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Lithopsian (talk | contribs) add an image which shows the zero agai main sequence |
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{{Main|Star formation|Protostar|Pre-main-sequence star|Stellar evolution#Main sequence stellar mass objects}}
[[File:Zams and tracks.png|thumb|left|Zero age main sequence and evolutionary tracks]]
[[File:The violent youth of solar proxies.jpg|thumb|The violent youth of stars like the Sun]]
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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