We may have found the smallest black hole we’ve ever seen. A search for black holes that orbit stars without devouring them has uncovered a black hole that is only 3.3 times the mass of the sun.
This black hole is about 10,000 light years away from Earth, where it orbits a giant star about once every 83 days. Despite its huge mass, the black hole is only about 20 kilometres across – roughly the length of Manhattan in New York.
Unlike most other systems with black holes orbiting stars, the black hole isn’t devouring gas from its partner, making it completely dark and difficult to spot. Todd Thompson at Ohio State University and his colleagues found it by looking at how the companion star wobbles due to the gravitational pull of the black hole. A similar process is used to look for exoplanets.
Whether this is truly the smallest black hole so far is up for debate – some have been seen with potentially lower masses, but with more uncertainty in the measurement. “It’s a mass range where we do not really have strong evidence for other black holes,” says Thompson.
This range, from about twice the mass of the sun to about five times its mass, is what astronomers refer to as the “mass gap” – bigger than the biggest neutron stars that we’ve seen, but smaller than the smallest black holes. Neutron stars and black holes both form from the collapses and explosions of giant stars, so their populations can tell us how those huge stars evolve and which ones are likely to explode in supernovae.
“Maybe in this gap, no black holes and no neutron stars form and that’s telling us about which stars explode and leave behind neutron stars, and which collapse into black holes,” says Thompson. “But if you find something in this gap, that tells you something interesting. Maybe there’s this whole other population that we weren’t seeing before that says something about how supernovae explode and how they might fail.”