A black dwarf is a theoretical astronomical object, constituting the remains of a Sun-sized star which has fused all of its original hydrogen and helium fuel to heavier elements such as carbon, oxygen and nitrogen and subsequently lost its remaining energy due to radiation. In other words, a black dwarf is a white dwarf that has cooled down so that it no longer emits heat or light. None are known to exist in our universe, as the time taken for a white dwarf to cool to such a degree is hypothesized to be longer than the age of the universe to date. It is possible, though, that some black dwarfs currently exist in other universes that are old enough to have them.

Even at the epoch when black dwarfs exist they may be extremely difficult to detect, emitting very little thermal radiation (if any) at a temperature not much above that of the cosmic microwave background radiation. They may be detectable through their gravitational influence.

Black dwarfs should not be confused with brown dwarfs, which are formed when gas contracts to form a star, but does not possess enough mass to initiate and sustain hydrogen nuclear fusion. "Brown dwarfs" were at times called "black dwarfs" in the 1960s.