Editing Stellar evolution (section)

==Star formation==
[[File:Starlifesimple.png|thumb|upright=1.4|Simplistic representation of the stages of stellar evolution]]
{{Main|Star formation}}

===Protostar===
{{Main|Protostar}}
[[Image:Stellar evolution L vs T.png|thumb|upright=1.2|right|Schematic of stellar evolution]]
Stellar evolution starts with the [[gravitational collapse]] of a [[giant molecular cloud]]. Typical giant molecular clouds are roughly {{convert|100|ly|km}} across and contain up to {{convert|6000000|solar mass|kg|lk=on}}. As it collapses, a giant molecular cloud breaks into smaller and smaller pieces. In each of these fragments, the collapsing gas releases [[Gravitational potential|gravitational potential energy]] as heat. As its temperature and pressure increase, a fragment condenses into a rotating ball of superhot gas known as a [[protostar]].<ref>{{harvtxt|Prialnik|2000|loc=Chapter 10}}</ref>  Filamentary structures are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, which are the precursors of stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control the detailed fragmentation manner of the filaments. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing comparable to the filament inner width, and embedded two protostars with gas outflows.<ref>{{cite journal |last1=Zhang |first1=Guo-Yin |last2=André |first2=Ph. |last3=Men'shchikov |first3=A. |last4=Wang |first4=Ke |title=Fragmentation of star-forming filaments in the X-shaped nebula of the California molecular cloud |journal=Astronomy and Astrophysics |date=1 October 2020 |volume=642 |pages=A76 |doi=10.1051/0004-6361/202037721 |bibcode=2020A&A...642A..76Z |issn=0004-6361|arxiv=2002.05984 |s2cid=211126855 }}</ref>

A protostar continues to grow by [[Accretion (astrophysics)|accretion]] of gas and dust from the molecular cloud, becoming a [[pre-main-sequence star]] as it reaches its final mass. Further development is determined by its mass. Mass is typically compared to the mass of the [[Sun]]: {{convert|1.0|solar mass|kg|abbr=on}} means 1 solar mass.

[[Protostar]]s are encompassed in dust, and are thus more readily visible at [[infrared]] wavelengths.
Observations from the [[Wide-field Infrared Survey Explorer]] (WISE) have been especially important for unveiling numerous galactic [[protostar]]s and their parent [[star cluster]]s.<ref name=wright>{{cite web|url=http://wise.ssl.berkeley.edu/ |title=Wide-field Infrared Survey Explorer Mission |publisher=NASA}}</ref><ref name=ma2013>Majaess, D. (2013). [http://adsabs.harvard.edu/abs/2013Ap%26SS.344..175M ''Discovering protostars and their host clusters via WISE''], ApSS, 344, 1 ([http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J%2Fother%2FApSS%2F344%2E175 ''VizieR catalog''])</ref>

===Brown dwarfs and sub-stellar objects===
{{Main|Brown dwarf}}
Protostars with masses less than roughly {{convert|0.08|solar mass|kg|abbr=on}} never reach temperatures high enough for [[nuclear fusion]] of hydrogen to begin. These are known as [[brown dwarf]]s. The [[International Astronomical Union]] defines brown dwarfs as stars massive enough to [[deuterium burning|fuse deuterium]] at some point in their lives (13 [[Jupiter mass]]es ({{Jupiter mass|link=y}}), 2.5&nbsp;&times;&nbsp;10<sup>28</sup>&nbsp;kg, or {{Solar mass|0.0125}}). Objects smaller than {{Jupiter mass|13}} are classified as [[sub-brown dwarf]]s (but if they orbit around another stellar object they are classified as planets).<ref>{{cite web|title=Working Group on Extrasolar Planets: Definition of a "Planet" |work=IAU position statement |date=2003-02-28 |url=http://www.dtm.ciw.edu/boss/definition.html |access-date=2012-05-30 |url-status=dead |archive-url=https://web.archive.org/web/20120204173630/http://www.dtm.ciw.edu/boss/definition.html |archive-date=February 4, 2012 }}</ref> Both types, deuterium-burning and not, shine dimly and fade away slowly, cooling gradually over hundreds of millions of years.

===Main sequence stellar mass objects===
{{Main|Main sequence}}
{{Annotated image|image-width=325|width=325
|caption=The evolutionary tracks of [[stars]] with different initial masses on the [[Hertzsprung–Russell diagram]]. The tracks start once the star has evolved to the [[main sequence]] and stop when [[Nuclear fusion|fusion]] stops (for massive stars) and at the end of the [[red-giant branch]] (for stars {{solar mass|1}} and less).<ref>{{harvtxt|Prialnik|2000|loc=Fig. 8.19, p. 174}}</ref><br/>A yellow track is shown for the [[Sun]], which will become a [[red giant]] after its main-sequence phase ends before expanding further along the [[asymptotic giant branch]], which will be the last phase in which the Sun undergoes fusion.
|imagemap=
<imagemap>
Image:Zams and tracks.png|325px
poly 380 230 780 530 750 560 350 260 [[Main Sequence]]
desc none
</imagemap>
|annotations=
{{Annotation|65|45|[[Wolf-Rayet star|WR]]}}
{{Annotation|170|35|[[Luminous blue variable|LBV]]}}
{{Annotation|230|35|[[Yellow hypergiant|YHG]]}}
{{Annotation|130|45|[[Blue supergiant|BSG]]}}
{{Annotation|270|65|[[Red supergiant|RSG]]}}
{{Annotation|290|100|[[Asymptotic giant branch|AGB]]}}
{{Annotation|270|130|[[Red giant|RG]]}}
}}
For a more-massive protostar, the core temperature will eventually reach 10 million [[kelvin]], initiating the [[proton–proton chain reaction]] and allowing [[hydrogen]] to fuse, first to [[deuterium]] and then to [[helium]]. In stars of slightly over {{convert|1|solar mass|kg|abbr=on}}, the carbon–nitrogen–oxygen fusion reaction ([[CNO cycle]]) contributes a large portion of the energy generation. The onset of nuclear fusion leads relatively quickly to a [[hydrostatic equilibrium]] in which energy released by the core maintains a high gas pressure, balancing the weight of the star's matter and preventing further gravitational collapse. The star thus evolves rapidly to a stable state, beginning the [[main sequence|main-sequence]] phase of its evolution.

A new star will sit at a specific point on the main sequence of the [[Hertzsprung–Russell diagram]], with the main-sequence [[spectral type]] depending upon the mass of the star. Small, relatively cold, low-mass [[red dwarf]]s fuse hydrogen slowly and will remain on the main sequence for hundreds of billions of years or longer, whereas massive, hot [[O-type main-sequence star|O-type stars]] will leave the main sequence after just a few million years. A mid-sized [[G-type main-sequence star|yellow dwarf]] star, like the Sun, will remain on the main sequence for about 10 billion years. The Sun is thought to be in the middle of its main sequence lifespan.

===Planetary system===
A star may gain a [[protoplanetary disk]], which furthermore can develop into a [[planetary system]].