Philipp Podsiadlowski

70 days. That's exactly how long the plateau, the steady glow after the blast, lasted for supernova SN 2018gj. Usually, these plateaus stretch a hundred days or more. This short plateau was the first crack in the star's mask. Images from the Hubble Space Telescope pinpointed the progenitor: a red supergiant. Enormous. Cold.

Massive stars end their lives in a grand explosion — a supernova. Their cores collapse into a neutron star (a super-dense ball where a teaspoon of matter weighs a billion tons) or a black hole. For a long time, the most common type of such outbursts was thought to be the domain of solitary stars. However, supernova SN 2018gj broke the rule.

Two burned-out stellar cores collide and merge into one. That's how R Coronae Borealis stars are born — rare luminaries where oxygen-18 is hundreds of times more abundant than in the Sun. But exactly how this happens has remained a puzzle.

Two spinning tops whirl so close they nearly touch. One is scorching blue, the other dim white. Together they complete a full orbit in twenty hours. This is the pair Feige 64: a blue star with a mass of just a third of the Sun's, and a white dwarf, the cooled core of a former star, one and a half times heavier than our Sun. The blue star seems too hot for its mass.