Hubble uncovers concentration of small black holes

Globular clusters are extremely dense stellar systems, in which stars are packed closely together. They are also typically very old--the globular cluster that is the focus of this study, NGC 6397, is almost as old as the Universe itself. It resides 7800 light-years away, making it one of the closest globular clusters to Earth. Because of its very dense nucleus, it is known as a core-collapsed cluster.