One of the giants of the Milky Way stands high overhead at nightfall. Deneb marks the tail of Cygnus, the swan. It’s a supergiant — about 15 times as massive as the Sun, and more than a hundred times as wide. And it shines more than 50,000 times brighter. Sometime in the next couple of million years, though, Deneb will get even brighter — but only for a while. The nuclear reactions in its core will stop, so the core will collapse. But its outer layers will blast into space as a supernova. For a few weeks, it’ll be the brightest object in the entire galaxy — brighter than the combined light of billions of normal stars. That fate awaits all supergiants. But some much smaller stars will also become supernovae: the dead cores of Sun-like stars. A white dwarf is about as massive as the Sun, but only as big as Earth. If it’s alone, then it’ll spend billions of years cooling and fading from view. If it has a companion, though, it could face a more spectacular ending. If the stars are close, t
