Black holes are antisocial: Not only do they gobble gas from the stars that orbit them, but according to theory, they also kick one another out of their homes. Now, however, astronomers say they have spotted two separate black holes that inhabit the same cluster of stars, suggesting more black holes await discovery—both in this cluster and in others.
Never before have astronomers found a black hole in a Milky Way star cluster. The star cluster in question is a so-called globular cluster, a tightly packed conglomeration containing hundreds of thousands of stars. All Milky Way globular clusters formed long ago, so their short-lived massive stars have died and become black holes. Each black hole weighs more than any of the many cluster stars that still shine and so should sink to the cluster's center. There, their large gravity ejects other black holes into the galaxy at large, leaving behind at most one black hole system.
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