A Black Hole Lightweight

Artist's rendition of a newly discovered low-mass black hole (left)-giant star (right) binary system

Jason Shults at The Ohio State University

When stars more massive than our Sun end their lives in powerful explosions called supernovae, what remains is either a neutron star, a densely packed ball of neutrons about twelve miles in diameter, or a black hole, an object whose gravitation is so strong that not even light can escape. According to theory, neutron stars cannot have more mass than about 2.5 times that of the Sun, while the lightest-weight black holes that have been previously detected have more than five times the mass of the Sun. Thus, there has been a conspicuous size gap in our knowledge of stellar evolution—until a recently reported discovery of a black hole with a mass 3.3 times that of the Sun.

Black holes have traditionally been discovered from the high-energy radiation generated by surrounding matter falling into them. However, a group of astronomers led by Todd Thompson at The Ohio State University looked for quiescent black holes in a mutual orbit with another star. Searching data from a survey called APOGEE, the Apache Point Observatory Galactic Evolution Experiment, which recorded the motions of around 100,000 stars in our galaxy, they identified a star named J05215658 in the constellation of Auriga that seemed to be orbiting an invisible companion at conspicuously high speed.

 

Detailed analysis showed that J05215658 is a binary system consisting of a visible giant star orbiting a black hole companion that is 3.3 times the mass of the Sun, smaller than any known black hole and bigger than any known neutron star. Further observations showed that the visible star’s brightness varied in synchrony with its orbit, indicating that the star rotates on its axis at the same rate as its orbit, much like our Moon.

 

In addition to identifying the first low-mass black hole, the astronomy team also developed a method that holds promise for detecting similar lightweight black holes, bridging the post-supernova size gap. According to Thompson, this “new population of black holes. . . would tell us more about which stars explode, which don’t, which form black holes, [and] which form neutron stars.” (Science)