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. Its progenitor, a red giant, was located in an ancient region of the galaxy where no new stars form, and after the explosion the glow faded too quickly. Computer modeling showed: such a picture arises only if two stars merged before the explosion.
It's like a car assembled from parts of different vehicles: an engine from one model, a chassis from another. The resulting hybrid behaves unexpectedly — and so a star after a merger acquires an unusual structure. This very scenario explained the oddities of SN 2018gj and turned the notion upside down: most massive stars are born in pairs, meaning mergers may be the rule, not the exception. Now astronomers have the key to searching for an entire hidden family of such explosions.
Most massive stars are born in pairs, not alone.