A black dwarf is a white dwarf that has sufficiently cooled to no longer emit 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 (13.8 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 for which it has sufficient temperature to fuse. What is left is then a dense ball of electron-degenerate matter that cools slowly by thermal radiation, eventually becoming a black dwarf. 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.
Because 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., § IIIE, IVA. Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K; 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 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 temperature that the cosmic background radiation will have 1037 years in the future., §IVB.
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^R. F. Jameson, M. R. Sherrington, and A. R. Giles (October 1983). "A failed search for black dwarfs as companions to nearby stars". Royal Astronomical Society205: 39–41. Bibcode:1983MNRAS.205P..39J.Cite uses deprecated parameters (help)
^brown dwarf, entry in The Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling, accessed online May 24, 2007.