Black dwarf

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 of 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. [§3, cite journal
author=Heger, A.; Fryer, C. L.; Woosley, S. E.; Langer, N.; Hartmann, D. H.
title=How Massive Single Stars End Their Life
journal=Astrophysical Journal
year=2003 | volume=591 | issue=1 | pages=288–300
url=http://adsabs.harvard.edu/abs/2003ApJ...591..288H
accessdate=2007-08-14 | doi = 10.1086/375341] What is left is then a dense piece of electron-degenerate matter which cools slowly by thermal radiation, eventually becoming a black dwarf.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|accessdate=2007-05-03] [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 | accessdate = 2006-08-04 ] 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. [cite web|url=http://adsabs.harvard.edu/abs/1999ASPC..165..362A|title=Baryonic Dark Matter: The Results from Microlensing Surveys|author=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|date=1999]

Since the far-future evolution of white dwarfs depends on physical questions, such as thenature of dark matter and the possibility and rate of proton decay, which are poorlyunderstood, it is not known precisely how long it will take white dwarfs to cool to blackness. [http://xxx.lanl.gov/abs/astro-ph/9701131v1 A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects] , Fred C. Adams and Gregory Laughlin, arXiv:astro-ph/9701131v1.] ^{, § IIIE, IVA.} Barrow and Tipler estimate that it would take 10¹⁵ years for a white dwarf to cool to 5 K [Table 10.2, "The Anthropic Cosmological Principle", John D. Barrow and Frank J. Tipler, Oxford: Oxford University Press, 1986. ISBN 0-19-282147-4.] ; 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.^{, § 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.^{, §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. [cite web|url=http://adsabs.harvard.edu/abs/1983MNRAS.205P..39J|title=A failed search for black dwarfs as companions to nearby stars|author=R. F. Jameson, M. R. Sherrington, and A. R. Giles|date=October, 1983|pages=pp 39-41] These objects are now generally called "brown dwarfs", a term coined in the 1970s. [ [http://www.daviddarling.info/encyclopedia/B/browndwarf.html brown dwarf] , entry in "The Encyclopedia of Astrobiology, Astronomy, and Spaceflight", David Darling, accessed online May 24, 2007.] Also, black dwarfs should not be confused with black holes or neutron stars.

ee also

* White dwarf
* Brown dwarf
* Degenerate matter
* Heat death of the universe

References