Editing Red-giant branch (section)

===Leaving the red-giant branch===
A degenerate core begins fusion explosively in an event known as the [[helium flash]], but externally there is little immediate sign of it. The energy is consumed in lifting the degeneracy in the core. The star overall becomes less luminous and hotter and migrates to the horizontal branch. All degenerate helium cores have approximately the same mass, regardless of the total stellar mass, so the helium fusion luminosity on the horizontal branch is the same. Hydrogen shell fusion can cause the total stellar luminosity to vary, but for most stars at near solar metallicity, the temperature and luminosity are very similar at the cool end of the horizontal branch. These stars form the [[red clump]] at about 5,000 K and {{solar luminosity|50}}. Less massive hydrogen envelopes cause the stars to take up a hotter and less luminous position on the horizontal branch, and this effect occurs more readily at low metallicity so that old metal-poor clusters show the most pronounced horizontal branches.<ref name=salaris2005/><ref name=dantona>{{cite journal|arxiv=astro-ph/0209331|bibcode=2002A&A...395...69D|doi=10.1051/0004-6361:20021220|title=Helium variation due to self-pollution among Globular Cluster stars|journal=Astronomy and Astrophysics|volume=395|pages=69–76|year=2002|last1=d'Antona|first1=F.|last2=Caloi|first2=V.|last3=Montalbán|first3=J.|last4=Ventura|first4=P.|last5=Gratton|first5=R.|s2cid=15262502}}</ref>

Stars initially more massive than {{solar mass|2}} have non-degenerate helium cores on the red-giant branch. These stars become hot enough to start triple-alpha fusion before they reach the tip of the red-giant branch and before the core becomes degenerate. They then leave the red-giant branch and perform a blue loop before returning to join the asymptotic giant branch. Stars only a little more massive than {{solar mass|2}} perform a barely noticeable blue loop at a few hundred {{solar luminosity}} before continuing on the AGB hardly distinguishable from their red-giant branch position. More massive stars perform extended blue loops which can reach 10,000 K or more at luminosities of {{solar luminosity|thousands of}}. These stars will cross the [[instability strip]] more than once and pulsate as [[Classical Cepheid variable|Type I (Classical) Cepheid variable]]s.<ref name=bono>{{cite journal|bibcode=2000ApJ...543..955B|arxiv=astro-ph/0006251|title=Intermediate-Mass Star Models with Different Helium and Metal Contents|journal=The Astrophysical Journal|volume=543|issue=2|pages=955|last1=Bono|first1=Giuseppe|last2=Caputo|first2=Filippina|last3=Cassisi|first3=Santi|last4=Marconi|first4=Marcella|last5=Piersanti|first5=Luciano|last6=Tornambè|first6=Amedeo|year=2000|doi=10.1086/317156|s2cid=18898755}}</ref>