Editing Horizontal branch (section)

==Evolution==
[[File:Evolutionary track 1m.svg|thumb|upright=1.2|The evolutionary track of a sun-like star, showing the horizontal branch and red clump region]]
After exhausting their core hydrogen, stars leave the [[main sequence]] and begin [[thermonuclear fusion|fusion]] in a hydrogen shell around the helium core and become [[giant star|giants]] on the [[red-giant branch]].  In stars with masses up to 2.3 times the mass of the [[Sun]] the helium core becomes a region of [[degenerate matter]] that does not contribute to the generation of [[energy]]. It continues to grow and increase in [[temperature]] as the [[hydrogen fusion]] in the shell contributes more [[helium]].<ref name=karttunen_oja2007/>

If the [[star]] has more than about 0.5 [[solar mass]]es,<ref>{{cite web|title=Post Main Sequence Stars|url=http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_postmain.html|publisher=Australia Telescope Outreach and Education|access-date=2 December 2012}}</ref><!-- previously listed as 0.26 solar mass, but unable to find citation --> the core eventually reaches the temperature necessary for the [[Stellar nucleosynthesis|fusion]] of helium into carbon through the [[triple-alpha process]]. The initiation of [[helium fusion]] begins across the core region, which will cause an immediate temperature rise and a rapid increase in the rate of [[Stellar nucleosynthesis|fusion]].  Within a few seconds the core becomes non-[[Degenerate matter|degenerate]] and quickly expands, producing an event called [[helium flash]]. Non-degenerate cores initiate fusion more smoothly, without a flash.  The output of this event is absorbed by the layers of [[plasma (physics)|plasma]] above, so the effects are not seen from the exterior of the star. The star now changes to a new [[Hydrostatic equilibrium|equilibrium]] state, and its evolutionary path switches from the [[red-giant branch]] (RGB) onto the horizontal branch of the [[Hertzsprung–Russell diagram]].<ref name=karttunen_oja2007/>

Stars initially between about {{solar mass|2.3}} and {{solar mass|8}} have larger helium cores that do not become degenerate.  Instead their cores reach the [[Schönberg–Chandrasekhar limit|Schönberg–Chandrasekhar mass]] at which they are no longer in hydrostatic or thermal equilibrium.  They then contract and heat up, which triggers helium fusion before the core becomes degenerate.  These stars also become hotter during core helium fusion, but they have different core masses and hence different luminosities from HB stars.  They vary in temperature during core helium fusion and perform a [[blue loop]] before moving to the asymptotic giant branch.  Stars more massive than about {{solar mass|8}} also ignite their core helium smoothly, and also go on to burn heavier elements as a [[red supergiant]].<ref name=salaris>{{cite journal|bibcode=2005essp.book.....S|title=Evolution of Stars and Stellar Populations|url=https://archive.org/details/evolutionofstars0000sala|url-access=registration|journal=Evolution of Stars and Stellar Populations|pages=400|last1=Salaris|first1=Maurizio|last2=Cassisi|first2=Santi|year=2005}}</ref>

Stars remain on the horizontal branch for around 100 million years, becoming slowly more luminous in the same way that main sequence stars increase luminosity as the [[virial theorem]] shows.  When their core helium is eventually exhausted, they progress to helium shell burning on the [[asymptotic giant branch]] (AGB).  On the AGB they become cooler and much more luminous.<ref name=karttunen_oja2007/>