70 days — that's how long the farewell glow of SN 2018gj lingered after its death. Usually, such explosions shine for about a hundred days. This star was clearly hiding something.

Images from the Hubble Space Telescope showed that before the blast, it was a red supergiant — a vast, cool star at the end of its life. Light analysis via spectroscopy — splitting into colors — and precise brightness measurements with photometry revealed two oddities. The hydrogen envelope was thin. And its host galaxy was old: new stars hadn't been born for 40 million years, so heavy elements were scarce.

The answer came from numerical simulations — computer models based on the laws of Fritz Zwicky, Cecilia Payne-Gaposchkin, and Hans Bethe. Hundreds of scenarios showed that the star was born from the merger of two companions. Their cores fused, their envelopes were shed — a giant with a thin layer emerged. It exploded later. Probability: over 99%.

Now astronomers know: a short glow is a sign of a double past. By measuring it, they can find other stars with secret lives.