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Stellar evolution
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===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>
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