Since the discovery of M101 ULX-1, also known as the 'pinwheel' galaxy (pictured), in Ursa Major, astronomers have pondered the source of the ultraluminous X-rays that emanate from it. As a binary system of a black hole and a star, it is either too bright for its estimated mass, or more massive than previously thought. To resolve this issue, it would help to measure the mass of the star — which is exactly what Jifeng Liu and co-workers have done, using the Gemini telescope.
The black hole could be an elusive 'intermediate mass' black hole. Such objects could coalesce to form supermassive black holes, but none of these intermediate-mass 'seeds' have been found so far. Liu and co-workers started with the companion star, which they confirmed is a massive, highly evolved Wolf–Rayet star that is extremely hot; its brightness suggests that it has roughly 19 solar masses. From there, they arrived at a black-hole mass of some 20–30 solar masses. Thus it is a 'stellar mass' black hole, but not by any means standard. The extreme luminosity remains a puzzle.