A white dwarf is a relatively small, dense type of star that is formed when a main sequence star burns all of its hydrogen and helium fuel but lacks the pressure and heat necessary to fuse carbon and oxygen. A white dwarf has a mass typically between 0.5 and 0.7 times that of the Sun, but a volume comparable to that of the Earth. White dwarfhood is the end state of stellar evolution for 97% of known stars.
The transformation of a star into a white dwarf begins when a main sequence star, around the mass of our Sun, burns up all its hydrogen fuel and starts being forced to fuse helium into carbon and oxygen. Because its core starts to build up with carbon and oxygen that cannot be fused, the fusion must take place on a shell outside the core. The immense gravity of the core pushes the hydrogen together and causes it to fuse much faster than before, increasing the luminosity of the star by a factor of 1,000 - 10,000 and increasing its radius to something comparable to Mars' orbit.
When all the hydrogen in the star is fused, gravity takes over and the star begins to fall in on itself. If the star is sufficiently massive, a supernova may occur. Otherwise, the excess material just floats away to form a planetary nebula, and only the super-dense core remains, which is the white dwarf. Because a white dwarf have no source of energy of its own, the only heat it produces is a remnant from its helium-fusing days. After billions of years, it is predicted that white dwarfs cool to become black dwarfs, lifeless stellar husks, although the age of the universe (13.7 billion years) has not been sufficient for this to occur yet.
White dwarfs make up 6% of all stars in our solar neighborhood. Because no nuclear reactions occur in their core, they aren't very bright, although they are observable with powerful telescopes. Sirius B, the companion of its more famous partner, Sirius A, also known as the Dog Star, is a white dwarf. The first white dwarf was observed by Friedrich Herschel on 31 January 1783, in a binary system, Eridani B and C.
