In a first, an image shows a dying star exploded twice to become a supernova

Astronomers found evidence supporting the double-detonation theory for some supernovas, observing two calcium shells racing from a 300-year-old supernova remnant in a nearby galaxy. This suggests a white dwarf first exploded its helium blanket before triggering a larger blast. The finding may explain why some type 1a supernovas occur below the expected mass threshold and could improve understanding of these cosmic events used to measure universal expansion.