A white dwarf is a small, dense star that remains after a medium-sized star, like our Sun, has exhausted its nuclear fuel. When these stars run out of energy, they shed their outer layers, leaving behind a hot core that gradually cools over time. White dwarfs are primarily composed of carbon and oxygen and are supported against gravitational collapse by electron degeneracy pressure. They are typically about the size of Earth but contain a mass comparable to that of the Sun. Eventually, they will fade and become black dwarfs, although this process takes billions of years.