A black hole in a low mass X-ray binary star
To provide a little bit of context, it is known that globular clusters should produce a lot of black holes, but it has long been believed that they should retain very few, if any of these black holes.  One reason for this is that black holes should receive a kick upon their formation.  The gravitational potential of a globular cluster is weak enough that this would generally be enough to kick the black hole out of the cluster.<p>For those that remain in the cluster, the black holes would be much more massive than the rest of the stars in the cluster.  Globular clusters undergo a phenomenon called mass segregation, where the most massive stars settle to the center of the cluster and lighter stars occupy the outer regions.  So over time the black holes would sink to the center of the cluster where they would interact and form binaries with each other.  These binary-binary interactions would, over time, kick single black holes out of the cluster.  Since these interactions happen on timescales that are much shorter than the lifetime of the cluster, the received wisdom about twenty years ago was that globular clusters should have only a couple of black holes left, if any at all.<p>In 2007, Thomas Maccarone (who is an author on this paper) discovered a black hole in a globular cluster in another galaxy.  It wasn&#x27;t until 2012 that any were discovered in a globular cluster around the Milky Way.  Now that computers have gotten fast enough that it has started to become possible to simulate the dynamics of globular clusters, theorists have found that globular clusters retain a much higher fraction of black holes than they originally thought.  The reason for this seems to be that the distribution in black hole masses leads the black hole population to remain better mixed with ordinary stars than was originally thought.