A white dwarf is a small, dense star that remains after a medium-sized star has exhausted its nuclear fuel. These stars are typically about the size of Earth but contain a mass similar to that of the Sun. As they no longer undergo fusion, they gradually cool and fade over time. White dwarfs are primarily composed of carbon and oxygen, formed from the remnants of the star's core. They are supported against gravitational collapse by electron degeneracy pressure, a quantum mechanical effect. Eventually, they may become black dwarfs when they cool completely, although this process takes longer than the current age of the universe.