Editing Main sequence (section)

== 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/>

When nuclear fusion of hydrogen becomes the dominant energy production process and the excess energy gained from gravitational contraction has been lost,<ref name=science293_5538/> the star lies along a [[curve]] on the [[Hertzsprung–Russell diagram]] (or HR diagram) called the standard main sequence. Astronomers will sometimes refer to this stage as "zero-age main sequence", or ZAMS.<ref name=zams_sao/><ref name=Hansen1999/> The ZAMS curve can be calculated using computer models of stellar properties at the point when stars begin hydrogen fusion. From this point, the brightness and surface temperature of stars typically increase with age.<ref name=clayton83/>

A star remains near its initial position on the main sequence until a significant amount of hydrogen in the core has been consumed, then begins to evolve into a more luminous star. (On the HR diagram, the evolving star moves up and to the right of the main sequence.) Thus the main sequence represents the primary hydrogen-burning stage of a star's lifetime.<ref name=tnc/>