Some of the brightest jewels in the universe are also some of the smallest: the exploding cores of dead stars. A star like the Sun spends most of its time “fusing” the hydrogen fuel in its core to make helium. It then spends a shorter time fusing the helium to make carbon and oxygen. After that, it can no longer continue the process. It expels its outer layers into space, leaving behind only its dead core, known as a white dwarf. It’s about as heavy as the Sun, but only as big as Earth. For the Sun itself, the story will end there. But if a white dwarf has a close companion, the story can continue. If the companion is a “normal” star, the white dwarf may steal gas from its surface. If enough gas builds up on the white dwarf, it can trigger a supernova — an explosion that blasts the star to bits. And if the companion is another white dwarf, then the two stars may spiral together, again triggering a supernova. These explosions all brighten and fade in a characteristic way. From that cycl
