Black dwarf

From Wikipedia, the free encyclopedia

For other uses, see Black dwarf (disambiguation).

A black dwarf is a hypothetical star, created when a white dwarf becomes sufficiently cool to no longer emit significant heat or light. Since the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe, 13.7 billion years, no black dwarfs are expected to exist in the universe yet, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe. A white dwarf is what remains of a main sequence star of low or medium mass (below approximately 9 to 10 solar masses), after it has either expelled or fused all the elements which it has sufficient temperature to fuse.[1] What is left is then a dense piece of electron-degenerate matter which cools slowly by thermal radiation, eventually becoming a black dwarf.[2][3] If black dwarfs were to exist, they would be extremely difficult to detect, since, by definition, they would emit very little radiation. One theory is that they might be detectable through their gravitational influence.[4]

Since the far-future evolution of white dwarfs depends on physical questions, such as the nature of dark matter and the possibility and rate of proton decay, which are poorly understood, it is not known precisely how long it will take white dwarfs to cool to blackness.[5], § IIIE, IVA. Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K[6]; however, if weakly interacting massive particles exist, it is possible that interactions with these particles will keep some white dwarfs much warmer than this for approximately 1025 years.[5], § IIIE. If the proton is 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 temperature that the cosmic background radiation will have 1037 years in the future.[5], §IVB.

The name black dwarf has also been applied to sub-stellar objects which do not have sufficient mass, approximately 0.08 solar masses, to maintain hydrogen-burning nuclear fusion.[7] These objects are now generally called brown dwarfs, a term coined in the 1970s.[8] Also, black dwarfs should not be confused with black holes or neutron stars.

[edit] See also

Look up Black dwarf in
Wiktionary, the free dictionary.

[edit] 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. doi:10.1086/375341. 
  2. ^ Johnson, Jennifer. Extreme Stars: White Dwarfs & Neutron Stars. Ohio State University. Retrieved on 2007-05-03.
  3. ^ Richmond, Michael. Late stages of evolution for low-mass stars. Rochester Institute of Technology. Retrieved on 2006-08-04.
  4. ^ Charles Alcock, Robyn A. Allsman, David Alves, Tim S. Axelrod, Andrew C. Becker, David Bennett, Kem H. Cook, Andrew J. Drake, Ken C. Freeman, Kim Griest, Matt Lehner, Stuart Marshall, Dante Minniti, Bruce Peterson, Mark Pratt, Peter Quinn, Alex Rodgers, Chris Stubbs, Will Sutherland, Austin Tomaney, Thor Vandehei, Doug L. Welch (1999). Baryonic Dark Matter: The Results from Microlensing Surveys.
  5. ^ a b c A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects, Fred C. Adams and Gregory Laughlin, arXiv:astro-ph/9701131v1.
  6. ^ Table 10.2, The Anthropic Cosmological Principle, John D. Barrow and Frank J. Tipler, Oxford: Oxford University Press, 1986. ISBN 0-19-282147-4.
  7. ^ R. F. Jameson, M. R. Sherrington, and A. R. Giles (October, 1983). A failed search for black dwarfs as companions to nearby stars pp 39-41.
  8. ^ brown dwarf, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, accessed online May 24, 2007.