Editing Red-giant branch (section)

===Subgiant phase===
After a main-sequence star has exhausted its core hydrogen, it begins to fuse hydrogen in a thick shell around a core consisting largely of helium. The mass of the helium core is below the [[Schönberg–Chandrasekhar limit]] and is in [[thermal equilibrium]], and the star is a [[subgiant]]. Any additional energy production from the shell fusion is consumed in inflating the envelope and the star cools but does not increase in luminosity.<ref name=catelan>{{cite conference|bibcode=2007AIPC..930...39C|arxiv=astro-ph/0703724|title=Structure and Evolution of Low-Mass Stars: An Overview and Some Open Problems|conference=GRADUATE SCHOOL IN ASTRONOMY: XI Special Courses at the National Observatory of Rio de Janeiro (XI CCE). |series=AIP Conference Proceedings|volume=930|pages=39–90|last1=Catelan|first1=Márcio|last2=Roig|first2=Fernando|last3=Alcaniz|first3=Jailson|last4=de la Reza|first4=Ramiro|last5=Lopes|first5=Dalton|year=2007|doi=10.1063/1.2790333|s2cid=15599804}}</ref>

Shell hydrogen fusion continues in stars of roughly solar mass until the helium core increases in mass sufficiently that it becomes [[degenerate matter|degenerate]]. The core then shrinks, heats up and develops a strong temperature gradient. The hydrogen shell, fusing via the temperature-sensitive [[CNO cycle]], greatly increases its rate of energy production and the stars is considered to be at the foot of the red-giant branch. For a star the same mass as the sun, this takes approximately 2 billion years from the time that hydrogen was exhausted in the core.<ref name=salaris2005>{{cite book|bibcode=2005essp.book.....S|title=Evolution of Stars and Stellar Populations|url=https://archive.org/details/evolutionofstars0000sala|url-access=registration|pages=400|last1=Salaris|first1=Maurizio|last2=Cassisi|first2=Santi|year=2005}}</ref>

Subgiants more than about {{solar mass|2}} reach the Schönberg–Chandrasekhar limit relatively quickly before the core becomes degenerate. The core still supports its own weight thermodynamically with the help of energy from the hydrogen shell, but is no longer in thermal equilibrium. It shrinks and heats causing the hydrogen shell to become thinner and the stellar envelope to inflate. This combination decreases luminosity as the star cools towards the foot of the RGB. Before the core becomes degenerate, the outer hydrogen envelope becomes opaque which causes the star to stop cooling, increases the rate of fusion in the shell, and the star has entered the RGB. In these stars, the subgiant phase occurs within a few million years, causing an apparent gap in the Hertzsprung–Russell diagram between [[B-type main-sequence star]]s and the RGB seen in young [[open cluster]]s such as [[Praesepe]]. This is the [[Hertzsprung gap]] and is actually sparsely populated with subgiant stars rapidly evolving towards red giants, in contrast to the short densely populated low-mass subgiant branch seen in older clusters such as [[ω Centauri]].<ref name=mermilliod>{{cite journal|bibcode=1981A&A....97..235M|title=Comparative studies of young open clusters. III – Empirical isochronous curves and the zero age main sequence|journal=Astronomy and Astrophysics|volume=97|pages=235|last1=Mermilliod|first1=J. C.|year=1981}}</ref><ref name=bedin>{{cite journal|bibcode=2004ApJ...605L.125B|arxiv=astro-ph/0403112|title=Ω Centauri: The Population Puzzle Goes Deeper|journal=The Astrophysical Journal|volume=605|issue=2|pages=L125|last1=Bedin|first1=Luigi R.|last2=Piotto|first2=Giampaolo|last3=Anderson|first3=Jay|last4=Cassisi|first4=Santi|last5=King|first5=Ivan R.|last6=Momany|first6=Yazan|last7=Carraro|first7=Giovanni|year=2004|doi=10.1086/420847|s2cid=2799751|url=https://zenodo.org/record/968404}}</ref>