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.
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Source: Phys.org