor]]s, individual stars have primarily been observed in the [[Local Group]],<ref name=gordon2016>{{Cite journal|last1=Gordon|first1=Michael S.|last2=Humphreys|first2=Roberta M.|last3=Jones|first3=Terry J.|date=July 2016|title=Luminous and Variable Stars in M31 and M33. III. The Yellow and Red Supergiants and Post-red Supergiant Evolution|journal=The Astrophysical Journal|language=en|volume=825|issue=1|pages=50|doi=10.3847/0004-637X/825/1/50|arxiv=1603.08003|bibcode=2016ApJ...825...50G|s2cid=119281102|issn=0004-637X |doi-access=free }}</ref> and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for the Milky Way galaxy) and its satellites.<ref>{{cite Gaia EDR3}}</ref> Individual stars such as Cepheid variables have been observed in the [[Messier 87|M87]]<ref>{{cite journal |bibcode=2020ApJS..246....3D |title=Clustering of Local Group Distances: Publication Bias or Correlated Measurements? VI. Extending to Virgo Cluster Distances |last1=De Grijs |first1=Richard |last2=Bono |first2=Giuseppe |journal=The Astrophysical Journal Supplement Series |year=2020 |volume=246 |issue=1 |page=3 |doi=10.3847/1538-4365/ab5711 |arxiv=1911.04312 |s2cid=207852888 |doi-access=free }}
</ref> and [[Messier 100|M100]] galaxies of the [[Virgo Cluster]],<ref>{{cite web
 | last1=Villard | first1=Ray | last2=Freedman | first2=Wendy L.
 | date=1994-10-26
 | url=http://hubblesite.org/newscenter/archive/releases/1994/1994/49/text/
 | title=Hubble Space Telescope Measures Precise Distance to the Most Remote Galaxy Yet
 | publisher=Hubble Site
 | access-date= 2007-08-05}}</ref> as well as luminous stars in some other relatively nearby galaxies.<ref>{{cite journal |bibcode=2020MNRAS.497.4834S |title=New luminous blue variable candidates in the NGC 247 galaxy |last1=Solovyeva |first1=Y. |last2=Vinokurov |first2=A. |last3=Sarkisyan |first3=A. |last4=Atapin |first4=K. |last5=Fabrika |first5=S. |last6=Valeev |first6=A. F. |last7=Kniazev |first7=A. |last8=Sholukhova |first8=O. |last9=Maslennikova |first9=O. |journal=Monthly Notices of the Royal Astronomical Society |year=2020 |volume=497 |issue=4 |page=4834 |doi=10.1093/mnras/staa2117 |doi-access=free |arxiv=2008.06215 |s2cid=221451751 }}
</ref> With the aid of [[gravitational lensing]], a single star (named [[Icarus (star)|Icarus]]) has been observed at 9&nbsp;billion light-years away.<ref name="NA-20180402">{{cite journal |author=Kelly, Patrick L. |display-authors=etal |title=Extreme magnification of an individual star at redshift 1.5 by a galaxy-cluster lens |date=2018-04-02 |journal=[[Nature (journal)|Nature]] |volume=2 |issue=4 |pages=334–342 |doi=10.1038/s41550-018-0430-3 |arxiv=1706.10279 |bibcode=2018NatAs...2..334K |s2cid=125826925 }}</ref><ref name="SPC-20180402">{{cite web |last=Howell |first=Elizabeth |title=Rare Cosmic Alignment Reveals Most Distant Star Ever Seen |url=https://www.space.com/40171-cosmic-alignment-reveals-most-distant-star-yet.html |date=2018-04-02|work=[[Space.com]] |access-date=2018-04-02 }}</ref>

==Designations==
{{Main|Stellar designation|Astronomical naming conventions|Star catalogue}}
The concept of a constellation was known to exist during the [[Babylon]]ian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the [[ecliptic]] and these became the basis of [[astrology]].<ref name=koch95/> Many of the more prominent individual stars were given names, particularly with [[Arabic]] or [[Latin language|Latin]] designations.

As well as certain constellations and the Sun itself, individual stars have their own [[mythology|myths]].<ref name="mythology">{{cite web
 | last= Coleman | first= Leslie S
 | url= http://frostydrew.org/papers.dc/papers/paper-myths/
 | title= Myths, Legends and Lore
 | publisher= Frosty Drew Observatory
 | access-date= 2012-06-15}}</ref> To the [[Ancient Greek religion|Ancient Greeks]], some "stars", known as [[planet]]s (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets [[Mercury (planet)|Mercury]], [[Venus]], [[Mars]], [[Jupiter]] and [[Saturn]] were taken.<ref name="mythology" /> ([[Uranus]] and [[Neptune]] were [[Greek mythology|Greek]] and [[Roman mythology|Roman gods]], but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)

Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer [[Johann Bayer]] created a series of star maps and applied Greek letters as [[Bayer designation|designations]] to the stars in each constellation. Later a numbering system based on the star's [[right ascension]] was invented and added to [[John Flamsteed]]'s star catalogue in his book ''"Historia coelestis Britannica"'' (the 1712 edition), whereby this numbering system came to be called ''[[Flamsteed designation]]'' or ''Flamsteed numbering''.<ref>{{cite web
 | url= http://www.iau.org/public/naming/ | title= Naming Astronomical Objects
 | publisher= [[International Astronomical Union]] (IAU)
 | access-date= 2009-01-30}}</ref><ref>{{cite web
 | url= http://spider.seds.org/spider/Misc/naming.html | title= Naming Stars
 | publisher= [[Students for the Exploration and Development of Space]] (SEDS)
 | access-date= 2009-01-30}}</ref>

The internationally recognized authority for naming celestial bodies is the [[International Astronomical Union]] (IAU).<ref name=space_law09/> The International Astronomical Union maintains the [[IAU Working Group on Star Names|Working Group on Star Names]] (WGSN)<ref name="WGSN">{{cite web | url=https://www.iau.org/science/scientific_bodies/working_groups/280/ | title=IAU Working Group on Star Names (WGSN)|access-date=2016-05-22}}</ref> which catalogs and standardizes proper names for stars.<ref>{{cite web |url=https://www.iau.org/public/themes/naming_stars/ |title=Naming Stars |access-date=2021-02-05}}</ref> A number of private companies sell names of stars which are not recognized by the IAU, professional astronomers, or the amateur astronomy community.<ref name=andersen10/> The [[British Library]] calls this an [[regulation|unregulated]] [[commercial enterprise]],<ref name=astrometry05/><ref name=bl_disclaimer/> and the [[New York City Department of Consumer and Worker Protection]] issued a violation against one such star-naming company for engaging in a deceptive trade practice.<ref name=pliat02/><ref name=sclafani19980508/>

== Units of measurement ==

Although stellar parameters can be expressed in [[International System of Units|SI units]] or [[Gaussian units]], it is often most convenient to express [[mass]], [[luminosity]], and [[radius|radii]] in solar units, based on the characteristics of the Sun. In 2015, the IAU defined a set of ''nominal'' solar values (defined as SI constants, without uncertainties) which can be used for quoting stellar parameters:

:{|
| [[solar luminosity|nominal solar luminosity]]
| ''{{solar luminosity}}'' = {{val|3.828|e=26|u=W}}<ref name="Prsa16">
  {{cite journal
   | last1=Prsa | first1=A. | last2=Harmanec | first2=P. | last3=Torres | first3=G. | last4=Mamajek | first4=E.
   | display-authors=etal
   | title=Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3| journal=Astronomical Journal
   | date=2016 | volume=152 | issue=2 | pages=41 | bibcode=2016AJ....152...41P
   | doi=10.3847/0004-6256/152/2/41 |arxiv=1605.09788| s2cid=55319250
  | doi-access=free }}</ref>
|-
| [[solar radius|nominal solar radius]]
| ''{{Solar radius}}'' = {{val|6.957|e=8|u=m}}<ref name="Prsa16"/>
|}

The [[solar mass]] {{solar mass}} was not explicitly defined by the IAU due to the large relative uncertainty ({{val|e=−4}}) of the [[Newtonian constant of gravitation]] ''G''. Since the product of the Newtonian constant of gravitation and solar mass
together (''G''{{solar mass}}) has been determined to much greater precision, the IAU defined the ''nominal'' solar mass parameter to be:

:{|
| nominal solar mass parameter:
| ''G{{solar mass}}'' = {{val|1.3271244|e=20|u=m3}}/s<sup>2</sup><ref name="Prsa16"/>
|}

The nominal solar mass parameter can be combined with the most recent (2014) CODATA estimate of the Newtonian constant of gravitation ''G'' to derive the solar mass to be approximately {{val|1.9885|e=30|u=kg}}. Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the 2015 IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.

Large lengths, such as the radius of a giant star or the [[Semi-major and semi-minor axes|semi-major axis]] of a binary star system, are often expressed in terms of the [[astronomical unit]]—approximately equal to the mean distance between the Earth and the Sun (150&nbsp;million km or approximately 93&nbsp;million miles). In 2012, the IAU defined the [[astronomical constant]] to be an exact length in meters: 149,597,870,700 m.<ref name="Prsa16"/>

==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|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 collapse]]
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|url-access=subscription }}</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>

==Distribution==
[[File:Sirius A and B artwork.jpg|left|thumb|Artist's impression of the [[Sirius]] system, a [[white dwarf]] star in orbit around an [[A-type main-sequence star]]]]

Stars are not spread uniformly across the universe but are normally grouped into galaxies along with interstellar gas and dust. A typical large galaxy like the Milky Way contains hundreds of billions of stars. There are more than 2&nbsp;trillion ({{val|e=12}}) galaxies, though most are less than 10% the mass of the Milky Way.<ref name="NYT-20161017">{{cite news |last=Fountain |first=Henry |title=Two Trillion Galaxies, at the Very Least |url=https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |date=2016-10-17 |work=[[The New York Times]] |access-date=2016-10-17 }}</ref> Overall, there are likely to be between {{val|e=22}} and {{val|e=24}} stars,<ref name="ESA-2019">{{cite web |author=Staff |title=How Many Stars Are There In The Universe? |url=https://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |date=2019 |work=[[European Space Agency]] |access-date=2019-09-21 }}</ref><ref name="ReferenceA">{{Cite book|chapter=The Structure of the Universe|doi=10.1007/978-1-4614-8730-2_10|title=The Fundamentals of Modern Astrophysics|pages=279–294|year=2015|last1=Marov|first1=Mikhail Ya.|isbn=978-1-4614-8729-6}}</ref> which are more stars than all the [[Sand|grains of sand]] on planet Earth.<ref name="SU-20020201">{{cite web |last=Mackie |first=Glen |title=To see the Universe in a Grain of Taranaki Sand |url=http://astronomy.swin.edu.au/~gmackie/billions.html |date=2002-02-01 |work=[[Centre for Astrophysics and Supercomputing]] |access-date=2017-01-28 }}</ref><ref name=Borenstein2010>{{cite news
 | first=Seth | last=Borenstein | date=2010-12-01
 | title=Universe's Star Count Could Triple | agency=Associated Press
 | url=https://www.nbcnews.com/id/wbna40454979 | access-date=2021-02-09}}</ref><ref>{{cite journal |last1=Van Dokkum |first1=Pieter G. |last2=Conroy |first2=Charlie |year=2010 |title=A substantial population of low-mass stars in luminous elliptical galaxies |journal=Nature |volume=468 |issue=7326 |pages=940–942 |arxiv=1009.5992 |bibcode=2010Natur.468..940V |doi=10.1038/nature09578 |pmid=21124316 |s2cid=205222998}}</ref> Most stars are within galaxies, but between 10 and 50% of the starlight in large [[galaxy cluster]]s may come from stars outside of any galaxy.<ref>{{cite news
 | title=Hubble Finds Intergalactic Stars
 | publisher=Hubble News Desk | date=1997-01-14
 | url=http://hubblesite.org/newscenter/archive/releases/1997/02/text/
 | access-date=2006-11-06}}</ref><ref>{{cite journal |last1=Puchwein |first1=Ewald |last2=Springel |first2=Volker |last3=Sijacki |first3=Debora|author3-link= Debora Šijački |last4=Dolag |first4=Klaus |title=Intracluster stars in simulations with active galactic nucleus feedback |journal=Monthly Notices of the Royal Astronomical Society |date=2010-08-01 |volume=406 |issue=2 |pages=936–951 |doi=10.1111/j.1365-2966.2010.16786.x|arxiv=1001.3018 |bibcode=2010MNRAS.406..936P |doi-access=free }}</ref><ref>{{cite journal |last1=Lin |first1=Yen-Ting |last2=Mohr |first2=Joseph J. |title=K-band Properties of Galaxy Clusters and Groups: Brightest Cluster Galaxies and Intracluster Light |journal=The Astrophysical Journal |date=2004-12-20 |volume=617 |issue=2 |pages=879–895 |doi=10.1086/425412 |bibcode=2004ApJ...617..879L|arxiv=astro-ph/0408557 |s2cid=119347770 }}</ref>

A multi-star system consists of two or more gravitationally bound stars that [[stellar dynamics|orbit each other]]. The simplest and most common multi-star system is a binary star, but systems of three or more stars exist. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.<ref>{{cite book
 | first1=Victor G. | last1=Szebehely
 | last2=Curran | first2=Richard B. | date=1985
 | title=Stability of the Solar System and Its Minor Natural and Artificial Bodies
 | publisher=Springer
 | isbn=978-90-277-2046-7}}</ref> Larger groups are called star clusters. These range from loose [[Stellar kinematics|stellar associations]] with only a few stars to [[open cluster]]s with dozens to thousands of stars, up to enormous [[globular cluster]]s with hundreds of thousands of stars. Such systems orbit their host galaxy. The stars in an open or globular cluster all formed from the same [[giant molecular cloud]], so all members normally have similar ages and compositions.<ref name='carroll_ostlie_ch13' />

Many stars are observed, and most or all may have originally formed in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, 80% of which are believed to be part of multiple-star systems. The proportion of single star systems increases with decreasing star mass, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, more than two thirds of stars in the Milky Way are likely single red dwarfs.<ref>{{cite press release
 | publisher=Harvard-Smithsonian Center for Astrophysics
 | date=2006-01-30
 | url=http://www.cfa.harvard.edu/news/2006/pr200611.html
 | title=Most Milky Way Stars Are Single
 | access-date=2006-07-16}}</ref> In a 2017 study of the [[Perseus molecular cloud]], astronomers found that most of the newly formed stars are in binary systems. In the model that best explained the data, all stars initially formed as binaries, though some binaries later split up and leave single stars behind.<ref>{{cite news |last1=Sanders |first1=Robert |title=New evidence that all stars are born in pairs |url=https://news.berkeley.edu/2017/06/13/new-evidence-that-all-stars-are-born-in-pairs/ |work=Berkeley News |date=2017-06-13}}</ref><ref>{{cite journal |last1=Sadavoy |first1=Sarah I. |last2=Stahler |first2=Steven W. |title=Embedded binaries and their dense cores |journal=Monthly Notices of the Royal Astronomical Society |date=August 2017 |volume=469 |issue=4 |pages=3881–3900 |doi=10.1093/mnras/stx1061 |doi-access=free | arxiv=1705.00049 | bibcode=2017MNRAS.469.3881S}}</ref>

[[File:Ngc6397 hst blue straggler.jpg|right|thumb|This view of [[NGC 6397]] includes stars known as [[blue straggler]]s for their location on the [[Hertzsprung–Russell diagram]].]]
The nearest star to the Earth, apart from the Sun, is [[Proxima Centauri]], {{convert|4.2465|ly|e12km|abbr=none}} away. Travelling at the orbital speed of the [[Space Shuttle]], {{convert|8|km/s|km/h | abbr=off}}, it would take about 150,000 years to arrive.<ref>{{val|3.99|e=13|u=km}} / ({{val|3|e=4|u=km/h}} × 24 × 365.25) = {{val|1.5|e=5|u=years}}.</ref> This is typical of stellar separations in [[disc (galaxy)|galactic discs]].<ref>{{cite journal |last1=Holmberg |first1=J. |last2=Flynn |first2=C. |title=The local density of matter mapped by Hipparcos |journal=Monthly Notices of the Royal Astronomical Society |volume=313 |issue=2 |date=2000 |pages=209–216 |bibcode=2000MNRAS.313..209H |doi= 10.1046/j.1365-8711.2000.02905.x |doi-access=free |arxiv= astro-ph/9812404|s2cid=16868380 }}</ref> Stars can be much closer to each other in the centres of galaxies<ref>{{Cite web|title=How close can stars get to each other in galaxy cores?|work=Astronomy.com|date=2006-01-01|first=David|last=Norby|accessdate=2022-09-11| url = https://astronomy.com/magazine/ask-astro/2006/01/how-close-can-stars-get-to-each-other-in-galaxy-cores}}</ref> and in globular clusters,<ref>{{Cite journal| issn = 1432-0754| volume = 27| issue = 1| last1 = Gratton| first1 = Raffaele| last2 = Bragaglia| first2 = Angela| last3 = Carretta| first3 = Eugenio| last4 = D'Orazi| first4 = Valentina| last5 = Lucatello| first5 = Sara| last6 = Sollima| first6 = Antonio| title = What is a globular cluster? An observational perspective| journal = The Astronomy and Astrophysics Review| date = 2019-05-15| page = 8|access-date=2022-09-11|url=https://link.springer.com/article/10.1007/s00159-019-0119-3| doi = 10.1007/s00159-019-0119-3| arxiv = 1911.02835| bibcode = 2019A&ARv..27....8G| s2cid = 207847491}}</ref> or much farther apart in [[galactic spheroid|galactic halos]].<ref>{{Cite web |title=Imagine the Universe! |url=https://imagine.gsfc.nasa.gov/features/cosmic/nearest_star_info.html |access-date=2023-02-08 |website=imagine.gsfc.nasa.gov}}</ref>

Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.<ref name= "DarkMatter">{{cite news |title= Astronomers: Star collisions are rampant, catastrophic |publisher= CNN News |date=2000-06-02 |url= http://archives.cnn.com/2000/TECH/space/06/02/stellar.collisions/ |archive-url= https://web.archive.org/web/20070107140146/http://archives.cnn.com/2000/TECH/space/06/02/stellar.collisions/ |archive-date= 2007-01-07 |access-date= 2014-01-21}}</ref> Such collisions can produce what are known as [[blue straggler]]s. These abnormal stars have higher surface temperatures and thus are bluer than stars at the [[main sequence turnoff]] in the cluster to which they belong; in standard stellar evolution, blue stragglers would already have evolved off the main sequence and thus would not be seen in the cluster.<ref>{{cite journal |display-authors= 1 |first1= J. C. Jr. |last1= Lombardi |last2= Warren |first2= J. S. |last3= Rasio |first3= F. A. |last4= Sills |first4= A. |last5= Warren |first5= A. R. |title= Stellar Collisions and the Interior Structure of Blue Stragglers |journal= The Astrophysical Journal |date= 2002 |volume= 568 |issue= 2 |pages= 939–953 |bibcode= 2002ApJ...568..939L |doi= 10.1086/339060|arxiv= astro-ph/0107388|s2cid= 13878176 }}</ref>

==Characteristics==
Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.

===Age===
{{main|Stellar age estimation}}

Most stars are between 1&nbsp;billion and 10&nbsp;billion years old. Some stars may even be close to 13.8&nbsp;billion years old—the observed [[age of the universe]]. The oldest star yet discovered, [[HD 140283]], nicknamed Methuselah star, is an estimated 14.46 ± 0.8&nbsp;billion years old.<ref name=Bond-140283 /> (Due to the uncertainty in the value, this age for the star does not conflict with the age of the universe, determined by the [[Planck (spacecraft)|Planck satellite]] as 13.799 ± 0.021).<ref name=Bond-140283 /><ref name="Planck 2015">{{cite journal
 |author=Planck Collaboration
 |year=2016
 |title=Planck 2015 results. XIII. Cosmological parameters (See Table 4 on page 31 of pfd).
 |arxiv=1502.01589
|bibcode = 2016A&A...594A..13P
 |doi=10.1051/0004-6361/201525830
 |volume=594
 |journal=Astronomy & Astrophysics
 |pages=A13|s2cid=119262962
 }}</ref>

The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.<ref>{{cite magazine
 | last1=Naftilan | first1=S. A. | last2=Stetson | first2=P. B.
 | date=2006-07-13
 | url=https://www.scientificamerican.com/article/how-do-scientists-determi/
 | title=How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe?
 | magazine=Scientific American
 | access-date=2007-05-11}}</ref><ref>{{cite journal |last1=Laughlin |first1=G. |last2=Bodenheimer |first2=P. |last3=Adams |first3=F. C. |date=1997 |title=The End of the Main Sequence |journal=The Astrophysical Journal |volume=482 |issue=1 |pages=420–432 |bibcode=1997ApJ...482..420L |doi=10.1086/304125 |doi-access=free}}</ref>

{| class="wikitable" style="text-align: center;"
|+ Lifetimes of stages of stellar evolution in billions of years<ref name=pols>{{cite journal|bibcode=1998MNRAS.298..525P|title=Stellar evolution models for Z = 0.0001 to 0.03|journal=Monthly Notices of the Royal Astronomical Society|volume=298|issue=2|pages=525|last1=Pols|first1=Onno R.|last2=Schröder|first2=Klaus-Peter|last3=Hurley|first3=Jarrod R.|last4=Tout|first4=Christopher A.|last5=Eggleton|first5=Peter P.|year=1998|doi=10.1046/j.1365-8711.1998.01658.x|doi-access=free}}</ref>
|-
! Initial Mass ({{solar mass|link=yes}})
!width="20%"| Main Sequence
!width="20%"| Subgiant
!width="20%"| First Red Giant
!width="20%"| Core He Burning
|-
| 1.0 || 9.33 || 2.57 || 0.76 || 0.13
|-
| 1.6 || 2.28 || 0.03 || 0.12 || 0.13
|-
| 2.0 || 1.20 || 0.01 || 0.02 || 0.28
|-
| 5.0 || 0.10 || 0.0004 || 0.0003 || 0.02
|}

===Chemical composition===
{{See also|Metallicity|Molecules in stars}}
When stars form in the present Milky Way galaxy, they are composed of about 71% hydrogen and 27% helium,<ref>{{cite book
 | first=Judith A. | last=Irwin | date=2007
 | title=Astrophysics: Decoding the Cosmos
 | publisher=John Wiley and Sons | isbn=978-0-470-01306-9
 | page=78| bibcode=2007adc..book.....I }}</ref> as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system.<ref>{{cite journal |last1=Fischer |first1=D. A. |last2=Valenti |first2=J. |date=2005 |title=The Planet-Metallicity Correlation |journal=The Astrophysical Journal |volume=622 |issue=2 |pages=1102–1117 |bibcode=2005ApJ...622.1102F |doi=10.1086/428383 |doi-access=free}}</ref>

{{as of|2005}} the star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.<ref>{{cite web
 | date=April 17, 2005 | url=https://www.sciencedaily.com/releases/2005/04/050417162354.htm | title=Signatures Of The First Stars
 | website=ScienceDaily | access-date=2006-10-10}}</ref> By contrast, the super-metal-rich star [[Mu Leonis|μ Leonis]] has nearly double the abundance of iron as the Sun, while the planet-bearing star [[14 Herculis]] has nearly triple the iron.<ref>{{cite journal
 | last1=Feltzing | first1=S. |author-link= Sofia Feltzing | last2=Gonzalez | first2=G.
 | title=The nature of super-metal-rich stars: Detailed abundance analysis of 8 super-metal-rich star candidates
 | journal=Astronomy & Astrophysics
 | date=2000 | volume=367 | issue=1 | pages=253–265
 | bibcode=2001A&A...367..253F
 | doi=10.1051/0004-6361:20000477| s2cid=16502974 | url=http://lup.lub.lu.se/search/ws/files/4419103/624132.pdf | doi-access=free }}</ref> Chemically [[peculiar star]]s show unusual abundances of certain elements in their spectrum; especially [[chromium]] and [[rare earth element]]s.<ref>{{cite book | first=David F. | last=Gray | date=1992 | title=The Observation and Analysis of Stellar Photospheres | pages=[https://archive.org/details/observationanaly0000gray/page/413 413–414] | publisher=Cambridge University Press | isbn=978-0-521-40868-4 | url=https://archive.org/details/observationanaly0000gray/page/413 }}</ref> Stars with cooler outer atmospheres, including the Sun, can form various diatomic and polyatomic molecules.<ref>{{cite book
 | first1=Uffe G. | last1=Jørgensen
 | title=Molecules in Astrophysics: Probes and Processes
 | chapter=Cool Star Models | volume=178
 | series=International Astronomical Union Symposia. Molecules in Astrophysics: Probes and Processes
 | editor-first=Ewine F. | editor-last=van Dishoeck
 | publisher=Springer Science & Business Media
 | year=1997 | isbn=978-0792345381 | page=446
 | chapter-url=https://books.google.com/books?id=VW50otz5v8sC&pg=PA446
}}</ref>
[[File:Star Size Comparison 2024 Update.jpg|thumb|Size comparison of some well-known [[supergiant]] and [[hypergiant]] stars, featuring [[Cygnus OB2-12]], [[V382 Carinae]], [[Betelgeuse]], [[VV Cephei]], and [[VY Canis Majoris]]]]

===Diameter===
{{Main|List of largest stars|List of smallest stars|Solar radius}}
Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that [[Twinkling|twinkle]] because of the effect of the Earth's atmosphere. The Sun is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is [[R Doradus]], with an [[angular diameter]] of only 0.057 [[arcsecond]]s.<ref>{{cite news
 | title=The Biggest Star in the Sky | publisher=ESO
 | date=1997-03-11 | url=http://www.eso.org/public/news/eso9706/
 | access-date=2006-07-10}}</ref>

The disks of most stars are much too small in [[Angular diameter|angular size]] to be observed with current ground-based optical telescopes, so [[Interferometry|interferometer]] telescopes are required to produce images of these objects. Another technique for measuring the angular size of stars is through [[occultation]]. By precisely measuring the drop in brightness of a star as it is occulted by the [[Moon]] (or the rise in brightness when it reappears), the star's angular diameter can be computed.<ref>{{cite journal
 | last1=Ragland | first1=S. | last2=Chandrasekhar | first2=T.
 | last3=Ashok | first3=N. M.
 | title=Angular Diameter of Carbon Star Tx-Piscium from Lunar Occultation Observations in the Near Infrared
 | journal=Journal of Astrophysics and Astronomy
 | date=1995 | volume=16 | page=332
 | bibcode=1995JApAS..16..332R}}</ref>

Stars range in size from neutron stars, which vary anywhere from 20 to {{convert|40|km|0|abbr=on}} in diameter, to [[supergiant]]s like [[Betelgeuse]] in the [[Orion constellation]], which has a diameter about 640 times that of the Sun<ref>{{Cite journal |last1=Mittag |first1=M. |last2=Schröder |first2=K.-P. |last3=Perdelwitz |first3=V. |last4=Jack |first4=D. |last5=Schmitt |first5=J. H. M. M. |date=January 2023 |title=Chromospheric activity and photospheric variation of &alpha; Ori during the great dimming event in 2020 |journal=Astronomy & Astrophysics |volume=669 |pages=A9 |doi=10.1051/0004-6361/202244924 |issn=0004-6361|arxiv=2211.04967 |bibcode=2023A&A...669A...9M }}</ref> with a much lower [[density]].<ref>{{cite web
 | last=Davis | first=Kate | date=2000-12-01
 | url=http://www.aavso.org/vstar/vsots/1200.shtml
 | title=Variable Star of the Month – December, 2000: Alpha Orionis
 | publisher=AAVSO | access-date=2006-08-13 | archive-url=https://web.archive.org/web/20060712000904/http://www.aavso.org/vstar/vsots/1200.shtml <!-- Bot retrieved archive --> | archive-date=2006-07-12}}</ref>

===Kinematics===
{{Main|Stellar kinematics}}
[[File:Pleiades large.jpg|thumb|The [[Pleiades]], an [[open cluster]] of stars in the [[constellation]] of [[Taurus (constellation)|Taurus]]. These stars share a common motion through space.<ref>{{cite journal
 | last=Loktin | first=A. V.
 | title=Kinematics of stars in the Pleiades open cluster
 | journal=Astronomy Reports | volume=50 | issue=9
 | pages=714–721 |date=September 2006
 | doi=10.1134/S1063772906090058 | bibcode=2006ARep...50..714L| s2cid=121701212
 }}</ref>]]

The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.<ref>{{cite book
 | title=The Origin of the Galaxy and Local Group
 | series=Saas-Fee Advanced Course 37 Swiss Society for Astrophysics and Astronomy
 | first1=Joss | last1=Bland-Hawthorn | first2=Kenneth | last2=Freeman | first3=Francesca | last3=Matteucci
 | date=2014 | page=114 | isbn=978-3642417207
 | publisher=Springer Berlin Heidelberg
 | editor-first=Ben | editor-last=Moore
 | chapter=Appendix B: Stellar Data: Sources and Techniques
 | chapter-url=https://books.google.com/books?id=pTTABAAAQBAJ&pg=PA114 }}</ref> The components of motion of a star consist of the [[radial velocity]] toward or away from the Sun, and the traverse angular movement, which is called its proper motion.<ref>{{cite book
 | title=Observational Astronomy
 | first1=D. Scott | last1=Birney | first2=Guillermo | last2=Gonzalez | first3=David | last3=Oesper
 | date=2006 | pages=72–79 | isbn=978-1316139400
 | publisher=Cambridge University Press
 | url=https://books.google.com/books?id=vrA-BAAAQBAJ&pg=PA73 }}</ref>

Radial velocity is measured by the [[doppler shift]] of the star's spectral lines and is given in units of km/[[second|s]]. The proper motion of a star, its parallax, is determined by precise astrometric measurements in units of milli-[[arc second]]s (mas) per year. With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.<ref>{{cite web
 | date=1999-09-10 | url=http://www.rssd.esa.int/index.php?project=HIPPARCOS
 | title=Hipparcos: High Proper Motion Stars
 | publisher=ESA | access-date=2006-10-10}}</ref>

When both rates of movement are known, the [[space velocity (astronomy)|space velocity]] of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.<ref>{{cite journal
 | last= Johnson | first= Hugh M.
 | title=The Kinematics and Evolution of Population I Stars
 | journal=Publications of the Astronomical Society of the Pacific | date=1957 | volume=69 | issue=406 | page=54
 | bibcode=1957PASP...69...54J
 | doi=10.1086/127012| doi-access=free}}</ref> A comparison of the kinematics of nearby stars has allowed astronomers to trace their origin to common points in giant molecular clouds; such groups with common points of origin are referred to as [[stellar association]]s.<ref>{{cite journal |last1=Elmegreen |first1=B. |last2=Efremov |first2=Y. N. |date=1999 |title=The Formation of Star Clusters |url=http://www.americanscientist.org/template/AssetDetail/assetid/15714/page/1 |journal=American Scientist |volume=86 |issue=3 |page=264 |bibcode=1998AmSci..86..264E |doi=10.1511/1998.3.264 |s2cid=209833510 |archive-url=https://web.archive.org/web/20050323072521/http://www.americanscientist.org/template/AssetDetail/assetid/15714/page/1 |archive-date=2005-03-23 |access-date=2006-08-23|url-access=subscription }}</ref>

===Magnetic field===
{{Main|Stellar magnetic field}}
[[File:suaur.jpg|thumb|Surface magnetic field of [[SU Aurigae|SU&nbsp;Aur]] (a young star of [[T Tauri star|T Tauri type]]), reconstructed by means of [[Zeeman–Doppler imaging]]]]

The [[magnetic field]] of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a [[dynamo theory|dynamo]], wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo. Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces [[starspot]]s, which are regions of strong magnetic fields and lower than normal surface temperatures. [[Coronal loop]]s are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona. The coronal loops can be seen due to the plasma they conduct along their length. [[Stellar flare]]s are bursts of high-energy particles that are emitted due to the same magnetic activity.<ref>{{cite web
 | last=Brainerd | first=Jerome James
 | date=2005-07-06 | url=http://www.astrophysicsspectator.com/topics/observation/XRayCorona.html
 | title=X-rays from Stellar Coronas
 | publisher=The Astrophysics Spectator
 | access-date= 2007-06-21}}</ref>

Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.<ref name= "Michelson Starspots"/> During the [[Maunder Minimum]], for example, the Sun underwent a 70-year period with almost no sunspot activity.<ref>{{Cite journal|last1=Carrasco|first1=V. M. S.|last2=Vaquero|first2=J. M.|last3=Gallego|first3=M. C.|last4=Muñoz-Jaramillo|first4=A.|last5=de Toma|first5=G.|last6=Galaviz|first6=P.|last7=Arlt|first7=R.|last8=Senthamizh Pavai|first8=V.|last9=Sánchez-Bajo|first9=F.|last10=Villalba Álvarez|first10=J.|last11=Gómez|first11=J. M.|date=2019|title=Sunspot Characteristics at the Onset of the Maunder Minimum Based on the Observations of Hevelius|journal=The Astrophysical Journal|language=en|volume=886|issue=1|pages=18|doi=10.3847/1538-4357/ab4ade|arxiv=2103.09495|bibcode=2019ApJ...886...18C|issn=1538-4357|doi-access=free}}</ref>

===Mass===
{{Main|Stellar mass}}
Stars have masses ranging from less than half the solar mass to over 200 solar masses (see [[List of most massive stars]]). One of the most massive stars known is [[Eta Carinae]],<ref>{{cite journal
 |first=Nathan
 |last=Smith
 |date=1998
 |url=http://www.astrosociety.org/pubs/mercury/9804/eta.html
 |title=The Behemoth Eta Carinae: A Repeat Offender
 |journal=Mercury Magazine
 |volume=27
 |issue=4
 |page=20
 |access-date=2006-08-13
 |url-status=dead
 |archive-url=https://web.archive.org/web/20060927091554/http://www.astrosociety.org/pubs/mercury/9804/eta.html
 |archive-date=2006-09-27
|bibcode=1998Mercu..27d..20S
 }}</ref> which, with 100–150&nbsp;times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests {{Solar mass|150}} as a rough upper limit for stars in the current era of the universe.<ref name= "weidner">{{Cite journal
 |title= Evidence for a fundamental stellar upper mass limit from clustered star formation
 |journal= Monthly Notices of the Royal Astronomical Society
 |date=2004-02-11 |pages= 187–191 |volume= 348 |issue= 1
 |doi= 10.1111/j.1365-2966.2004.07340.x
 |first1= C. |last1= Weidner |first2= P. |last2= Kroupa
 |doi-access= free
 |bibcode=2004MNRAS.348..187W|arxiv = astro-ph/0310860 |s2cid= 119338524
 }}</ref> This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud. Several stars in the [[R136]] cluster in the [[Large Magellanic Cloud]] have been measured with larger masses,<ref name= "hainich">{{Cite journal | doi = 10.1051/0004-6361/201322696| title = The Wolf-Rayet stars in the Large Magellanic Cloud| journal = Astronomy & Astrophysics| volume = 565| pages = A27| year = 2014| last1 = Hainich | first1 = R.| last2 = Rühling | first2 = U.| last3 = Todt | first3 = H.| last4 = Oskinova | first4 = L. M.| last5 = Liermann | first5 = A.| last6 = Gräfener | first6 = G.| last7 = Foellmi | first7 = C.| last8 = Schnurr | first8 = O.| last9 = Hamann | first9 = W.-R. | arxiv = 1401.5474
| bibcode = 2014A&A...565A..27H| s2cid = 55123954}}</ref> but it has been determined that they could have been created through the collision and merger of massive stars in close binary systems, sidestepping the {{Solar mass|150}} limit on massive star formation.<ref name=banerjee>{{Cite journal |title= The emergence of super-canonical stars in R136-type starburst clusters |journal= Monthly Notices of the Royal Astronomical Society |date=2012-10-21 |pages= 1416–1426 |volume= 426 |issue= 2 |doi= 10.1111/j.1365-2966.2012.21672.x |first1= Sambaran |last1= Banerjee |first2= Pavel |last2= Kroupa |first3= Seungkyung |last3= Oh |doi-access= free |bibcode=2012MNRAS.426.1416B|arxiv = 1208.0826 |s2cid= 119202197 }}</ref>

[[File:Ngc1999.jpg|thumb|The [[reflection nebula]] [[NGC 1999]] is brilliantly illuminated by [[V380 Orionis]]. The black patch of sky is a vast hole of empty space and not a [[dark nebula]] as previously thought.]]
The first stars to form after the Big Bang may have been larger, up to {{Solar mass|300}},<ref>{{cite news
 | title=Ferreting Out The First Stars
 | publisher=Harvard-Smithsonian Center for Astrophysics
 | date=2005-09-22 | url=http://www.cfa.harvard.edu/news/2005/pr200531.html
 | access-date=2006-09-05}}</ref> due to the complete absence of elements heavier than [[lithium]] in their composition. This generation of supermassive [[population III stars]] is likely to have existed in the very early universe (i.e., they are observed to have a high redshift), and may have started the production of chemical elements heavier than [[hydrogen]] that are needed for the later formation of planets and life. In June 2015, astronomers reported evidence for Population III stars in the [[Cosmos Redshift 7]] galaxy at {{math|''z'' {{=}} 6.60}}.<ref name="AJ-20150604">{{cite journal
|last1=Sobral |first1=David |last2=Matthee |first2=Jorryt |last3=Darvish |first3=Behnam |last4=Schaerer |first4=Daniel |last5=Mobasher |first5=Bahram |last6=Röttgering |first6=Huub J. A. |last7=Santos |first7=Sérgio |last8=Hemmati |first8=Shoubaneh |title=Evidence For POPIII-Like Stellar Populations In The Most Luminous LYMAN-α Emitters At The Epoch Of Re-Ionisation: Spectroscopic Confirmation |date=2015-06-04|journal=[[The Astrophysical Journal]] | doi = 10.1088/0004-637x/808/2/139|arxiv = 1504.01734 |bibcode = 2015ApJ...808..139S |volume=808 |issue=2 |pages=139|s2cid=18471887 }}</ref><ref name="NYT-20150617">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Astronomers Report Finding Earliest Stars That Enriched Cosmos |url=https://www.nytimes.com/2015/06/18/science/space/astronomers-report-finding-earliest-stars-that-enriched-cosmos.html |date=2015-06-17 |work=[[The New York Times]] |access-date=2015-06-17}}</ref>

With a mass only 80&nbsp;times that of Jupiter ({{Jupiter mass}}), [[2MASS J0523-1403]] is the smallest known star undergoing nuclear fusion in its core.<ref name=SIMBAD>{{cite web
 |title=2MASS J05233822-1403022
 |url=http://simbad.u-strasbg.fr/simbad/sim-id?Name=2MASS%20J05233822-1403022&Ident=%40788130&submit=submit
 |publisher=SIMBAD – [[Centre de Données astronomiques de Strasbourg]]
 |access-date=2013-12-14}}</ref> For stars with metallicity similar to the Sun, the theoretical minimum mass the star can have and still undergo fusion at the core, is estimated to be about 75 {{Jupiter mass}}.<ref name="boss_planets_or_what">{{cite web
 |first=Alan
 |last=Boss
 |date=2001-04-03
 |url=http://www.carnegieinstitution.org/News4-3,2001.html
 |title=Are They Planets or What?
 |publisher=Carnegie Institution of Washington
 |access-date=2006-06-08
 |archive-url=https://web.archive.org/web/20060928065124/http://www.carnegieinstitution.org/News4-3%2C2001.html
 |archive-date=2006-09-28
 |url-status=dead
}}</ref><ref name="minimum">{{cite magazine
 |last=Shiga
 |first=David
 |date=2006-08-17
 |url=http://www.newscientistspace.com/article/dn9771-mass-cutoff-between-stars-and-brown-dwarfs-revealed.html
 |archive-url=https://web.archive.org/web/20061114221813/http://space.newscientist.com/article/dn9771-mass-cutoff-between-stars-and-brown-dwarfs-revealed.html
 |archive-date=2006-11-14
 |title=Mass cut-off between stars and brown dwarfs revealed
 |magazine=New Scientist
 |access-date=2006-08-23
 |url-status=dead
}}</ref> When the metallicity is very low, the minimum star size seems to be about 8.3% of the solar mass, or about 87 {{Jupiter mass}}.<ref name="minimum" /><ref>{{cite news
 | title=Hubble glimpses faintest stars
 | publisher=BBC | date=2006-08-18
 | url=http://news.bbc.co.uk/2/hi/science/nature/5260008.stm
 | access-date=2006-08-22
 | first=Elli
 | last=Leadbeater}}</ref> Smaller bodies called [[brown dwarf]]s, occupy a poorly defined grey area between stars and [[gas giant]]s.<ref name="boss_planets_or_what" /><ref name="minimum" />

The combination of the radius and the mass of a star determines its surface gravity. Giant stars have much lower surface gravity than do main-sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the [[absorption line]]s.<ref name="new cosmos" />

===Rotation===
{{Main|Stellar rotation}}

The rotation rate of stars can be determined through [[spectroscopy|spectroscopic measurement]], or more exactly determined by tracking their [[starspot]]s. Young stars can have a rotation greater than 100&nbsp;km/s at the equator. The B-class star [[Achernar]], for example, has an equatorial velocity of about 225&nbsp;km/s or greater, causing its [[equatorial bulge|equator to bulge outward]] and giving it an equatorial diameter that is more than 50% greater than between the poles. This rate of rotation is just below the critical velocity of 300&nbsp;km/s at which speed the star would break apart.<ref>{{cite news
 | title=Flattest Star Ever Seen | publisher=ESO
 | date=2003-06-11 | url=http://www.eso.org/public/news/eso0316/
 | access-date=2006-10-03}}</ref> By contrast, the Sun rotates once every 25–35 days depending on latitude,<ref>{{cite web|url=https://www.nasa.gov/mission_pages/sunearth/science/solar-rotation.html|title=Solar Rotation Varies by Latitude|date=2013-01-23|publisher=NASA}}</ref> with an equatorial velocity of 1.93&nbsp;km/s.<ref>{{cite journal|title=Spectroscopic Determinations of Solar Rotation|last1=Howard|first1=R.|last2=Harvey|first2=J.|journal=Solar Physics|volume=12|issue=1|pages=23–51|bibcode=1970SoPh...12...23H|year=1970|doi=10.1007/BF02276562|s2cid=122140471}}</ref> A main-sequence star's magnetic field and the stellar wind serve to slow its rotation by a significant amount as it evolves on the main sequence.<ref>{{cite web
 | last=Fitzpatrick | first=Richard
 | date=2006-02-13 | url=http://farside.ph.utexas.edu/teaching/plasma/lectures/lectures.html
 | archive-url=https://web.archive.org/web/20100104142353/http://farside.ph.utexas.edu/teaching/plasma/lectures/lectures.html
 | archive-date=2010-01-04
 | title=Introduction to Plasma Physics: A graduate course
 | publisher=The University of Texas at Austin
 | access-date=2006-10-04}}</ref>

[[Degenerate star]]s have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.<ref>{{cite journal |last= Villata |first= Massimo |title= Angular momentum loss by a stellar wind and rotational velocities of white dwarfs |journal= Monthly Notices of the Royal Astronomical Society |date= 1992 |volume= 257 |issue= 3 |pages= 450–454 |bibcode= 1992MNRAS.257..450V| doi = 10.1093/mnras/257.3.450 |doi-access= free }}</ref> In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the [[Crab nebula]], for example, rotates 30 times per second.<ref>{{cite news
 | title=A History of the Crab Nebula | publisher=ESO
 | date=1996-05-30 | url=http://hubblesite.org/newscenter/archive/releases/1996/22/astrofile/
 | access-date=2006-10-03}}</ref> The rotation rate of the pulsar will gradually slow due to the emission of radiation.<ref>{{cite web|url=http://www.jb.man.ac.uk/distance/frontiers/pulsars/section1.html|title=Properties of Pulsars|publisher=Jodrell Bank Observatory, University of Manchester|work=Frontiers of Modern Astronomy|access-date=2018-08-17}}</ref>

===Temperature===
The surface temperature of a main-sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's [[color index]].<ref name="astronomynotes">{{cite web
 |url=http://www.astronomynotes.com/starprop/s5.htm
 |title=Properties of Stars: Color and Temperature
 |access-date=2007-10-09 |last=Strobel |first=Nick
 |date=2007-08-20 |work=Astronomy Notes
 |publisher=Primis/McGraw-Hill, Inc.
 |archive-url=https://web.archive.org/web/20070626090138/http://www.astronomynotes.com/starprop/s5.htm
 |archive-date=2007-06-26}}</ref> The temperature is normally given in terms of an [[effective temperature]], which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. The effective temperature is only representative of the surface, as the temperature increases toward the core.<ref>{{cite web
 | first=Courtney | last=Seligman | work=Self-published
 | url=http://cseligman.com/text/stars/heatflowreview.htm
 | title =Review of Heat Flow Inside Stars
 | access-date= 2007-07-05}}</ref> The temperature in the core region of a star is several million [[kelvin]]s.<ref name="aps_mss" />

The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual [[absolute magnitude]] and absorption features, is used to classify a star (see classification below).<ref name="new cosmos" />

Massive main-sequence stars can have surface temperatures of 50,000&nbsp;K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600&nbsp;K; but they have a high luminosity due to their large exterior surface area.<ref name=zeilik>{{cite book |last1=Zeilik |first1=Michael A. |last2=Gregory |first2=Stephan A. |title=Introductory Astronomy & Astrophysics |edition=4th |date=1998 |publisher=Saunders College Publishing |isbn=978-0-03-006228-5 |page=321}}</ref>

==Radiation==
[[File:Eta Carinae.jpg|thumb|upright=1.2|[[Eta Carinae]] is an unstable [[blue hypergiant]] star, roughly 100 times more massive than the Sun, over 700 times wider, and 4 million times more luminous. In a 19th century event termed the Great Eruption, Eta Carinae brightened and violently ejected mass to form the surrounding [[Homunculus Nebula]] (pictured).]]
The energy produced by stars, a product of nuclear fusion, radiates to space as both [[electromagnetic radiation]] and [[particle radiation]]. The particle radiation emitted by a star is manifested as the stellar wind,<ref>{{cite news
 | last=Koppes | first=Steve
 | title=University of Chicago physicist receives Kyoto Prize for lifetime achievements in science
 | publisher=The University of Chicago News Office
 | date=2003-06-20 | url=http://www-news.uchicago.edu/releases/03/030620.parker.shtml
 | access-date=2012-06-15}}</ref> which streams from the outer layers as electrically charged [[proton]]s and [[alpha particle|alpha]] and [[beta particle]]s. A steady stream of almost massless neutrinos emanate directly from the star's core.<ref name='carroll_ostlie_ch11'>{{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 11|date= 2017 }}</ref>

The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single [[atomic nucleus]] of a new heavier element, [[gamma ray]] [[photon]]s are released from the nuclear fusion product. This energy is converted to other forms of [[electromagnetic energy]] of lower frequency, such as visible light, by the time it reaches the star's outer layers.<ref name='carroll_ostlie_ch10'>{{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 10|year= 2017 }}</ref>

The color of a star, as determined by the most intense [[frequency]] of the visible light, depends on the temperature of the star's outer layers, including its [[photosphere]].<ref>{{cite web
 |url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html
 |title=The Colour of Stars
 |publisher=Australian Telescope Outreach and Education
 |access-date=2006-08-13
 |url-status=dead
 |archive-url=https://web.archive.org/web/20120318151427/http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html
 |archive-date=2012-03-18
}}</ref> Besides visible light, stars emit forms of electromagnetic radiation that are invisible to the [[human eye]]. In fact, stellar electromagnetic radiation spans the entire [[electromagnetic spectrum]], from the longest [[wavelength]]s of [[radio frequency|radio waves]] through [[infrared]], visible light, [[ultraviolet]], to the shortest of [[X-ray]]s, and gamma rays. From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.<ref name='carroll_ostlie_ch11' />

Using the [[stellar spectrum]], astronomers can determine the surface temperature, [[surface gravity]], metallicity and [[rotation]]al velocity of a star. If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be calculated for stars in [[Binary star|binary systems]] by measuring their orbital velocities and distances. [[Gravitational microlensing]] has been used to measure the mass of a single star.<ref>{{cite news
 | title=Astronomers Measure Mass of a Single Star – First Since the Sun
 | publisher=Hubble News Desk | date=2004-07-15 | url=http://hubblesite.org/newscenter/archive/releases/2004/24/text/
 | access-date=2006-05-24}}</ref>) With these parameters, astronomers can estimate the age of the star.<ref>{{cite journal
 | last1=Garnett | first1=D. R. | last2=Kobulnicky | first2=H. A.
 | title=Distance Dependence in the Solar Neighborhood Age-Metallicity Relation
 | journal=The Astrophysical Journal | date=2000
 | volume=532
 | issue= 2 | pages=1192–1196
 | doi= 10.1086/308617
| bibcode=2000ApJ...532.1192G|arxiv= astro-ph/9912031| s2cid=18473242 }}</ref>

===Luminosity===
The luminosity of a star is the amount of light and other forms of [[radiant energy]] it radiates per unit of time. It has units of [[power (physics)|power]]. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface. The rapidly rotating star [[Vega]], for example, has a higher [[energy flux]] (power per unit area) at its poles than along its equator.<ref>{{cite news
 | author=Staff
 | date=2006-01-10
 | title=Rapidly Spinning Star Vega has Cool Dark Equator
 | publisher=National Optical Astronomy Observatory
 | url=http://www.noao.edu/outreach/press/pr06/pr0603.html
 | access-date=2007-11-18
 | archive-date=2019-05-24
 | archive-url=https://web.archive.org/web/20190524103812/https://www.noao.edu/outreach/press/pr06/pr0603.html
 | url-status=dead
 }}</ref>

Patches of the star's surface with a lower temperature and luminosity than average are known as [[sunspot|starspots]]. Small, ''dwarf'' stars such as the Sun generally have essentially featureless disks with only small starspots. ''Giant'' stars have much larger, more obvious starspots,<ref name= "Michelson Starspots">{{cite journal
 |last1=Michelson |first1=A. A. |last2=Pease |first2=F. G.
 |title=Starspots: A Key to the Stellar Dynamo |journal=Living Reviews in Solar Physics
 |volume=2 |issue=1 |pages=8 |date=2005
 |url=http://solarphysics.livingreviews.org/Articles/lrsp-2005-8/|bibcode=2005LRSP....2....8B |doi=10.12942/lrsp-2005-8 |doi-access=free }}</ref> and they exhibit strong stellar [[limb darkening]]. That is, the brightness decreases towards the edge of the stellar disk.<ref>{{cite journal
 |last1=Manduca |first1=A. |last2=Bell |first2=R. A. |last3=Gustafsson |first3=B.
 |title=Limb darkening coefficients for late-type giant model atmospheres
 |journal=Astronomy and Astrophysics |date=1977 |volume=61 |issue=6 |pages=809–813
 |bibcode=1977A&A....61..809M}}</ref> Red dwarf [[flare star]]s such as [[UV Ceti]] may possess prominent starspot features.<ref>{{cite journal   |last1=Chugainov |first1=P. F.
 |title=On the Cause of Periodic Light Variations of Some Red Dwarf Stars
 |journal=Information Bulletin on Variable Stars |date=1971 |volume=520 |pages=1–3 |bibcode=1971IBVS..520....1C}}</ref>

===Magnitude===
{{Main|Apparent magnitude|Absolute magnitude}}
The apparent [[brightness]] of a star is expressed in terms of its [[apparent magnitude]]. It is a function of the star's luminosity, its distance from Earth, the [[Extinction (astronomy)|extinction]] effect of [[interstellar dust]] and gas, and the altering of the star's light as it passes through Earth's atmosphere. Intrinsic or absolute magnitude is directly related to a star's luminosity, and is the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years).<ref name="Lawrence2019">{{cite book |author=Lawrence |first=J. L. |url=https://books.google.com/books?id=YTyUDwAAQBAJ&pg=PA252 |title=Celestial Calculations: A Gentle Introduction to Computational Astronomy |date=2019 |publisher=Massachusetts Institute of Technology Press |isbn=978-0-262-53663-9 |pages=252}}</ref>

{| class="wikitable" style="float: right; margin-left: 1em;"
|+ Number of stars brighter than magnitude
!Apparent<br />magnitude
!Number&nbsp;<br />of&nbsp;stars<ref>{{cite web |url= http://www.nso.edu/PR/answerbook/magnitude.html |archive-url= https://web.archive.org/web/20080206074842/http://www.nso.edu/PR/answerbook/magnitude.html |archive-date= 2008-02-06 |title= Magnitude |publisher= National Solar Observatory – Sacramento Peak |access-date= 2006-08-23}}</ref>
|- style="text-align: center;"
||0
||4
|- style="text-align: center;"
||1
||15
|- style="text-align: center;"
||2
||48
|- style="text-align: center;"
||3
||171
|- style="text-align: center;"
||4
||513
|- style="text-align: center;"
||5
||1,602
|- style="text-align: center;"
||6
||4,800
|- style="text-align: center;"
||7
||14,000
|}

Both the apparent and absolute magnitude scales are [[logarithmic units]]: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times<ref name="luminosity">{{cite web|url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_luminosity.html |title=Luminosity of Stars |publisher=Australian Telescope Outreach and Education |access-date=2006-08-13 |url-status=dead |archive-url=https://web.archive.org/web/20140809120004/http://www.atnf.csiro.au/outreach//education/senior/astrophysics/photometry_specparallax.html |archive-date=2014-08-09 }}</ref> (the [[nth root|5th root]] of 100 or approximately 2.512). This means that a [[first magnitude star]] (+1.00) is about 2.5 times brighter than a [[second magnitude star|second magnitude]] (+2.00) star, and about 100 times brighter than a [[sixth magnitude star]] (+6.00). The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.<ref name="Nicolson1999">{{cite book |author=Nicolson |first=Iain |url=https://books.google.com/books?id=5iacbufd4kEC&pg=PA134 |title=Unfolding Our Universe |date=1999 |publisher=Cambridge University Press |isbn=978-0-521-59270-3 |pages=134}}</ref>

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (Δ''L'') between two stars is calculated by subtracting the magnitude number of the brighter star (''m''<sub>b</sub>) from the magnitude number of the fainter star (''m''<sub>f</sub>), then using the difference as an exponent for the base number 2.512; that is to say:

:<math> \Delta{m} = m_\mathrm{f} - m_\mathrm{b} </math>
:<math>2.512^{\Delta{m}} = \Delta{L}</math>

Relative to both luminosity and distance from Earth, a star's absolute magnitude (''M'') and apparent magnitude (''m'') are not equivalent;<ref name="luminosity" /> for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.

The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while [[Canopus]], the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, the latter star appears the brighter of the two. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.<ref>{{cite book|title=Astounding Science Fact & Fiction|url=https://books.google.com/books?id=zLkOAQAAIAAJ|year=1960|publisher=Street & Smith|page=7}}</ref>

The [[List of most luminous stars|most luminous known stars]] have absolute magnitudes of roughly −12, corresponding to 6&nbsp;million times the luminosity of the Sun.<ref>{{cite journal |last1=Bestenlehner |first1=Joachim M. |last2=Crowther |first2=Paul A. |last3=Caballero-Nieves |first3=Saida M. |last4=Schneider |first4=Fabian R. N. |last5=Simón-Díaz |first5=Sergio |last6=Brands |first6=Sarah A. |last7=de Koter |first7=Alex |last8=Gräfener |first8=Götz |last9=Herrero |first9=Artemio |last10=Langer |first10=Norbert |last11=Lennon |first11=Daniel J. |last12=Maíz Apellániz |first12=Jesus |last13=Puls |first13=Joachim |last14=Vink |first14=Jorick S. |date=2020-10-17 |title=The R136 star cluster dissected with Hubble Space Telescope/STIS – II. Physical properties of the most massive stars in R136 |journal=Monthly Notices of the Royal Astronomical Society |volume=499 |issue=2 |pages=1918–1936 |arxiv=2009.05136 |bibcode=2020MNRAS.499.1918B |doi=10.1093/mnras/staa2801 |doi-access=free}}</ref> Theoretically, the least luminous stars are at the lower limit of mass at which stars are capable of supporting nuclear fusion of hydrogen in the core; stars just above this limit have been located in the [[NGC 6397]] cluster. The faintest red dwarfs in the cluster are absolute magnitude 15, while a 17th absolute magnitude white dwarf has been discovered.<ref>{{cite web
 | date=August 17, 2006 | url=http://hubblesite.org/newscenter/archive/releases/2006/37/image/a/
 | title=Faintest Stars in Globular Cluster NGC 6397
 | publisher=HubbleSite | access-date=2006-06-08}}</ref><ref>{{cite journal |last1=Richer |first1=H. B. |title=Probing the Faintest Stars in a Globular Star Cluster |journal=Science |date=18 August 2006 |volume=313 |issue=5789 |pages=936–940 |doi=10.1126/science.1130691|pmid=16917054 |arxiv=astro-ph/0702209 |bibcode=2006Sci...313..936R |s2cid=27339792 }}</ref>

==Classification==
{{Main|Stellar classification}}
{| class="wikitable" style="float: right; text-align: center; margin-left: 1em;"
|+ Surface temperature ranges for<br />different stellar classes<ref>{{cite web
 | last=Smith | first=Gene | date=1999-04-16
 | url=http://casswww.ucsd.edu/public/tutorial/Stars.html
 | title=Stellar Spectra
 | publisher=University of California, San Diego
 | access-date=2006-10-12}}</ref>
! Class
! Temperature
! Sample star
|- style="background: {{star-color|O}}"
| O
| 33,000&nbsp;K&nbsp;or&nbsp;more
| [[Zeta Ophiuchi]]
|- style="background: {{star-color|B}}"
| B
| 10,500–30,000&nbsp;K
| [[Rigel]]
|- style="background: {{star-color|A}}"
| A
| 7,500–10,000&nbsp;K
| [[Altair]]
|- style="background: {{star-color|F}}"
| F
| 6,000–7,200&nbsp;K
| [[Procyon|Procyon&nbsp;A]]
|- style="background: {{star-color|G}}"
| G
| 5,500–6,000&nbsp;K
| [[Sun]]
|- style="background: {{star-color|K}}"
| K
| 4,000–5,250&nbsp;K
| [[Epsilon Indi]]
|- style="background: {{star-color|M}}"
| M
| 2,600–3,850&nbsp;K
| [[Proxima Centauri]]
|}

The current stellar classification system originated in the early 20th century, when stars were classified from ''A'' to ''Q'' based on the strength of the [[hydrogen line]].<ref>{{cite journal
 | last=Fowler | first=A.
 | title=The Draper Catalogue of Stellar Spectra
 | journal=Nature
 | date=April 1891 | volume=45 | issue=1166 | pages=427–428 | doi= 10.1038/045427a0 |bibcode= 1892Natur..45..427F| doi-access=free }}</ref> It was thought that the hydrogen line strength was a simple linear function of temperature. Instead, it was more complicated: it strengthened with increasing temperature, peaked near 9000&nbsp;K, and then declined at greater temperatures. The classifications were since reordered by temperature, on which the modern scheme is based.<ref name=carlos>{{cite book
 | first1=Carlos | last1=Jaschek | last2=Jaschek | first2=Mercedes
 | date=1990 | title=The Classification of Stars
 | publisher=Cambridge University Press | pages=31–48
 | isbn=978-0-521-38996-9}}</ref>

Stars are given a single-letter classification according to their spectra, ranging from type ''O'', which are very hot, to ''M'', which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: ''O, B, A, F, G, K'', and ''M''. A variety of rare spectral types are given special classifications. The most common of these are types ''L'' and ''T'', which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes ''O0'' and ''O1'' may not exist.<ref name="spectrum">{{cite web
 | first=Alan M
 | last=MacRobert
 | url=http://www.skyandtelescope.com/howto/basics/3305876.html
 | title=The Spectral Types of Stars
 | publisher=Sky and Telescope
 | access-date=2006-07-19
 | archive-date=2013-10-22
 | archive-url=https://web.archive.org/web/20131022124237/http://www.skyandtelescope.com/howto/basics/3305876.html
 | url-status=dead
 }}</ref>

In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity. These range from ''0'' ([[hypergiant]]s) through ''III'' ([[giant star|giants]]) to ''V'' (main-sequence dwarfs); some authors add ''VII'' (white dwarfs). Main-sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.<ref name="spectrum" /> The Sun is a main-sequence ''G2V'' yellow dwarf of intermediate temperature and ordinary size.<ref name="RixHay2015">{{cite book |last1=Rix |first1=Erika |url=https://books.google.com/books?id=7pCKCgAAQBAJ&pg=PA43 |title=Solar Sketching: A Comprehensive Guide to Drawing the Sun |last2=Hay |first2=Kim |last3=Russell |first3=Sally |last4=Handy |first4=Richard |date=2015 |publisher=Springer |isbn=978-1-4939-2901-6 |page=43}}</ref>

There is additional nomenclature in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum. For example, an "''e''" can indicate the presence of emission lines; "''m''" represents unusually strong levels of metals, and "''var''" can mean variations in the spectral type.<ref name="spectrum" />

White dwarf stars have their own class that begins with the letter ''D''. This is further sub-divided into the classes ''DA'', ''DB'', ''DC'', ''DO'', ''DZ'', and ''DQ'', depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature.<ref>{{cite web
 | url= http://www.physics.uq.edu.au/people/ross/ph3080/whitey.htm
 | archive-url= https://web.archive.org/web/20091008115925/http://www.physics.uq.edu.au/people/ross/ph3080/whitey.htm
 | archive-date= 2009-10-08
 | title= White Dwarf (wd) Stars
 | publisher= White Dwarf Research Corporation
 | access-date= 2006-07-19}}</ref>

==Variable stars==
{{Main|Variable star}}
[[File:Mira 1997.jpg|thumb|[[Mira]], an oscillating variable star on the asymptotic giant branch, is a red giant nearing the end of its life, noted for its asymmetrical appearance.]]

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes [[Cepheid variable|Cepheid and Cepheid-like stars]], and long-period variables such as [[Mira variable|Mira]].<ref name="variables">{{cite web |url=http://www.aavso.org/types-variables |title=Types of Variable |date=2010-05-11 |publisher=AAVSO |access-date=2010-08-20 |archive-date=17 October 2018 |archive-url=https://web.archive.org/web/20181017170335/http://www.aavso.org/types-variables |url-status=dead }}</ref>

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.<ref name="variables" /> This group includes protostars, Wolf-Rayet stars, and flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties. This group includes [[nova]]e and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type Ia supernova.<ref name="iben" /> The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.<ref>{{cite web
 | date =2004-11-01 | url= http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html
 | title= Cataclysmic Variables
 | publisher= NASA Goddard Space Flight Center
 | access-date= 2006-06-08}}</ref> Some novae are recurrent, having periodic outbursts of moderate amplitude.<ref name="variables" />

Stars can vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.<ref name="variables" /> A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.1 to 3.4 over a period of 2.87 days.<ref name=gcvs>{{cite journal|bibcode=2009yCat....102025S|title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)|journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S|volume=1|display-authors=etal|last1=Samus|first1=N. N.|last2=Durlevich|first2=O. V.|year=2009}}</ref>

==Structure==
{{Main|Stellar structure}}
[[Image:Star types.svg|left|thumb|upright=1.4|Internal structures of main-sequence stars with masses indicated in solar masses, convection zones with arrowed cycles, and radiative zones with red flashes. Left to right, a [[red dwarf]], a [[Yellow dwarf star|yellow dwarf]], and a [[Stellar classification|blue-white main-sequence star]]]]

The interior of a stable star is in a state of [[hydrostatic equilibrium]]: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure [[gradient]] within the star. The [[pressure gradient]] is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main-sequence or giant star is at least on the order of {{val|e=7|u=K}}. The resulting temperature and pressure at the hydrogen-burning core of a main-sequence star are sufficient for [[nuclear fusion]] to occur and for sufficient energy to be produced to prevent further collapse of the star.<ref name="hansen">{{cite book |last1=Hansen |first1=Carl J. |last2=Kawaler |first2=Steven D. |last3=Trimble |first3=Virginia |pages=[https://archive.org/details/springer_10.1007-978-1-4419-9110-2/page/n44 32]–33 |title=Stellar Interiors |url=https://archive.org/details/springer_10.1007-978-1-4419-9110-2 |publisher=Springer |date=2004 |isbn=978-0-387-20089-7}}</ref><ref name="Schwarzschild">{{cite book|title=Structure and Evolution of the Stars|last=Schwarzschild|first=Martin|date=1958|publisher=Princeton University Press|isbn=978-0-691-08044-4}}<!-- Book republished by Dover as ISBN 0-486-61479-4, but ISBN in the cite book template is the one as published by Prin. Univ. Press--></ref>

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant, and energy production ceases at the core. Instead, for stars of more than {{Solar mass|0.4}}, fusion occurs in a slowly expanding shell around the [[degenerate matter|degenerate]] helium core.<ref>{{cite web |url= http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html |title= Formation of the High Mass Elements |publisher= Smoot Group |access-date= 2006-07-11}}</ref>

In addition to hydrostatic equilibrium, the interior of a stable star will maintain an energy balance of [[thermal equilibrium]]. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.<ref name="HuangYu1998">{{cite book |last1=Huang |first1=R. Q. |url=https://books.google.com/books?id=TInvAAAAMAAJ |title=Stellar Astrophysics |last2=Yu |first2=K. N. |publisher=Springer |year=1998 |isbn=978-981-3083-36-3 |page=70}}</ref>

The [[radiation zone]] is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone. In this region the plasma will not be perturbed, and any mass motions will die out. Where this is not the case, then the plasma becomes unstable and convection will occur, forming a [[convection zone]]. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high [[opacity (optics)|opacity]] (making radiatative heat transfer inefficient) as in the outer envelope.<ref name="Schwarzschild" />

The occurrence of convection in the outer envelope of a main-sequence star depends on the star's mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.<ref name="imagine">{{cite web |date= 2006-09-01 |url= http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html |title= What is a Star? |publisher= NASA |access-date= 2006-07-11}}</ref> Red dwarf stars with less than {{Solar mass|0.4}} are convective throughout, which prevents the accumulation of a helium core.<ref name="late stages" /> For most stars the convective zones will vary over time as the star ages and the constitution of the interior is modified.<ref name="Schwarzschild" />

[[File:Sun parts big.jpg|thumb|upright=1.6|A cross-section of the [[Sun]]]]
The photosphere is that portion of a star that is visible to an observer. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate into space. It is within the photosphere that [[sun spots]], regions of lower than average temperature, appear.<ref name="NewcombHolden1887">{{cite book|author1=Simon Newcomb|author2=Edward Singleton Holden|title=Astronomy for High Schools and Colleges|url=https://books.google.com/books?id=87YXAAAAIAAJ&pg=PA278|year=1887|publisher=H. Holt|pages=278}}</ref>

Above the level of the photosphere is the stellar atmosphere. In a main-sequence star such as the Sun, the lowest level of the atmosphere, just above the photosphere, is the thin [[chromosphere]] region, where [[Solar spicule|spicules]] appear and [[solar flare|stellar flares]] begin. Above this is the transition region, where the temperature rapidly increases within a distance of only {{convert|100|km|0|abbr=on}}. Beyond this is the [[stellar corona|corona]], a volume of super-heated plasma that can extend outward to several million kilometres.<ref>{{cite press release
 | publisher= ESO | date=2001-08-01
 | title= The Glory of a Nearby Star: Optical Light from a Hot Stellar Corona Detected with the VLT
 | url= http://www.eso.org/public/news/eso0127/
 | access-date= 2006-07-10}}</ref> The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.<ref name= "imagine" /> Despite its high temperature, the corona emits very little light, due to its low gas density.<ref>{{Cite web |title=What Is the Sun's Corona? {{!}} NASA Space Place – NASA Science for Kids |url=https://spaceplace.nasa.gov/sun-corona/en/#:~:text=Why?,the%20surface%20of%20the%20Sun. |access-date=2023-11-21 |website=spaceplace.nasa.gov}}</ref> The corona region of the Sun is normally only visible during a [[solar eclipse]].

From the corona, a stellar wind of plasma particles expands outward from the star, until it interacts with the interstellar medium. For the Sun, the influence of its [[solar wind]] extends throughout a bubble-shaped region called the [[heliosphere]].<ref>{{cite journal | display-authors= 1
 | last1= Burlaga | first1= L. F. | last2= Ness | first2= N. F. | last3= Acuña | first3= M. H. | last4= Lepping | first4= R. P. | last5= Connerney | first5= J. E. P. | last6= Stone | first6= E. C. | last7= McDonald | first7= F. B.
 | title= Crossing the Termination Shock into the Heliosheath: Magnetic Fields
 | journal= Science | date= 2005 | volume= 309
 | issue= 5743 | pages= 2027–2029 | doi= 10.1126/science.1117542
 | pmid= 16179471 | bibcode= 2005Sci...309.2027B| s2cid= 5998363 }}</ref>

==Nuclear fusion reaction pathways==
{{Main|Stellar nucleosynthesis}}
{{Multiple image|direction=vertical|align=right|image1=Fusion in the Sun.svg|image2=CNO Cycle.svg|caption1=Overview of the proton–proton chain|caption2=The carbon-nitrogen-oxygen cycle}}

When nuclei fuse, the mass of the fused product is less than the mass of the original parts. This lost mass is converted to electromagnetic energy, according to the [[mass–energy equivalence]] relationship <math> E=mc^2 </math>.<ref name="sunshine">{{cite web
 | last= Bahcall | first= John N.
 | date=2000-06-29
 | url= http://nobelprize.org/nobel_prizes/physics/articles/fusion/index.html
 | title= How the Sun Shines | publisher= Nobel Foundation
 | access-date= 2006-08-30}}</ref> A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition.

The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result, the core temperature of main-sequence stars only varies from 4&nbsp;million kelvin for a small M-class star to 40&nbsp;million kelvin for a massive O-class star.<ref name="aps_mss">{{cite web
 | date=2005-02-16 | url= http://www.astrophysicsspectator.com/topics/stars/MainSequence.html
 | title= Main Sequence Stars
 | publisher= The Astrophysics Spectator
 | access-date= 2006-10-10}}</ref>

In the Sun, with a 16-million-kelvin core, hydrogen fuses to form helium in the [[proton–proton chain reaction]]:<ref name="synthesis">{{cite journal |display-authors= 1 |last1= Wallerstein |first1= G. | author1-link=George Wallerstein |last2= Iben
 |first2= I. Jr. |last3= Parker |first3= P. |last4= Boesgaard |first4= A. M. |last5= Hale
 |first5= G. M. |last6= Champagne |first6= A. E. |last7= Barnes |first7= C. A. |last8= KM-dppeler
 |first8= F. |last9= Smith |first9= V. V. |last10= Hoffman |first10= R. D. |last11= Timmes
 |first11= F. X. |last12= Sneden |first12= C. |last13= Boyd |first13= R. N. |last14= Meyer
 |first14= B. S. |last15= Lambert |first15= D. L.
 |title= Synthesis of the elements in stars: forty years of progress |journal= Reviews of Modern Physics
 |date= 1999 |volume= 69 |issue= 4 |pages= 995–1084
 |url= http://authors.library.caltech.edu/10255/1/WALrmp97.pdf |access-date= 2006-08-04
 |doi= 10.1103/RevModPhys.69.995 |bibcode= 1997RvMP...69..995W|hdl= 2152/61093 |hdl-access= free }}</ref>

:4[[Hydrogen-1|<sup>1</sup>H]] → 2[[deuterium|<sup>2</sup>H]] + 2[[positron|e<sup>+</sup>]] + 2[[neutrino|ν<sub>e</sub>]](2 x 0.4 M[[electronvolt|eV]])
:2[[positron|e<sup>+</sup>]] + 2[[positron|e]]<sup>−</sup> → 2[[photon|γ]] (2 x 1.0 MeV)
:2<sup>1</sup>H + 2<sup>2</sup>H → 2[[Helium-3|<sup>3</sup>He]] + 2[[photon|γ]] (2 x 5.5 MeV)
:2<sup>3</sup>He → [[Helium-4|<sup>4</sup>He]] + 2<sup>1</sup>H (12.9 MeV)

There are a couple other paths, in which {{sup|3}}He and {{sup|4}}He combine to form {{sup|7}}Be, which eventually (with the addition of another proton) yields two {{sup|4}}He, a gain of one.

All these reactions result in the overall reaction:

:4<sup>1</sup>H → <sup>4</sup>He + 2γ + 2ν<sub>e</sub> (26.7 MeV)

where γ is a gamma ray photon, ν<sub>e</sub> is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts. Each individual reaction produces only a tiny amount of energy, but because enormous numbers of these reactions occur constantly, they produce all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.

In more massive stars, helium is produced in a cycle of reactions [[catalyst|catalyzed]] by carbon called the [[CNO cycle|carbon-nitrogen-oxygen cycle]].<ref name="synthesis" />

In evolved stars with cores at 100&nbsp;million kelvin and masses between 0.5 and {{Solar mass|10}}, helium can be transformed into carbon in the [[triple-alpha process]] that uses the intermediate element [[beryllium]]:<ref name="synthesis" />

:<sup>4</sup>He + <sup>4</sup>He + 92 keV → [[isotopes of beryllium|<sup>8*</sup>Be]]
:<sup>4</sup>He + <sup>8*</sup>Be + 67 keV → <sup>12*</sup>C
:<sup>12*</sup>C → [[carbon-12|<sup>12</sup>C]] + γ + 7.4 MeV

For an overall reaction of:

[[File:Kernfusionen0 en.png|thumb|upright=1.4|Overview of consecutive fusion processes in massive stars]]
:3<sup>4</sup>He → <sup>12</sup>C + γ + 7.2 MeV

In massive stars, heavier elements can be burned in a contracting core through the [[neon-burning process]] and [[oxygen-burning process]]. The final stage in the stellar nucleosynthesis process is the [[silicon-burning process]] that results in the production of the stable isotope iron-56.<ref name="synthesis" /> Any further fusion would be an endothermic process that consumes energy, and so further energy can only be produced through gravitational collapse.

{| class="wikitable" style="float: left; margin-right: 1.0em;"
|+ Duration of the main phases of fusion for a {{solar mass|20}} star<ref>{{cite journal |last1= Woosley |first1= S. E. |last2= Heger |first2= A. |last3= Weaver |first3= T. A. |title= The evolution and explosion of massive stars |journal= Reviews of Modern Physics |date= 2002 |volume= 74 |issue= 4 |pages= 1015–1071 |bibcode= 2002RvMP...74.1015W |doi= 10.1103/RevModPhys.74.1015}}</ref>
|-
!valign="bottom"| Fuel<br />material
!valign="bottom"| Temperature<br />(million kelvins)
!valign="bottom"| Density<br />({{val|u=kg/cm3}})
!valign="bottom"| Burn duration<br />(τ in years)
|- style="text-align:center;"
|| H
|| 37
|| 0.0045
|| 8.1&nbsp;million
|- style="text-align:center;"
|| He
|| 188
|| 0.97
|| 1.2&nbsp;million
|- style="text-align:center;"
|| C
|| 870
|| 170
|| 976
|- style="text-align:center;"
|| Ne
|| 1,570
|| 3,100
|| 0.6
|- style="text-align:center;"
|| O
|| 1,980
|| 5,550
|| 1.25
|- style="text-align:center;"
|| S/Si
|| 3,340
|| 33,400
|| {{convert|11.5|days|years|disp=out|abbr=values|sigfig=3}} (~11.5 days)
|}
{{Clear}}

==See also==
* [[List of proper names of stars]]
* [[Outline of astronomy]]
* [[Sidereal time]]
* [[Star clock]]s
* [[Star count]]
* [[Stars in fiction]]

==References==
{{reflist|30em|refs=
<ref name=koch95>{{cite book |last1= Koch-Westenholz |first1= Ulla |last2= Koch |first2= Ulla Susanne |date= 1995 |title= Mesopotamian astrology: an introduction to Babylonian and Assyrian celestial divination |page= 163 |volume= 19 |series= Carsten Niebuhr Institute Publications |publisher= Museum Tusculanum Press |isbn= 978-87-7289-287-0}}</ref>
<ref name= space_law09>{{cite book |last1= Lyall |first1= Francis |last2= Larsen |first2= Paul B. |title= Space Law: A Treatise |url= https://archive.org/details/spacelaw00lyal |url-access= limited |page= [https://archive.org/details/spacelaw00lyal/page/n190 176] |publisher= Ashgate Publishing, Ltd. |date= 2009 |isbn= 978-0-7546-4390-6 |chapter= Chapter 7: The Moon and Other Celestial Bodies}}</ref>
<ref name=astrometry05>{{cite web |title= Star naming |date= 2005 |publisher= Scientia Astrophysical Organization. |url= http://www.astrometry.org/starnaming.php |access-date= 2010-06-29 |url-status=dead |archive-url= https://web.archive.org/web/20100617054639/http://www.astrometry.org/starnaming.php |archive-date= 2010-06-17 }}</ref>
<ref name= bl_disclaimer>{{cite web |title= Disclaimer: Name a star, name a rose and other, similar enterprises |work= British Library |publisher= The British Library Board |url= http://www.bl.uk/names.html |archive-url= https://web.archive.org/web/20100119033625/http://www.bl.uk/names.html |archive-date= 2010-01-19 |access-date= 2010-06-29}}</ref>
<ref name= andersen10>{{cite web |first= Johannes |last= Andersen |title= Buying Stars and Star Names |publisher= International Astronomical Union |url= http://www.iau.org/public/buying_star_names/ |access-date= 2010-06-24}}</ref>
<ref name= pliat02>{{cite book |first= Philip C. |last= Plait |author-link= Phil Plait| date= 2002 |title= Bad astronomy: misconceptions and misuses revealed, from astrology to the moon landing "hoax" |pages= [https://archive.org/details/badastronomymisc00plai_621/page/n247 237]–240 |publisher= John Wiley and Sons |isbn= 978-0-471-40976-2|title-link= Bad astronomy: misconceptions and misuses revealed, from astrology to the moon landing "hoax" }}</ref>
<ref name=sclafani19980508>{{cite news |first= Tom |last= Sclafani |date=1998-05-08 |title= Consumer Affairs Commissioner Polonetsky Warns Consumers: "Buying A Star Won't Make You One" |publisher= National Astronomy and Ionosphere Center, Aricebo Observatory |url= http://www.naic.edu/~gibson/starnames/isr_news.html |archive-url= https://web.archive.org/web/20060111052632/http://www.naic.edu/~gibson/starnames/isr_news.html |url-status=dead |archive-date=2006-01-11 |access-date= 2010-06-24 }}</ref>
<ref name=Bond-140283>
{{cite journal
 |title=HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang
 |date=2013
 |author1=H. E. Bond |author2=E. P. Nelan |author3=D. A. VandenBerg |author4=G. H. Schaefer |author5=D. Harmer
 |doi=10.1088/2041-8205/765/1/L12
 |journal=[[The Astrophysical Journal Letters]]
 |volume=765 |pages=L12 |issue=1
 |arxiv=1302.3180
 |bibcode=2013ApJ...765L..12B
|s2cid=119247629
 }}</ref>
}}

==External links==
{{Wiktionary}}
{{commons category|Stars}}
* {{cite web |title= How To Decipher Classification Codes |publisher= Astronomical Society of South Australia |url= http://www.assa.org.au/sig/variables/classifications.asp |access-date= 2010-08-20}}
* {{cite web |last= Kaler |first= James |title= Portraits of Stars and their Constellations |publisher= University of Illinois |url= http://stars.astro.illinois.edu/sow/sow.html |access-date= 2010-08-20}}
* {{cite book |first1= Cliff |last1= Pickover |author-link= Cliff Pickover |date= 2001 |title= The Stars of Heaven |publisher= Oxford University Press |isbn= 978-0-19-514874-9 |url-access= registration |url= https://archive.org/details/starsofheaven00pick }}
* {{cite web |display-authors= 1 |last1= Prialnick |first1= Dina |last2= Wood |first2= Kenneth |last3= Bjorkman |first3= Jon |last4= Whitney |first4= Barbara |last5= Wolff |first5= Michael |last6= Gray |first6= David |last7= Mihalas |first7= Dimitri |title= Stars: Stellar Atmospheres, Structure, & Evolution |date= 2001 |publisher= University of St. Andrews |url= http://www-star.st-and.ac.uk/~kw25/teaching/stars/stars.html |access-date= 2010-08-20 |archive-date= 11 February 2021 |archive-url= https://web.archive.org/web/20210211123306/http://www-star.st-and.ac.uk/~kw25/teaching/stars/stars.html |url-status= dead }}
* {{cite web |title= Query star by identifier, coordinates or reference code |work= SIMBAD |publisher= Centre de Données astronomiques de Strasbourg |url= http://simbad.u-strasbg.fr/simbad/sim-fid |access-date= 2010-08-20}}

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