The Milky Way teems with black holes — about 100 million of them.
But there’s no reason to fear. “It may sound like a big number, but by astronomical standards, it’s a pretty small number,” says physicist Daniel Holz of the University of Chicago. The number of stars in the Milky Way, for example, is about a thousand times larger.
Scientists from the University of California, Irvine calculated the galaxy’s black hole population as part of a new census that estimates the numbers of cosmic chasms in galaxies big and small. The analysis, in press in Monthly Notices of the Royal Astronomical Society, quantified stellar-mass black holes, which form when a star collapses. Such objects can have masses tens of times that of the sun.
To draw up the celestial inventory, the researchers combined a variety of information about stars and galaxies. A star’s size and composition — the proportion of heavy elements it contains — determine whether it can form a black hole, and how big the black hole will be. And given a galaxy‘s size, scientists can estimate the number and properties of stars within, allowing researchers to deduce the number of black holes and their sizes.
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