Found at the center of galaxies, supermassive black holes have the mass of more than 100 000 suns. Stellar-mass black holes, widely observed in binary systems in which a black hole siphons material from its partner star, are typically less than 50 solar masses. But observations of black holes with masses somewhere in between have been relatively sparse and sometimes contentious, and the ways that those intermediate-mass black holes might form remain a subject of debate. New star-by-star simulations by Michiko Fujii (University of Tokyo) and colleagues lend support to one possibility: The birth of stars in dense clusters can lead to the formation of those mysterious midsize black holes.
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