Lone black holes probably litter the Galaxy, but they’re extremely hard to spot. Now astronomers have, for the first time, seen an isolated black hole, wandering unattached across the Milky Way. Black holes are typically glimpsed as they interact with other objects, such as companion stars. Studying solo black holes — a separate class of cosmic object — should help scientists to understand how they form, and how abundant they are.
Black holes are so massive that not even light can escape their gravitational pull, so by their nature they are invisible. They remain ghostly unless they interact with other stellar objects, by super-heating matter from a companion star, for example, or colliding to generate gravitational waves that ripple across the Universe.
Solo black holes are thought to be common, forming whenever a single star of more than 20 solar masses or so reaches the end of its life. “There should be 100 million such black holes in the galaxy, they should be everywhere, but it’s very hard to find them,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, who led the team that made the discovery.
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