Ablack dwarf is a theoreticalstellar remnant, specifically awhite 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 currentage 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 theuniverse's age.[1]
A white dwarf is what remains of amain sequence star of low or medium mass (below approximately 9 to 10 solar masses (M☉)) after it has either expelled orfused all theelements for which it has sufficient temperature to fuse.[1] What is left is then a dense sphere ofelectron-degenerate matter that cools slowly bythermal radiation, eventually becoming a black dwarf.[8][9]
If black dwarfs were to exist, they would be challenging to detect because, by definition, they would emit very littleradiation. They would, however, be detectable through theirgravitational influence.[10] Variouswhite dwarfs cooled below 3,900 K (3,630 °C; 6,560 °F) (equivalent to M0spectral class) were found in 2012 by astronomers usingMDM Observatory's 2.4 meter telescope. They are estimated to be 11 to 12 billion years old.[11]
Because the far-future evolution of stars depends on physical questions which are poorly understood, such as the nature ofdark matter and the possibility and rate ofproton 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.[12]: §§IIIE, IVA Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K (−268.15 °C; −450.67 °F);[13] however, ifweakly interacting massive particles (WIMPs) exist, interactions with these particles may keep some white dwarfs much warmer than this for approximately 1025 years.[12]: §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 theeffective surface temperature of an old one-solar-mass white dwarf to approximately 0.06 K (−273.09 °C; −459.56 °F). Although cold, this is thought to be hotter than thecosmic microwave background radiation temperature 1037 years in the future.[12]
It is speculated that some massive black dwarfs may eventually producesupernova explosions. These will occur ifpycnonuclear (density-based) fusion processes much of the star tonickel-56, which decays into iron via emitting apositron. This would lower theChandrasekhar 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 of101100 years, while the least massive to explode would be about 1.16 solar masses and would take of the order1032000 years, totaling around 1% of all black dwarfs. One major caveat is thatproton decay would decrease the mass of a black dwarf far more rapidly than pycnonuclear processes occur, preventing any supernova explosions.[14]
Once theSun stops fusinghelium in its core and ejects its layers in aplanetary nebula in about 8 billion years, it will become awhite dwarf and, over trillions of years, eventually no longer emit any light. After that, the Sun will not be visible to the equivalent of thenaked 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 1015 (1 quadrillion) years, though it could take much longer than this, ifweakly 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.[15]
^Darling, David."brown dwarf".The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. David Darling. RetrievedMay 24, 2007 – via daviddarling.info.
^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.
