Editing White dwarf (section)

=== Stars with low to medium mass ===
If the mass of a main-sequence star is between {{solar mass|0.5 and 8}},<ref name=Brown2011/><ref name=Woolsey2002/> its core will become sufficiently hot to fuse helium into [[carbon]] and [[oxygen]] via the [[triple-alpha process]], but it will never become sufficiently hot to fuse carbon into [[neon]]. Near the end of the period in which it undergoes fusion reactions, such a star will have a carbon–oxygen core that does not undergo fusion reactions, surrounded by an inner helium-burning shell and an outer hydrogen-burning shell. On the Hertzsprung–Russell diagram, it will be found on the asymptotic giant branch. It will then expel most of its outer material, creating a [[planetary nebula]], until only the carbon–oxygen core is left. This process is responsible for the carbon–oxygen white dwarfs that form the vast majority of observed white dwarfs.<ref name="sj" /><ref name="vd1">{{cite web |url=http://www.vikdhillon.staff.shef.ac.uk/teaching/phy213/phy213_lowmass.html |title=The evolution of low-mass stars |archive-url=https://web.archive.org/web/20121107125754/http://www.vikdhillon.staff.shef.ac.uk/teaching/phy213/phy213_lowmass.html |archive-date=7 November 2012 |author=Dhillon, Vik |series=lecture notes, Physics 213 |publisher=University of Sheffield |access-date=3 May 2007}}</ref><ref name="vd2">{{cite web |url=http://www.vikdhillon.staff.shef.ac.uk/teaching/phy213/phy213_highmass.html |title=The evolution of high-mass stars |archive-url=https://web.archive.org/web/20121107125747/http://www.vikdhillon.staff.shef.ac.uk/teaching/phy213/phy213_highmass.html |archive-date=7 November 2012 |author=Dhillon, Vik |series=lecture notes, Physics 213 |publisher=University of Sheffield |access-date=3 May 2007}}</ref>

White dwarfs with a mass greater than {{val|1.05|u=Solar mass}} are termed ultramassive white dwarfs. When formed in single-star systems, these are expected to have an oxygen-neon core. However, a significant fraction (~20%) of ultramassive white dwarfs are formed through white dwarf mergers. In this case the result is a carbon-oxygen ultramassive white dwarf.<ref name=Camisassa_et_al_2021>{{cite journal
 | title=Forever young white dwarfs: When stellar ageing stops
 | last1=Camisassa | first1=María E. | last2=Althaus | first2=Leandro G.
 | last3=Torres | first3=Santiago | last4=Córsico | first4=Alejandro H.
 | last5=Rebassa-Mansergas | first5=Alberto | last6=Tremblay | first6=Pier-Emmanuel
 | last7=Cheng | first7=Sihao | last8=Raddi | first8=Roberto
 | display-authors=1 | journal=Astronomy & Astrophysics
 | volume=649 | at=id. L7 | date=May 2021
 | doi=10.1051/0004-6361/202140720 | arxiv=2008.03028
 | bibcode=2021A&A...649L...7C }}</ref>