Editing White dwarf (section)

== Formation ==
White dwarfs are thought to represent the end point of [[stellar evolution]] for main-sequence stars with masses from about {{solar mass|0.07 to 10}}.<ref name="cosmochronology" /><ref name="evo">
{{cite journal
 |last1=Heger |first1=A.
 |last2=Fryer |first2=C. L.
 |last3=Woosley |first3=S. E.
 |last4=Langer |first4=N.
 |last5=Hartmann |first5=D. H.
 |date=2003
 |title=How Massive Single Stars End Their Life
 |journal=The Astrophysical Journal
 |volume=591 |issue=1
 |pages=288–300
 |arxiv= astro-ph/0212469
 |bibcode=2003ApJ...591..288H
 |doi= 10.1086/375341
 |s2cid=59065632
}}</ref> The composition of the white dwarf produced will depend on the initial mass of the star. Current galactic models suggest the Milky Way galaxy currently contains about ten billion white dwarfs.<ref>{{cite journal |doi=10.1088/1742-6596/172/1/012004 |title=The galactic population of white dwarfs |journal=Journal of Physics |series=Conference Series |volume=172 |page=012004 |year=2009 |last1=Napiwotzki |first1=Ralf |issue=1 |bibcode=2009JPhCS.172a2004N |arxiv=0903.2159|s2cid=17521113 }}</ref>

=== Stars with very low mass ===
If the mass of a main-sequence star is lower than approximately half a [[solar mass]], it will never become hot enough to ignite and fuse helium in its core.<ref name=Brown2011>{{cite journal |first1=J. M. |last1=Brown |first2=M. |last2=Kilic |first3=W. R. |last3=Brown |first4=S. J. |last4=Kenyon |date=2011 |title=The binary fraction of low-mass white dwarfs |journal=The Astrophysical Journal |volume=730 |number=67 |page=67 |doi=10.1088/0004-637X/730/2/67|arxiv=1101.5169 |bibcode=2011ApJ...730...67B }}</ref> It is thought that, over a lifespan that considerably exceeds the age of the universe ({{circa}} 13.8&nbsp;billion years),<ref name=aou>
{{cite journal
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 |last3=Doré     |first3=O.       |last4=Nolta    |first4=M.R.
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 |last21=Wollack |first21=E.      |last22=Wright  |first22=E.L.
 |display-authors=6
 |year=2007
 |title=Wilkinson Microwave Anisotropy Probe (WMAP) three year results: Implications for cosmology
 |journal=The Astrophysical Journal Supplement Series
 |volume=170  |issue=2  |pages=377–408
 |arxiv=astro-ph/0603449 |bibcode=2007ApJS..170..377S
 |doi=10.1086/513700  |s2cid=1386346
}}
</ref> such a star will eventually burn all its hydrogen, for a while becoming a [[Blue dwarf (red-dwarf stage)|blue dwarf]], and end its evolution as a helium white dwarf composed chiefly of [[helium-4]] nuclei.<ref>
{{cite journal
 |last1=Laughlin |first1=G.
 |last2=Bodenheimer |first2=P.
 |last3=Adams |first3=Fred C.
 |date=1997
 |title=The End of the Main Sequence
 |journal=The Astrophysical Journal
 |volume=482 |issue=1
 |pages=420–432
 |bibcode=1997ApJ...482..420L
 |doi=10.1086/304125
 |doi-access=free
}}</ref> Due to the very long time this process takes, it is not thought to be the origin of the observed helium white dwarfs. Rather, they are thought to be mostly the product of mass loss in binary systems.<ref name="rln" /><ref name="apj606_L147" /><ref name="he2" /><ref name="sj">{{cite web |url=http://star.arm.ac.uk/~csj/pus/astnow/astnow.html |title=Stars Beyond Maturity |archive-url=https://web.archive.org/web/20150404004046/http://star.arm.ac.uk/~csj/pus/astnow/astnow.html |archive-date=4 April 2015 |author=Jeffery, Simon |access-date=3 May 2007}}</ref><ref>
{{cite journal
 |last1=Sarna |first1=M. J.
 |last2=Ergma |first2=E.
 |last3=Gerškevitš |first3=J.
 |journal=Astronomische Nachrichten
 |date=2001
 |title=Helium core white dwarf evolution – including white dwarf companions to neutron stars
 |volume=322 |issue=5–6 |pages=405–410
 |bibcode=2001AN....322..405S
 |doi= 10.1002/1521-3994(200112)322:5/6<405::AID-ASNA405>3.0.CO;2-6
}}</ref><ref>
{{cite journal
 |last1=Benvenuto |first1=O. G.
 |last2=De Vito |first2=M. A.
 |date=2005
 |title=The formation of helium white dwarfs in close binary systems – II
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=362 |issue=3 |pages=891–905
 |bibcode=2005MNRAS.362..891B
 |doi= 10.1111/j.1365-2966.2005.09315.x |doi-access=free
}}</ref> Proposals to explain those helium white dwarfs that are not part of binary systems include mass loss due to a large planetary companion, stars being stripped of material by companions exploding as supernovae, and various types of stellar mergers.<ref>
{{cite journal
 |last1=Nelemans |first1=G.
 |last2=Tauris |first2=T. M.
 |title=Formation of undermassive single white dwarfs and the influence of planets on late stellar evolution
 |date=1998
 |journal=Astronomy and Astrophysics
 |volume=335 |pages=L85
 |arxiv= astro-ph/9806011
 |bibcode=1998A&A...335L..85N
}}</ref><ref>
{{cite journal
 |first1=Xianfei |last1=Zhang
 |first2=Philip D. |last2=Hall
 |first3=C. Simon |last3=Jeffery
 |first4=Shaolan |last4=Bi
 |year=2018
 |title=Evolution models of helium white dwarf–main-sequence star merger remnants: the mass distribution of single low-mass white dwarfs
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=474
 |pages=427–432
 |doi=10.1093/mnras/stx2747
 |doi-access=free
 |arxiv=1711.03285
}}</ref>

=== 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>

=== Stars with medium to high mass ===
If a star is massive enough, its core will eventually become sufficiently hot to fuse carbon to neon, and then to fuse neon to iron. Such a star will not become a white dwarf, because the mass of its central, non-fusing core, initially supported by electron degeneracy pressure, will eventually exceed the largest possible mass supportable by degeneracy pressure. At this point the core of the star will [[gravitational collapse|collapse]] and it will explode in a [[core-collapse supernova]] that will leave behind a remnant neutron star, [[black hole]], or possibly a more exotic form of [[compact star]].<ref name="evo" /><ref>
{{cite journal
 |bibcode=2005JPhG...31S.651S
 |arxiv=astro-ph/0412215
 |doi= 10.1088/0954-3899/31/6/004
 |title=Strange quark matter in stars: A general overview
 |date=2005
 |last1=Schaffner-Bielich |first1=Jürgen
 |journal=Journal of Physics G: Nuclear and Particle Physics
 |volume=31 |issue=6 |pages=S651–S657
 |s2cid=118886040
}}</ref> Some main-sequence stars, of perhaps {{solar mass|8 to 10}}, although sufficiently massive to [[Carbon-burning process|fuse carbon to neon and magnesium]], may be insufficiently massive to [[Neon-burning process|fuse neon]]. Such a star may leave a remnant white dwarf composed chiefly of [[oxygen]], neon, and [[magnesium]], provided that its core does not collapse, and provided that fusion does not proceed so violently as to blow apart the star in a [[supernova]].<ref>
{{cite journal
 |title=Evolution of 8–10&nbsp;solar mass stars toward electron capture supernovae. I – Formation of electron-degenerate O + NE + MG cores
 |date=1984
 |last1=Nomoto |first1=K.
 |journal=The Astrophysical Journal
 |volume=277 |page=791
 |bibcode=1984ApJ...277..791N
 |doi= 10.1086/161749 |doi-access=free
}}</ref><ref name=Woolsey2002>
{{cite journal
 |bibcode=2002RvMP...74.1015W
 |doi= 10.1103/RevModPhys.74.1015
 |title=The evolution and explosion of massive stars
 |date=2002
 |last1=Woosley |first1=S. E.
 |last2=Heger |first2=A.
 |last3= Weaver
 |first3= T. A.
 |journal=Reviews of Modern Physics
 |volume=74 |issue=4 |pages=1015–1071
}}</ref> Although a few white dwarfs have been identified that may be of this type, most evidence for the existence of such comes from the novae called ''ONeMg'' or ''neon'' novae. The spectra of these [[nova]]e exhibit abundances of neon, magnesium, and other intermediate-mass elements that appear to be only explicable by the accretion of material onto an oxygen–neon–magnesium white dwarf.<ref name="oxne" /><ref>
{{cite journal
 |bibcode=2004A&A...421.1169W
 |arxiv= astro-ph/0404325
 |doi= 10.1051/0004-6361:20047154
 |title=Chandra and FUSE spectroscopy of the hot bare stellar core H?1504+65
 |date=2004
 |last1=Werner |first1=K.
 |last2=Rauch |first2=T.
 |last3=Barstow |first3=M. A.
 |last4=Kruk |first4=J. W.
 |journal=Astronomy and Astrophysics
 |volume=421 |issue=3 |pages=1169–1183
 |s2cid= 2983893
}}</ref><ref>
{{cite journal
 |bibcode=1994ApJ...425..797L
 |doi= 10.1086/174024
 |title=On the interpretation and implications of nova abundances: An abundance of riches or an overabundance of enrichments
 |date=1994
 |last1=Livio |first1=Mario
 |last2=Truran |first2=James W.
 |journal=The Astrophysical Journal
 |volume=425 |page=797
 |doi-access=free
}}</ref>

=== Type Iax supernova ===
[[Type Ia supernova#Type Iax|Type Iax supernovae]], that involve helium accretion by a white dwarf, have been proposed to be a channel for transformation of this type of stellar remnant. In this scenario, the carbon detonation produced in a Type Ia supernova is too weak to destroy the white dwarf, expelling just a small part of its mass as ejecta, but produces an asymmetric explosion that kicks the star, often known as a ''[[zombie star]]'', to the high speeds of a [[hypervelocity star]]. The matter processed in the failed detonation is re-accreted by the white dwarf with the heaviest elements such as [[iron]] falling to its core where it accumulates.<ref name="ironcore">
{{cite journal
 |bibcode=2012ApJ...761L..23J
 |doi=10.1088/2041-8205/761/2/L23
 |title=Failed-detonation Supernovae: Subluminous Low-velocity Ia Supernovae and their Kicked Remnant White Dwarfs with Iron-rich Cores
 |date=2012
 |last1=Jordan |first1=George C. IV.
 |last2=Perets |first2=Hagai B.
 |last3=Fisher |first3=Robert T.
 |last4=van Rossum |first4=Daniel R.
 |journal=The Astrophysical Journal Letters
 |volume=761 |issue=2
 |pages=L23
 |arxiv = 1208.5069 |s2cid=119203015
}}</ref> These ''iron-core'' white dwarfs would be smaller than the carbon–oxygen kind of similar mass and would cool and crystallize faster than those.<ref name="ironcore2">
{{cite journal
 |bibcode=2000MNRAS.312..531P
 |doi=10.1046/j.1365-8711.2000.03236.x
 |title=The evolution of iron-core white dwarfs
 |date=2000
 |last1=Panei |first1=J. A.
 |last2=Althaus |first2=L. G.
 |last3=Benvenuto |first3=O. G.
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=312 |issue=3
 |pages=531–539
 |doi-access=free
 |arxiv = astro-ph/9911371 |s2cid=17854858
}}</ref>