What's the smallest a black hole can be? About 4.5 times the mass of the Sun, according to Will Farr of Northwestern University in Evanston, Illinois, and his colleagues.
Farr's team obtained data on the masses of 20 known black holes that are sucking gases away from their partners in binary star systems. Using these data, the researchers created an expected mass distribution for all stellar-mass black holes and found that 99% would have a mass greater than 4.5 solar masses.
This presents a problem for theories predicting a smooth distribution between black holes and neutron stars, which have a theoretical limit of three times the mass of the Sun.
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Mind the black-hole gap. Nature 479, 152 (2011). https://doi.org/10.1038/479152b
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DOI: https://doi.org/10.1038/479152b
