Editing Stellar evolution (section)

===Low-mass stars===
What happens after a low-mass star ceases to produce energy through fusion has not been directly observed; the [[universe]] is around 13.8 billion years old, which is less time (by several orders of magnitude, in some cases) than it takes for fusion to cease in such stars.

Recent astrophysical models suggest that [[red dwarf]]s of {{Solar mass|0.1}} may stay on the main sequence for some six to twelve trillion years, gradually increasing in both [[temperature]] and [[luminosity]], and take several hundred billion years more to collapse, slowly, into a [[white dwarf]].<ref name="S&T 22">{{cite journal| title=Why the Smallest Stars Stay Small| journal=Sky & Telescope|date=November 1997| issue=22}}</ref><ref>{{cite journal| journal=Astronomische Nachrichten| volume= 326| issue=10| pages= 913–919| date= 2005| title=M dwarfs: planet formation and long term evolution| first=F. C.|last= Adams| author2= P. Bodenheimer| author3=G. Laughlin|bibcode=2005AN....326..913A|doi=10.1002/asna.200510440| doi-access=free}}</ref>  Such stars will not become red giants as the whole star is a [[convection zone]] and it will not develop a degenerate helium core with a shell burning hydrogen.  Instead, hydrogen fusion will proceed until almost the whole star is helium.

Slightly more [[massive star]]s do expand into [[red giant]]s, but their helium cores are not massive enough to reach the temperatures required for helium fusion so they never reach the tip of the red-giant branch.  When hydrogen shell burning finishes, these stars move directly off the red-giant branch like a post-[[Asymptotic giant branch|asymptotic-giant-branch]] (AGB) star, but at lower luminosity, to become a white dwarf.<ref name="endms" />  A star with an initial mass about {{Solar mass|0.6}} will be able to reach temperatures high enough to fuse helium, and these "mid-sized" stars go on to further stages of evolution beyond the red-giant branch.<ref name="lejeune">{{cite journal|last1=Lejeune|first1=T|last2=Schaerer|first2=D|year=2001|title=Database of Geneva stellar evolution tracks and isochrones for <math>(UBV)_\mathsf{J}(RI)_\mathsf{C} JHKLL'M</math>, HST-WFPC2, Geneva and Washington photometric systems|journal=Astronomy & Astrophysics|volume=366|issue=2|pages=538–546|bibcode=2001A&A...366..538L|doi=10.1051/0004-6361:20000214|arxiv=astro-ph/0011497|s2cid=6708419}}</ref>