A white dwarf star that circles a black hole every 28 minutes may have the closest orbit of its kind ever seen in our Galaxy.

The system, called 47 Tuc X9, is some 4.5 kiloparsecs away. It was already thought to contain two objects orbiting each other, one of them probably a black hole, but the identity of the second object was uncertain. Arash Bahramian at the University of Alberta in Edmonton and his colleagues analysed X-ray and radio observations of the system from Earth- and space-based telescopes. They discovered that the system has high oxygen levels and noted a change in X-ray brightness roughly every half an hour. The researchers inferred that a white dwarf — a dense remnant of a Sun-like star — is orbiting the black hole at a distance of about 2.5 times that between Earth and the Moon.

The black hole has probably been sucking material from the star for tens of millions of years, but the star is unlikely to be engulfed by the black hole, say the authors.

Mon. Not. R. Astron. Soc. 467, 2199–2216 (2017)