Black dwarf

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.77 billion years), no black dwarfs are expected to exist in the universe as of now, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe.[1]

For other uses, see Black dwarf (disambiguation).
Not to be confused with black hole or black star (semiclassical gravity).
Stellar evolution.

The name "black dwarf" has also been applied to hypothetical late-stage cooled brown dwarfs – substellar objects that do not have sufficient mass (less than approximately 0.08 M☉) to maintain hydrogen-burning nuclear fusion.[2][3][4][5]

Black dwarfs should not be confused with black holes or black stars.

Formation

A white dwarf is what remains of a main-sequence star of low or medium mass (below approximately 9 to 10 solar masses (M☉)) after it has either expelled or fused all the elements for which it has sufficient temperature to fuse.[1] What is left is then a dense sphere of electron-degenerate matter that cools slowly by thermal radiation, eventually becoming a black dwarf.[6][7] If black dwarfs were to exist, they would be extremely difficult to detect, because, by definition, they would emit very little radiation. They would, however, be detectable through their gravitational influence.[8] Various white dwarfs cooled below 3900 K (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.[9]

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, it is not known precisely how long it will take white dwarfs to cool to blackness.[10]:§§IIIE, IVA Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K;[11] however, if weakly interacting massive particles (WIMPs) exist, it is possible that interactions with these particles will keep some white dwarfs much warmer than this for approximately 1025 years.[10]:§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 1037 years, Adams and Laughlin calculate that proton decay will raise the effective surface temperature of an old one-solar-mass white dwarf to approximately 0.06 K. Although cold, this is thought to be hotter than the cosmic background radiation temperature 1037 years in the future.[10]:§IVB

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 iron, which would lower the Chandrasekhar limit for some black dwarfs below their actual mass. If this point is reached, then it would collapse and initiate runaway nuclear fusion. The most massive to explode would be near 1.35 solar masses and would take of the order of 101100 years, while the least massive to explode would be about 1.16 solar masses and would take of the order 1032000 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.[12]

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 will 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 about 1015 (1 quadrillion) years, though it could take much longer than this, if weakly interacting massive particles (WIMPs) exist, as described above.

See also
  • Degenerate matter – Collection of free, non-interacting particles with a pressure and other physical characteristics determined by quantum mechanical effects
  • Heat death of the universe – Possible fate of the universe

References
  1. §3, Heger, A.; Fryer, C.L.; Woosley, S.E.; Langer, N.; Hartmann, D.H. (2003). "How massive single stars end their life". Astrophysical Journal. 591 (1): 288–300. arXiv:astro-ph/0212469. Bibcode:2003ApJ...591..288H. doi:10.1086/375341. S2CID 59065632.
  2. Jameson, R. F.; Sherrington, M. R.; Giles, A.R. (October 1983). "A failed search for black dwarfs as companions to nearby stars". Monthly Notices of the Royal Astronomical Society. 205: 39–41. Bibcode:1983MNRAS.205P..39J. doi:10.1093/mnras/205.1.39P.
  3. Kumar, Shiv S. (1962). "Study of Degeneracy in Very Light Stars". Astronomical Journal. 67: 579. Bibcode:1962AJ.....67S.579K. doi:10.1086/108658.
  4. Darling, David. "brown dwarf". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. David Darling. Retrieved May 24, 2007 – via daviddarling.info.
  5. Tarter, Jill (2014), "Brown is not a color: Introduction of the term 'Brown Dwarf'", in Joergens, Viki (ed.), 50 Years of Brown Dwarfs – From Prediction to Discovery to Forefront of Research, Astrophysics and Space Science Library, 401, Springer, pp. 19–24, doi:10.1007/978-3-319-01162-2_3, ISBN 978-3-319-01162-2
  6. Johnson, Jennifer. "Extreme Stars: White Dwarfs & Neutron Stars" (PDF). Ohio State University. Retrieved 3 May 2007.
  7. Richmond, Michael. "Late stages of evolution for low-mass stars". Rochester Institute of Technology. Retrieved 4 August 2006.
  8. Alcock, Charles; Allsman, Robyn A.; Alves, David; Axelrod, Tim S.; Becker, Andrew C.; Bennett, David; et al. (1999). "Baryonic Dark Matter: The Results from Microlensing Surveys". In the Third Stromlo Symposium: The Galactic Halo. 165: 362. Bibcode:1999ASPC..165..362A.
  9. "12 Billion-year-old white-dwarf stars only 100 light-years away". spacedaily.com. Norman, Oklahoma. 16 April 2012. Retrieved 10 January 2020.
  10. Adams, Fred C. & Laughlin, Gregory (April 1997). "A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects". Reviews of Modern Physics. 69 (2): 337–372. arXiv:astro-ph/9701131. Bibcode:1997RvMP...69..337A. doi:10.1103/RevModPhys.69.337. S2CID 12173790.
  11. Table 10.2, Barrow, John D.; Tipler, Frank J. (1986). The Anthropic Cosmological Principle 1st edition 1986 (revised 1988). Oxford University Press. ISBN 978-0-19-282147-8. LCCN 87028148.
  12. Caplan, M. E. (2020). "Black dwarf supernova in the far future". Monthly Notices of the Royal Astronomical Society. 497 (4): 4357–4362. arXiv:2008.02296. Bibcode:2020MNRAS.497.4357C. doi:10.1093/mnras/staa2262. S2CID 221005728.
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