HALFWAY across the universe, a star lies dead. You write it off as routine, the sort of thing that happens a million times in this crummy neighbourhood. Only slowly do you realise how this case could shake cosmology to its core.

Something like this unfolded in March 2017 when, on a routine patrol of the night sky, David Sand

White dwarfs pack the sun’s mass into just the volume of Earth. They often come in pairs, with one star feeding off the other, sucking material from it. Eat too much, and this vampire star can exceed the critical density at which carbon atoms in its core fuse into heavier elements. “It’s basically a runaway thermonuclear bomb,” says Mark Sullivan at the University of Southampton, UK. Within seconds, the star becomes billions of tonnes of radioactive shrapnel. As this decays over weeks and months, it gives off heat and radiation visible halfway across the universe.

Extensive observations of SN 2017cbv, however, suggested that the exploding star’s mysterious companion was not another white dwarf, but a larger star. This matters because we assumed type Ia supernovae all had the same trigger, and therefore a uniform brightness, meaning how they look to us depends only on …

at the University of Arizona came across something new. At first glance, it was just another type Ia supernova, the fiery end of an over-bloated white dwarf star.