{{short description|Theoretical stellar remnant}} {{hatnote group| {{Other uses}} {{About|the hypothetical cold remnant|the supernova remnant|black hole|the remnant in semiclassical theory|black star (semiclassical gravity)}} }}
{{Infobox astronomical formation| |thing=Hypothetical class of future stellar remnant |Mass=Up to Chandrasekhar limit (non-rotating) |temp=Under 798 K (Draper point) |luminosity=negligible (visual)<br/>≤ 23 kW/m{{sup|2}} (total) |head=|name=Black dwarf}} 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 dwarfs is one observational limit on the 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 dwarfs{{snd}}substellar objects with insufficient mass (less than approximately 0.07 {{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>
==Composition== A black dwarf would be mainly composed of carbon and oxygen, but it may also have trace amounts of other elements like neon and magnesium.<ref>{{Cite web|url=https://news.illinoisstate.edu/2020/08/black-dwarf-supernova-isu-physicist-calculates-when-the-last-supernova-ever-will-happen/|title=‘Black dwarf supernova’: ISU physicist calculates when the last supernova ever will happen|website=News.illinoisstate.edu|access-date=May 24, 2026}}</ref><ref>{{cite web|url=https://homework.study.com/explanation/what-is-a-black-dwarf-made-of.html|title=What is a black dwarf made of?|website=Homework.study.com|access-date=2026-05-24}}</ref>
==Formation== A white dwarf is what remains of a main sequence star of low or medium mass (below approximately 9 to 10 solar masses ({{Solar mass|link=y}})) after it has either expelled or fused all the elements for which it has sufficient temperature to fuse.<ref name="2003ApJ...591..288H" /> What is left is then a dense sphere of 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 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 dwarfs cooled below {{convert|3900|K|C F}} (equivalent to M0 spectral class) were found in 2012 by astronomers using MDM Observatory's 2.4 meter telescope. They are estimated to be 11 to 12 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 Billion-year-old white-dwarf stars only 100 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 particles (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 surface temperature of an old one-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 (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, or the maximum mass of a stable white dwarf star, 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>
==Future of the Sun== Once the Sun stops fusing helium in its core and ejects its layers in a planetary nebula in about 8 billion years, it will become a white dwarf and, over trillions of years, eventually no longer emit any light. After that, the Sun will not be visible to the equivalent of the 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 particles (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 |article-number=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}} * {{anl|Carbon planet}} * {{Annotated link|Terrestrial planet}} * {{Annotated link|Chthonian planet}}
==References== {{reflist|25em}}
{{white dwarf}} {{Star}} {{Portal bar|Astronomy|Stars|Outer space}} {{DEFAULTSORT:Black Dwarf}} + Category:Stellar evolution Category:Hypothetical stars