Star surrounded by colourful gas clouds — artist’s impression.

A binary system includes a star (centre; artist’s impression) and an unseen black hole. Such a system can evolve into a pairing of lopsided black holes. Credit: Katarzyna Drewniany

Astronomy and astrophysics

The odd couple: how a pair of mismatched black holes formed

A merger between two black holes of vastly different masses might have stemmed from a relationship between two unequal stars.

In an unprecedented sighting in 2019, gravitational-wave observatories detected the merger of two black holes of substantially different sizes, one of them more than three times as massive as the other. Now astrophysicists have calculated that the black holes could have originated as a binary star system.

Aleksandra Olejak at the Polish Academy of Sciences in Warsaw and her collaborators refined existing computer models for the evolution of binary star systems, which consist of two stars orbiting each other. The team found that if the two stars have certain initial masses, then they will end their lives by collapsing into two black holes of very uneven sizes, which will then continue to orbit each other until they merge.

Previously, only black holes that formed independently and later came to orbit each other were seen as plausible players in such a lopsided event. In particular, researchers had expected that in a binary star system, the smaller black hole’s formation would often cause the system to break up prematurely: the star’s collapse was predicted to produce a lopsided supernova explosion that would kick the resulting black hole out of the partnership.