Black dwarf

A black dwarf is a hypothetical small, very dense, cold, dead star. It is made mostly of carbon. This dark star is what remains after a red giant star loses its outer layers, forming a planetary nebula and then a white dwarf. The nuclear core of a black dwarf is depleted. Black dwarfs are about the size of a standard planet (but tremendously heavier).

At this point in the history of the Universe, no black dwarf stars exist; it is uncertain if they ever will, considering the process of a white dwarf turning into a black dwarf may take longer than the age of the Universe itself.