Black dwarf
Theoretical cooled white dwarf not yet existing in the universe.
Wikipedia / Wikimedia Commons
A black dwarf is a hypothetical stellar remnant that would form when a white dwarf cools enough to stop giving off significant heat or light. The time needed for a white dwarf to reach this state is calculated to be much longer than the current age of the universe, which is 13.79 billion years, so no black dwarfs are expected to exist today. The coolest white dwarfs observed provide a limit on how old the universe can be. The term "black dwarf" has also been used for a hypothetical late-stage cooled brown dwarf, a substellar object with less than about 0.07 solar masses, which cannot sustain hydrogen fusion.
A black dwarf would be made mostly of carbon and oxygen, with trace amounts of elements like neon and magnesium.
A white dwarf forms from a low- or medium-mass star (below about 9 to 10 solar masses) after it has expelled or fused all the elements it can. What remains is a dense sphere of electron-degenerate matter that slowly cools by radiating heat, eventually becoming a black dwarf. If black dwarfs existed, they would be hard to detect because they emit very little radiation, but they could be found through their gravitational pull. In 2012, astronomers using the MDM Observatory’s 2.4-meter telescope found white dwarfs cooler than 3,900 K (equivalent to spectral class M0), estimated to be 11 to 12 billion years old.
Exactly how long it takes a white dwarf to cool to blackness is uncertain because it depends on poorly understood physics, such as the nature of dark matter and whether protons decay (which has not been proven). Barrow and Tipler estimate it would take 10^15 years for a white dwarf to cool to 5 K. However, if weakly interacting massive particles (WIMPs) exist, they could keep some white dwarfs warmer for about 10^25 years. If protons are unstable, the energy from their decay would also warm white dwarfs. For a hypothetical proton lifetime of 10^37 years, Adams and Laughlin calculate that proton decay would raise the surface temperature of an old one-solar-mass white dwarf to about 0.06 K, which is still hotter than the cosmic microwave background radiation expected at that future time.
Some massive black dwarfs might eventually explode as supernovae. This could happen if pycnonuclear (density-based) fusion converts much of the star to nickel-56, which decays into iron by emitting a positron. This process would lower the Chandrasekhar l
- type
- Theoretical stellar remnant
- composition
- Mainly carbon and oxygen, with trace amounts of neon and magnesium
- formation
- Remnant of a low- or medium-mass star (below approximately 9 to 10 solar masses) after fusion ceases
- current_existence
- None expected, as cooling time exceeds universe age of 13.79 billion years
- detection_method
- Gravitational influence, as they emit very little radiation
- estimated_cooling_time
- At least 10^15 years to cool to 5 K, possibly longer if WIMPs exist
Lore & Background
A black dwarf forms from a white dwarf, which is the remnant of a main-sequence star of low or medium mass (below about 9 to 10 solar masses) after it has expelled or fused all elements for which it has sufficient temperature. The white dwarf is a dense sphere of electron-degenerate matter that cools slowly by thermal radiation, eventually becoming a black dwarf. If black dwarfs were to exist, they would be challenging to detect because they emit very little radiation, but they would be detectable through their gravitational influence. In 2012, astronomers using MDM Observatory's 2.4 meter telescope found various white dwarfs cooled below 3,900 K, estimated to be 11 to 12 billion years old.
Reader's Guide
Black dwarfs are significant as the ultimate fate of most stars, including the Sun, and as a probe of fundamental physics. Because the far-future evolution of stars depends on poorly understood questions such as the nature of dark matter and the possibility of proton decay, the exact time for white dwarfs to cool to blackness is uncertain. Barrow and Tipler estimate it would take 10^15 years to cool to 5 K, but if weakly interacting massive particles (WIMPs) exist, interactions may keep some white dwarfs warmer for about 10^25 years. If protons decay, that energy could also warm them. Some massive black dwarfs may eventually produce supernova explosions via pycnonuclear fusion, though proton decay could prevent this. The Sun will become a white dwarf in about 8 billion years and eventually a black dwarf after at least 10^15 years. These phenomena offer a method to verify the existence of WIMPs and black dwarfs.
Did You Know?
- No black dwarfs are expected to exist in the universe at the present time because the cooling time exceeds the current age of the universe (13.79 billion years).
- The coolest white dwarfs found in 2012 were estimated to be 11 to 12 billion years old and had temperatures below 3,900 K.
- If weakly interacting massive particles (WIMPs) exist, they may keep some white dwarfs much warmer for approximately 10^25 years.
- Some massive black dwarfs may explode as supernovae after up to 10^32000 years, but proton decay could prevent this.
More in Stars And Stellar Phenomena 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record