Black dwarf

A black dwarf is a hypothetical star, created when a white dwarf becomes sufficiently cool to no longer emit significant heat or light. Since the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe, 13.7 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.
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The theoretical cold remnant of a dead, low mass star. After a red giant has produced a planetary nebula and nuclear fusion has stopped in its core, the remains will collapse and form a white dwarf star. The white dwarf is very hot and will gradually radiate this heat energy into space. Since it is no longer producing energy, once this residue has been radiated it will be become a cold, dark body known as a black dwarf. The cooling times for a white dwarf are so large, however, that the universe may not yet be old enough to contain any black dwarfs. Detecting them is a huge problem, since they are so small and do not radiate any energy.