Some White Dwarfs Explode in Two Stages

Supernovae are usually a one-time event—once a star explodes, that’s it. But some white dwarfs blow up in two stages. In a “double detonation,” a white dwarf steals helium from a companion until the helium layer on its surface ignites. That first explosion sends a shockwave inward that triggers a second detonation in the core, and the whole star is destroyed. For years this was only a theory. In 2025, astronomers using ESO’s Very Large Telescope found two shells of calcium in the supernova remnant SNR 0509-67.5, the first visual evidence that the mechanism really happens.