Black dwarf

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits 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.

The name "black dwarf" has also been applied to substellar objects that do not have sufficient mass, less than approximately 0.08 M_☉, to maintain hydrogen-burning nuclear fusion. These objects are now generally called brown dwarfs, a term coined in the 1970s. Black dwarfs should not be confused with black holes, black stars, or neutron stars.

A white dwarf is what remains of a main-sequence star of low or medium mass (below approximately 9 to 10 solar masses (M_☉)) 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 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 extremely difficult to detect, because, by definition, they would emit very little radiation. They would, however, be detectable through their gravitational influence. Various white dwarfs cooled below 3900 K (M0 spectral class) have been found in 2012 by astronomers using MDM Observatory's 2.4-meter telescope. They are estimated to be 11 to 12 billion years old.

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