White Dwarfs

White dwarfs are the most common stellar remnants and the fate of about 97% of stars. They form after a star with less than about eight times the sun's mass runs out of viable nuclear fuel in its core. Lacking the gravity to hold on to its outer layers, they are ejected by stellar winds and radiation pressure from the core. The leftover, Earth-sized core is the white dwarf, composed mostly of carbon and oxygen.

Unable to carry out nuclear fusion, white dwarfs resist collapsing under their own gravity due to quantum pressure exerted by electrons. However, electron degeneracy pressure can only support white dwarfs up to about 1.44 times the sun's mass—the Chandrasekhar limit. If a white dwarf gains mass and approaches this limit, carbon fusion ignites in a runaway thermonuclear explosion that obliterates the entire star and briefly outshines all others in its galaxy.

For their predictable peak luminosity, these supernovae are considered standard candles—tools for measuring cosmic distances. In 1998, scientists used these exploding stars to measure the universe's expansion and uncovered that it was accelerating. White dwarfs that don't accumulate matter or explode will gradually crystallize and cool into black dwarfs over quadrillions of years.