Editing Stellar evolution (section)

====Horizontal branch====
{{Main|Horizontal branch|Red clump}}
In the helium cores of stars in the 0.6 to 2.0 solar mass range, which are largely supported by [[electron degeneracy pressure]], helium fusion will ignite on a timescale of days in a [[helium flash]]. In the nondegenerate cores of more massive stars, the ignition of helium fusion occurs relatively slowly with no flash.<ref>{{harvtxt|Ryan|Norton|2010|page=125}}</ref> The nuclear power released during the helium flash is very large, on the order of 10<sup>8</sup> times the [[Solar luminosity|luminosity of the Sun]] for a few days<ref name="RyanNorton115" /> and 10<sup>11</sup> times the luminosity of the Sun (roughly the luminosity of the [[Milky Way Galaxy]]) for a few seconds.<ref name="Prialnik151">{{harvtxt|Prialnik|2000|page=151}}</ref> However, the energy is consumed by the thermal expansion of the initially degenerate core and thus cannot be seen from outside the star.<ref name="RyanNorton115" /><ref name="Prialnik151" /><ref name="Deupree1996">{{cite journal|last1= Deupree|first1=R. G.|title= A Reexamination of the Core Helium Flash|journal= The Astrophysical Journal|volume=471|issue= 1|date= 1996-11-01|pages= 377–384|doi= 10.1086/177976|bibcode= 1996ApJ...471..377D|citeseerx= 10.1.1.31.44|s2cid=15585754 }}</ref> Due to the expansion of the core, the hydrogen fusion in the overlying layers slows and total energy generation decreases. The star contracts, although not all the way to the main sequence, and it migrates to the [[horizontal branch]] on the Hertzsprung–Russell diagram, gradually shrinking in radius and increasing its surface temperature.
[[File:The life cycle of a Sun-like star (annotated).jpg|thumb|upright=1.35|left|The change in size with time of a Sun-like star]]

Core helium flash stars evolve to the red end of the horizontal branch but do not migrate to higher temperatures before they gain a degenerate carbon-oxygen core and start helium shell burning.  These stars are often observed as a [[red clump]] of stars in the colour-magnitude diagram of a cluster, hotter and less luminous than the red giants. Higher-mass stars with larger helium cores move along the horizontal branch to higher temperatures, some becoming unstable pulsating stars in the yellow [[instability strip]] ([[RR Lyrae variables]]), whereas some become even hotter and can form a blue tail or blue hook to the horizontal branch. The morphology of the horizontal branch depends on parameters such as metallicity, age, and helium content, but the exact details are still being modelled.<ref name=parameters>{{Cite journal | last1 = Gratton | first1 = R. G. | last2 = Carretta | first2 = E. | last3 = Bragaglia | first3 = A. | last4 = Lucatello | first4 = S. | last5 = d'Orazi | first5 = V. | title = The second and third parameters of the horizontal branch in globular clusters | doi = 10.1051/0004-6361/200912572 | journal = Astronomy and Astrophysics | volume = 517 | pages = A81 | year = 2010 |arxiv = 1004.3862 |bibcode = 2010A&A...517A..81G | s2cid = 55701280 }}</ref>