When questioned with What Is A Black Dwarf? we could simply answer that is a dead white one, but we feel it needs a little more explanation.
As a white dwarf cools, theory suggests that it will get to a point where it doesn’t emit heat or light any longer. To reach this stage it is thought that it would take longer than the 13,8 billion years that the Universe has existed, consequently it is thought no black dwarfs yet exist in our Universe.
A white dwarf is a main sequence star of lower or medium mass, that consists of electron-degenerate matter that cools very slowly through thermal radiation to become a black dwarf.
Whilst we assume that no black dwarfs exist due to the time required for them to form, if they do exist already they would be incredibly difficult to detect due to the fact that they are emitting very small amounts of radiation if at all. Having said that, identifying them by sight may be tricky, but they could be detected through the influence of their gravity upon other objects that we can see.
Some white dwarfs have been identified with temperatures below 3900 kelvin and astronomers suggest that their ages are in the region of 11-12 billion years. The science isn’t sufficiently well understood at this stage to establish how long it will take a white dwarf to cool to a black dwarf. We don’t fully understand the physical issues along with dark matter and the rate of proton decay.
Cooling of the white dwarfs may be influenced by energy created by decay. It is thought that during proton decay, energy would be released to keep the star warm for a longer period.
The use of the name Black Dwarf, has also been used in the past for sub-stellar low mass objects but these are generally now called brown dwarfs.