Two black holes found in a star cluster

摘要

The Milky Way globular cluster M22, a massive, dense star cluster containing close to a million stars, is starting to reveal its secrets. On page 71 of this issue, Strader et al.1 present long-exposure radio images of M22, obtained by the Karl G. Jan-sky Very Large Array of radio antennas, showing two previously unknown sources of radio waves. The sources are candidates for 'stellar-mass' black holes, with masses 10-20 times that of the Sun. These are the first possible black holes to be detected in massive, old Milky Way star clusters such as M22. Futhermore, by virtue of the fact that two black holes were found, and were found in the cluster centre, the discovery provides insight into the dynamic evolution of the black-hole population.