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== Formation and evolution ==
{{Star formation}}
{{Main|Star formation|Protostar|Pre-main-sequence star|Stellar evolution#Main sequence stellar mass objects}}
[[File:Hot and brilliant O stars in star-forming regions.jpg|thumb|left|upright=1.2|Hot and brilliant [[O-type main-sequence star]]s in star-forming regions. These are all regions of star formation that contain many hot young stars including several bright stars of spectral type O.<ref>{{cite news |title=The Brightest Stars Don't Live Alone |newspaper=ESO Press Release |url=https://www.eso.org/public/news/eso1230/ |access-date=27 July 2012}}</ref>]]
When a [[protostar]] is formed from the [[Jeans instability|collapse]] of a [[giant molecular cloud]] of gas and dust in the local [[interstellar medium]], the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium, and trace amounts of other elements, by mass.<ref name=asr34_1/> The initial mass of the star depends on the local conditions within the cloud. (The mass distribution of newly formed stars is described empirically by the [[initial mass function]].)<ref name=science295_5552/> During the initial collapse, this [[pre-main-sequence star]] generates energy through gravitational contraction. Once sufficiently dense, stars begin converting hydrogen into helium and giving off energy through an [[exothermic]] [[nuclear fusion]] process.<ref name=tnc/>
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