On August 6, 1181, astronomers in China and Japan spotted a new star in the sky. It shone for 185 days, then faded. Today, in its place lies a white dwarf — a tiny stellar core left behind after an explosion. Sometimes such objects explode as a supernova. The dimmest of these explosions are puzzling: what triggers them? Back in the 1930s, Subrahmanyan Chandrasekhar showed that a white dwarf cannot be too heavy, and Fritz Zwicky predicted that their collapse gives birth to a supernova. The explosion can happen in two ways. The first is like a dance, where a white dwarf circles a companion, siphoning its gas, and then detonates, while the companion survives. The second is like two drops merging: two white dwarf stars collide and fuse into one, annihilating each other without a trace.

Astronomers checked both scenarios for the remnant of 1181. They scanned the area around the supernova remnant (the site of the former explosion) out to a radius comparable to a lunar disk on the sky. They used the Gaia satellite and ground-based images from Pan-STARRS1. If a surviving donor star were there, it should have been bright enough — calculations showed it would be no fainter than a certain limit. But all candidates were ruled out: some were too far away, others had the wrong color. The instruments' sensitivity was more than enough to spot such a companion. It simply isn't there.

So, the supernova of 1181 was born from the collision and merger of two white dwarf stars. Previously, it was thought that all such explosions required a companion to feed them, but this case opens up a second path — merger. This is important: supernova explosions serve as cosmic rulers for measuring distances, and if we confuse their types, errors creep into calculations. Today, a lone white dwarf — all that remains of the historic flash — floats in darkness, a reminder that sometimes the dance of stars ends in eternal union.