White dwarfs are the former cores of low mass stars such as the Sun. Essentially, no nuclear reactions are taking place. These objects are merely taking a long, long time to cool down. There are a lot of white dwarfs in our galaxy. Amoung the first white dwarfs to be recognized was the faint companion of the brilliant blue-white star Sirius. A white dwarf can be no more than 1.4 solar masses, the Chandrasekhar limit. If the white dwarf is in a binary system, it may be possible to get recurring explosions on the surface. A typical white dwarf is approximately the size of Earth, but that value depends on the mass. It is primarily made of degenerate gas. White dwarfs are the end result of a red giant phase for low mass stars. As the interior becomes more and more unstable, the outer layers are no longer held by gravity. Finally the star sheds its outer layers out into space. This ejected material surrounds the stellar core as what's called a planetary nebula, such as the famous Ring Nebula or the Owl Nebula. Planetary nebulæ are given that name because they do look a bit like planets in the telescope, not because they have anything to do with planets. The exposed stellar core is now observed as a white dwarf. More than eight solar masses? Then your star will likely become a neutron star.