Astronomers have identified the smallest known black hole. The puny object weighs only 3.8 times the Sun's mass and spans just 24 kilometres across.
The black hole is believed to have formed from the collapse of a massive star when it ran out of fuel.
Astronomers are not sure what the smallest possible mass is for black holes formed this way, but they estimate that it is somewhere between 1.7 and 2.7 times the Sun's mass. Less massive objects are expected to collapse into dense neutron stars instead of black holes.
"This black hole is really pushing the limits," says Nikolai Shaposhnikov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, who carried out the study with Lev Titarchuk, also of Goddard. "For many years, astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question."
The black hole they studied is part of a binary system called XTE J1650-500, where the black hole and an ordinary star orbit around each other. Gas stolen from the ordinary star heats up and emits X-rays as it spirals into the black hole.
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