nova

Long ago, when a 'new star' suddenly appeared in the sky where none had been, observers called it a nova — Latin for 'new'. But a nova is not a new star at all; it is an old, dead white dwarf that has briefly flared thousands of times brighter, fooling the eye into thinking a star was born. The white dwarf was always there, too faint to see; the sudden burst of light is the giveaway of a thermonuclear explosion on its surface.

Here is the mechanism. A white dwarf in a close binary steadily pulls hydrogen-rich gas off its companion star. That stolen gas piles up on the white dwarf's surface, growing denser and hotter, until — like a layer of fuel reaching its flash point — it ignites in a runaway nuclear fusion explosion across the whole surface. The blast flings the accumulated material into space and brightens the system by a factor of thousands to a million for days to months, then it fades. Crucially, the white dwarf survives, and over years to millennia the gas piles up again, so many novae recur.

Novae matter both as cosmic distance markers and as chemical factories, seeding space with elements forged in the surface flash. But the deepest reason they matter is the contrast with their explosive cousins. A nova is a surface skin burning off, leaving the white dwarf intact; a Type Ia supernova is the entire white dwarf detonating once it nears the Chandrasekhar mass, leaving nothing behind. The two are easy to confuse by name but are utterly different in scale — a supernova is roughly a million times brighter and is a true stellar death.