Among the star clusters in our galaxy, Palomar 5 is one of the most unique. It’s described as the “fluffiest” cluster in the halo of the Milky Way, with an average distance between stars in the cluster only a few light-years apart. Palomar 5 also has a unique stellar stream associated with it, spanning more than 20 degrees across the sky. A recent paper published by a group of astronomers and astrophysicists led by the University of Barcelona has shown that both of those distinguishing features of Palomar 5 are likely the result of a large black hole population.
The team believes there could be a population of more than 100 black holes at the center of the cluster. Professor Mark Gieles, the lead author of the paper, says that the number of black holes in the cluster is about three times higher than expected based on the number of stars in the cluster. He says that number of black holes means that more than 20 percent of the total cluster mass is made up of black holes. Each of the black holes in the cluster has a mass of about 20 times that of the sun.
Gieles says the black holes in Palomar 5 formed in supernova explosions at the end of the lives of massive stars when the cluster was still very young. Palomar 5 is of interest to astronomers because they want to understand how stellar streams such as the one it has form. Scientists don’t understand how the streams form, but one idea is that they are disrupted star clusters. Out of the 30 tidal streams discovered in the Milky Way halo so far, only Palomar 5 has a stellar system associated with it.
Study authors simulated the orbits and evolution of each star from the formation of the cluster until final dissolution. They varied the initial properties of the cluster until a good match with observations of the stream and cluster was found. They believe Palomar 5 formed with a lower blackhole fraction, but stars escaped more efficiently than black holes, so the blackhole fraction gradually increased. They also believe that before the cluster completely dissolves in approximately a billion years, it will consist entirely of black holes.