A Type Ia supernova is the thermonuclear detonation of a white dwarf that has gained too much matter from a companion star. Because the explosion occurs near a consistent mass limit, these events reach a predictable peak brightness, which makes them standard candles for measuring cosmic distance. Comparing how faint distant ones appear against their redshift is how two teams discovered in 1998 that the expansion of the universe is accelerating, the finding that established dark energy. Exactly what triggers the detonation is still debated.