A star that is the remnant core of a star that has completed fusion in its core. The sun will become a white dwarf. White dwarfs are typically composed primarily of carbon, have about the radius of the earth, and do not significantly evolve further. Often found in planetary nebulae.
It can be within a wide range of temperatures between 100,000 and 4,000 Kelvin. They have no internal heat sources but instead they gradually radiate their residual energy until they become black dwarfs. The upper mass limit of a white dwarf is 1.4 solar masses (the Chandrasekhar Limit). Larger than this and the white dwarf would become a neutron star, composed of baryon degenerate matter. White dwarfs in binary star systems can produce novae and type I supernovae.
The Sun will one day evolve into a white dwarf with a diameter of 10 000 km.