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One cluster, but two black holes

The current consensus is that a typical globular star cluster can accommodate just one stellar-mass black hole. Many more will be created, but all but one will be ejected following dynamical interactions. New observations of the Milky Way globular cluster M22, however, reveal the presence of two radio sources in one cluster, with properties typical of accreting black holes each more than ten times the mass of the Sun. This suggests that the ejection of black holes is not as efficient as predicted by most models. And it may not stop there: the authors speculate that there could be a population of tens of black holes in M22, either as single black holes or in binaries where there is no mass transfer taking place.

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