Editing White dwarf (section)

{{short description|Type of stellar remnant composed mostly of electron-degenerate matter}}
{{hatnote group|
{{redirect-distinguish|Degenerate dwarf|Degenerate star}}
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{{Use dmy dates|date=December 2020}}
[[File:Sirius A and B Hubble photo.editted.PNG|thumb|Image of [[Sirius]]&nbsp;A and Sirius&nbsp;B taken by the [[Hubble Space Telescope]]. Sirius&nbsp;B, which is a white dwarf, can be seen as a faint point of light to the lower left of the much brighter Sirius&nbsp;A.]]

A '''white dwarf''' is a [[Compact star|stellar core remnant]] composed mostly of [[electron-degenerate matter]]. A white dwarf is very [[density|dense]]: in an [[Earth]] sized volume, it packs a mass that is comparable to the [[Sun]]. No [[nuclear fusion]] takes place in a white dwarf; what light it radiates is from its residual [[heat]].<ref name="osln" /> The nearest known white dwarf is [[Sirius B]], at 8.6&nbsp;light years, the smaller component of the Sirius [[binary star]]. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun.<ref>
{{cite web
 |last=Henry |first=T.J.  |author-link=Todd J. Henry
 |date=1 January 2009
 |title=The one hundred nearest star systems
 |publisher=[[Research Consortium on Nearby Stars]]
 |url=http://www.astro.gsu.edu/RECONS/TOP100.posted.htm
 |access-date=21 July 2010  |url-status=live
 |archive-url=https://web.archive.org/web/20071112173559/http://www.chara.gsu.edu/RECONS/TOP100.posted.htm
 |archive-date=12 November 2007
}}
</ref> The unusual faintness of white dwarfs was first recognized in 1910.<ref name="schatzman"/>{{rp|page=1}} The name ''white dwarf'' was coined by [[Willem Jacob Luyten]] in 1922.

White dwarfs are thought to be the final [[stellar evolution|evolutionary state]] of stars whose [[mass]] is not high enough to become a [[neutron star]] or [[black hole]]. This includes over 97% of the stars in the [[Milky Way]].<ref name="cosmochronology"/>{{rp|§1}} After the [[hydrogen]]-[[stellar nucleosynthesis|fusing]] period of a [[main sequence|main-sequence star]] of [[Stellar mass|low or intermediate mass]] ends, such a star will expand to a [[red giant]] and fuse [[helium]] to [[carbon]] and [[oxygen]] in its core by the [[triple-alpha process]]. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon (around {{val|e=9|u=K}}), an inert mass of carbon and oxygen will build up at its center. After such a star sheds its outer layers and forms a [[planetary nebula]], it will leave behind a core, which is the remnant white dwarf.<ref name="rln">
{{cite web
 |last=Richmond  |first=M.
 |title=Late stages of evolution for low-mass stars
 |series=Lecture notes, Physics&nbsp;230
 |publisher=[[Rochester Institute of Technology]]
 |url=http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html
 |access-date=3 May 2007  |url-status=live
 |archive-url=https://web.archive.org/web/20170904224040/http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html
 |archive-date=4 September 2017
}}
</ref> Usually, white dwarfs are composed of carbon and oxygen ('''CO white dwarf'''). If the mass of the progenitor is between 7 and 9&nbsp;[[solar mass]]es ({{solar mass|link=y}}), the core temperature will be sufficient to fuse carbon but not [[neon]], in which case an oxygen–neon–[[magnesium]] ('''ONeMg''' or '''ONe''') white dwarf may form.<ref name="oxne">
{{cite conference
 |last1=Werner   |first1=K.     |last2=Hammer |first2=N.J.
 |last3=Nagel    |first3=T.     |last4=Rauch  |first4=T.
 |last5=Dreizler |first5=S.
 |year=2005
 |title=On possible oxygen / neon white dwarfs: H1504+65 and the white dwarf donors in ultracompact X-ray binaries
 |conference=14th European Workshop on White Dwarfs
 |volume=334  |page=165
 |arxiv=astro-ph/0410690
 |bibcode=2005ASPC..334..165W
}}
</ref> Stars of very low mass will be unable to fuse helium; hence, a helium white dwarf<ref name="apj606_L147">
{{cite journal
 |last1=Liebert    |first1=James  |last2=Bergeron |first2=P.
 |last3=Eisenstein |first3=D.     |last4=Harris   |first4=H. C.
 |last5=Kleinman   |first5=S. J.  |last6=Nitta    |first6=A.
 |last7=Krzesinski |first7=J.
 |year=2004
 |title=A helium white dwarf of extremely low mass
 |journal=[[The Astrophysical Journal]]
 |volume=606  |issue=2  |pages=L147
 |arxiv=astro-ph/0404291  |bibcode=2004ApJ...606L.147L
 |s2cid=118894713   |doi=10.1086/421462 
}}
</ref><ref name="he2">
{{cite press release
 |date=17 April 2007
 |title=Cosmic weight loss: The lowest mass white dwarf
 |publisher=[[Harvard-Smithsonian Center for Astrophysics]]
 |url=http://spaceflightnow.com/news/n0704/17whitedwarf
 |access-date=20 April 2007  |url-status=live
 |archive-url=https://web.archive.org/web/20070422034650/http://spaceflightnow.com/news/n0704/17whitedwarf/
 |archive-date=22 April 2007
}}
</ref> may be formed by mass loss in an [[interacting binary star]] system.<ref>{{cite journal
 | title=Evolution of Helium White Dwarfs of Low and Intermediate Masses
 | last1=Althaus | first1=L. G. | last2=Benvenuto | first2=O. G.
 | journal=The Astrophysical Journal
 | volume=477 | issue=1 | pages=313–334 | date=March 1997
 | doi=10.1086/303686 | bibcode=1997ApJ...477..313A }}</ref>

Because the material in a white dwarf no longer undergoes fusion reactions, it lacks a heat source to support it against [[gravitational collapse]]. Instead, it is supported only by [[electron degeneracy pressure]], causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the [[Chandrasekhar limit]]{{Mdash}}  approximately 1.44&nbsp;times {{solar mass|link=y}}{{Mdash}} beyond which electron degeneracy pressure cannot support it. A carbon–oxygen white dwarf which approaches this limit, typically by mass transfer from a companion star, may explode as a [[Type Ia supernova]] via a process known as [[carbon detonation]];<ref name=osln>
{{cite web
 |last=Johnson  |first=J.
 |year=2007
 |title=Extreme stars: White dwarfs & neutron stars
 |type=Lecture notes |series=Astronomy&nbsp;162
 |publisher=[[Ohio State University]]
 |url=http://www.astronomy.ohio-state.edu/~jaj/Ast162/lectures/notesWL22.html
 |access-date=17 October 2011  |url-status=live
 |archive-url=https://web.archive.org/web/20120331194342/http://www.astronomy.ohio-state.edu/~jaj/Ast162/lectures/notesWL22.html
 |archive-date=31 March 2012
}}
</ref><ref name="rln"/> [[SN 1006]] is a likely example.

A white dwarf, very hot when it forms, gradually cools as it radiates its energy. This radiation, which initially has a high [[color temperature]], lessens and reddens over time. Eventually a white dwarf will cool enough that its material will begin to crystallize into a cold [[black dwarf]].<ref name="cosmochronology">
{{cite journal
 |last1=Fontaine |first1=G.
 |last2=Brassard |first2=P.
 |last3=Bergeron |first3=P.
 |year=2001
 |title=The potential of white dwarf cosmochronology
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=113  |issue=782  |pages=409–435
 |bibcode=2001PASP..113..409F
 |doi=10.1086/319535  |doi-access=free
}}
</ref> The oldest known white dwarfs still radiate at temperatures of a few thousand [[kelvin]]s, which establishes an observational limit on the maximum possible [[age of the universe]].<ref name="evo" />