View source for Black dwarf

{{short description|Theoretical stellar remnant}}
{{hatnote group|
{{Other uses}}
{{distinguish|black hole|black star (semiclassical gravity)}}
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[[File:Star Life Cycle Chart.jpg|thumb|upright=1.8|Diagram of [[stellar evolution]], showing the various stages of stars with different masses]]

A '''black dwarf''' is a theoretical [[stellar remnant]], specifically a [[white dwarf]] that has cooled sufficiently to no longer emit significant [[heat]] or [[light]]. Because the time required for a [[white dwarf]] to reach this state is calculated to significantly exceed the current [[age of the universe]] (13.79 billion years), no black dwarfs are expected to exist in the universe at the present time. The temperature of the coolest [[White dwarf|white dwarfs]] is one observational limit on the [[Universe|universe's]] age.<ref name="2003ApJ...591..288H">{{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. |display-authors=2 |year=2003 |title=How Massive Single Stars End Their Life |url=https://iopscience.iop.org/article/10.1086/375341 |journal=The Astrophysical Journal |volume=591 |issue=1 |pages=288–300 |arxiv=astro-ph/0212469 |bibcode=2003ApJ...591..288H |doi=10.1086/375341 |access-date=25 March 2022 |s2cid=59065632}}</ref>

The name "black dwarf" has also been applied to hypothetical late-stage cooled [[brown dwarf]]s{{snd}}[[substellar object]]s with insufficient mass (less than approximately 0.07&nbsp;{{Solar mass|link=y}}) to maintain [[hydrogen]]-burning [[nuclear fusion]].<ref>{{cite journal |title=A failed search for black dwarfs as companions to nearby stars |first1=R. F. |last1=Jameson |first2=M. R. |last2=Sherrington |first3=A.R. |last3=Giles |date=October 1983 |pages=39–41 |bibcode=1983MNRAS.205P..39J |volume=205 |journal=Monthly Notices of the Royal Astronomical Society |doi=10.1093/mnras/205.1.39P|doi-access=free }}</ref><ref>{{cite journal |last=Kumar |first=Shiv S. |title=Study of Degeneracy in Very Light Stars |journal=Astronomical Journal |volume=67 |page=579 |date=1962 |doi=10.1086/108658 |bibcode=1962AJ.....67S.579K|doi-access=free }}</ref><ref>{{cite encyclopedia |url=http://www.daviddarling.info/encyclopedia/B/browndwarf.html |title=brown dwarf |encyclopedia=The Encyclopedia of Astrobiology, Astronomy, and Spaceflight |first=David |last= Darling | publisher=David Darling |via=daviddarling.info |access-date=May 24, 2007}}</ref><ref name=JillTarter2014>{{cite book |last=Tarter |first=Jill |title=50 Years of Brown Dwarfs |chapter=Brown is Not a Color: Introduction of the Term 'Brown Dwarf' |pages=19–24 |editor-last=Joergens |editor-first=Viki |series=Astrophysics and Space Science Library |volume=401 |publisher=Springer |date=2014 |isbn=978-3-319-01162-2 |chapter-url= https://www.springer.com/astronomy/book/978-3-319-01161-5 |doi=10.1007/978-3-319-01162-2_3}}</ref>

==Formation==
A white dwarf is what remains of a [[main sequence]] star of low or medium mass (below approximately 9 to 10&nbsp;solar masses ({{Solar mass|link=y}})) after it has either expelled or [[nuclear fusion|fused]] all the [[chemical element|element]]s for which it has sufficient temperature to fuse.<ref name="2003ApJ...591..288H" /> What is left is then a dense sphere of [[Degenerate matter#Electron degeneracy|electron-degenerate matter]] that cools slowly by [[thermal radiation]], eventually becoming a black dwarf.<ref name="on">{{cite web |url=http://www.astronomy.ohio-state.edu/~jaj/Ast162/lectures/notesWL22.pdf |title=Extreme Stars: White Dwarfs & Neutron Stars |first=Jennifer |last=Johnson |publisher=[[Ohio State University]] |access-date=2007-05-03 |df=dmy-all}}</ref><ref>{{cite web |last=Richmond |first=Michael |url=http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html |title=Late stages of evolution for low-mass stars |publisher=Rochester Institute of Technology |access-date=2006-08-04 |df=dmy-all}}</ref> 

If black dwarfs were to exist, they would be challenging to detect because, by definition, they would emit very little [[radiation]]. They would, however, be detectable through their [[gravity|gravitational]] influence.<ref>{{cite journal |title=Baryonic Dark Matter: The Results from Microlensing Surveys |first1=Charles |last1=Alcock |first2=Robyn A. |last2=Allsman |first3=David |last3=Alves |first4=Tim S. |last4=Axelrod |first5=Andrew C. |last5=Becker |first6=David |last6=Bennett |first7=Kem H. |last7=Cook |first8=Andrew J. |last8=Drake |first9=Ken C. |last9=Freeman |first10=Kim |last10=Griest |first11=Matt |last11=Lehner |first12=Stuart |last12=Marshall |first13=Dante |last13=Minniti |first14=Bruce |last14=Peterson |first15= Mark |last15=Pratt |first16=Peter |last16=Quinn |first17=Alex |last17=Rodgers |first18=Chris |last18=Stubbs |first19=Will |last19=Sutherland |first20=Austin |last20=Tomaney |first21=Thor |last21=Vandehei |first22=Doug L. |last22=Welch |display-authors=6 |year=1999 |bibcode=1999ASPC..165..362A |volume=165 |page=362 |journal=In the Third Stromlo Symposium: The Galactic Halo}}</ref> Various [[White dwarf|white dwarfs]] cooled below {{convert|3900|K|C F}} (equivalent to M0 [[Stellar classification|spectral class]]) were found in 2012 by astronomers using [[MDM Observatory]]'s 2.4&nbsp;meter telescope. They are estimated to be 11 to 12&nbsp;billion years old.<ref name=2examples>{{cite web |url=http://www.spacedaily.com/reports/12_Billion_Year_Old_White_Dwarf_Stars_Only_100_Light_Years_Away_999.html |title=12&nbsp;Billion-year-old white-dwarf stars only 100&nbsp;light-years away |work=spacedaily.com |date=April 16, 2012 |place=Norman, Oklahoma |access-date=January 10, 2020 |df=dmy-all}}</ref>

Because the far-future evolution of stars depends on physical questions which are poorly understood, such as the nature of [[dark matter]] and the possibility and rate of [[proton decay]] (which is yet to be proven to exist), it is not known precisely how long it would take white dwarfs to cool to blackness.<ref name="Adams">{{Cite journal |title=A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects |journal=Reviews of Modern Physics |volume=69 |issue=2 |pages=337–372 |doi=10.1103/RevModPhys.69.337 |first1=Fred C. |last1=Adams |first2=Gregory |last2=Laughlin |name-list-style=amp |arxiv=astro-ph/9701131 |bibcode=1997RvMP...69..337A |date=April 1997|s2cid=12173790 }}</ref>{{rp|§§IIIE, IVA}}  Barrow and Tipler estimate that it would take 10<sup>15</sup> years for a white dwarf to cool to {{convert|5|K|C F|abbr=on}};<ref>Table 10.2, {{BarrowTipler1986}}</ref> however, if [[weakly interacting massive particle]]s (WIMPs) exist, interactions with these particles may keep some white dwarfs much warmer than this for approximately 10<sup>25</sup> years.<ref name="Adams"/>{{rp|§IIIE}} If protons are not stable, white dwarfs will also be kept warm by energy released from proton decay. For a hypothetical proton lifetime of 10<sup>37</sup> years, Adams and Laughlin calculate that proton decay will raise the [[effective temperature|effective surface temperature]] of an old one-[[solar mass|solar-mass]] white dwarf to approximately {{convert|0.06|K|C F|2|abbr=on}}. Although cold, this is thought to be hotter than the [[cosmic microwave background radiation]] temperature 10<sup>37</sup> years in the future.<ref name= "Adams" />

It is speculated that some massive black dwarfs may eventually produce [[supernova]] explosions. These will occur if [[Pycnonuclear fusion|pycnonuclear]] (density-based) fusion processes much of the star to [[nickel-56]], which decays into iron via emitting a [[positron]]. This would lower the [[Chandrasekhar limit]] for some black dwarfs below their actual mass. If this point is reached, it would then collapse and initiate runaway nuclear fusion. The most massive to explode would be just below the Chandrasekhar limit at around 1.41 solar masses and would take of the order of {{val|e=1100|u=years}}, while the least massive to explode would be about 1.16 solar masses and would take of the order {{val|e=32000||u=years}}, totaling around 1% of all black dwarfs. One major caveat is that [[proton decay]] would decrease the mass of a black dwarf far more rapidly than pycnonuclear processes occur, preventing any supernova explosions.<ref name=caplan2020>{{cite journal |doi=10.1093/mnras/staa2262 |title=Black dwarf supernova in the far future |year=2020 |last1=Caplan |first1=M. E. |journal=Monthly Notices of the Royal Astronomical Society |volume=497 |issue=4 |pages=4357–4362 |doi-access=free |arxiv=2008.02296 |bibcode=2020MNRAS.497.4357C |s2cid=221005728 }}</ref>

A more recent research has investigated "the evaporation rate and decay time of a non-rotating star of constant density due to spacetime curvature-induced pair production and apply this to compact stellar remnants such as neutron stars and white dwarfs". In particular, the authors find that the characteristic evaporation time scale for white dwarfs is ≥ {{val|e=78|u=years}}, much lower than previous estimations.<ref name=falcke2025>{{cite journal | arxiv=2410.14734 | last1=Falcke | first1=Heino | last2=Wondrak | first2=Michael F. | last3=van Suijlekom | first3=Walter D. | title=An upper limit to the lifetime of stellar remnants from gravitational pair production | journal=Journal of Cosmology and Astroparticle Physics | date=2025 | issue=5 | page=023 | doi=10.1088/1475-7516/2025/05/023 | bibcode=2025JCAP...05..023F }}</ref> However, other authors have pointed out that gravitational pair creation cannot take place for objects such as white or black dwarfs, so the lifetime of a black dwarf would probably be the one previously estimated.<ref name=ferreiro2024>{{cite journal |doi=10.1103/PhysRevLett.133.229001 |title=Comment on ''Gravitational Pair Production and Black Hole Evaporation'' |year=2024 |last1=Ferreiro |first1=A. |last2=Navarro-Salas |first2=J. |last3=Pla |first3=S. |journal=Physical Review Letters |volume=133 |issue=22 |pages=229001|doi-access=free }}</ref>

==Future of the Sun==
Once the [[Sun]] stops fusing helium in its core and ejects its layers in a [[planetary nebula]] in about 8&nbsp;billion years, it will become a [[white dwarf]] and also, over trillions of years, eventually will no longer emit any light. After that, the Sun will not be visible to the equivalent of the [[naked eye|naked human eye]], removing it from optical view even if the gravitational effects are evident. The estimated time for the Sun to cool enough to become a black dwarf is at least 10<sup>15</sup> (1 quadrillion) years, though it could take much longer than this, if [[weakly interacting massive particle]]s (WIMPs) exist, as described above. The described phenomena are considered a promising method of verification for the existence of WIMPs and black dwarfs.<ref>{{Cite journal |last1=Kouvaris |first1=Chris |last2=Tinyakov |first2=Peter |date=2011-04-14 |title=Constraining asymmetric dark matter through observations of compact stars |url=https://link.aps.org/doi/10.1103/PhysRevD.83.083512 |journal=Physical Review D |language=en |volume=83 |issue=8 |pages=083512 |doi=10.1103/PhysRevD.83.083512 |arxiv=1012.2039 |bibcode=2011PhRvD..83h3512K |s2cid=55279522 |issn=1550-7998}}</ref> 

==See also==
{{Wiktionary}}
* {{annotated link|Degenerate matter}}
* {{annotated link|Heat death of the universe}}

==References==
{{reflist|25em}}

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