0823 GMT December 11, 2019
The discovery came after an international team of astronomers developed a new way to search for black holes, BBC Science Focus Magazine reported.
These cosmic entities — which have a gravitational pull so strong that not even light can escape — can form when massive stars collapse at the end of their life. The stellar black holes discovered so far have a mass at least five times that of the Sun.
If the dying star is below a certain mass, however, it will collapse into a small, dense neutron star. Neutron stars are generally no bigger than about twice the mass of the Sun — any bigger, and they’d collapse into a black hole. This leaves a gap between the biggest neutron stars and the smallest black holes, which has remained unfilled — until now.
The new technique makes use of the fact that black holes can often be found in a binary system — two stars locked together in mutual orbit. When one of the stars dies and becomes a black hole, it can stay in the system, its presence revealed by changes in the living star’s light spectrum as it orbits its invisible companion.
The researchers used data from APOGEE (Apache Point Observatory Galactic Evolution Experiment), which collected light spectra from around 100,000 stars across the Milky Way.
The team honed in on 200 stars that looked like they might be orbiting a black hole. Further data-crunching then revealed a giant red star orbiting a low-mass black hole, estimated to be about 3.3 times the mass of the Sun.
“What we’ve done here is come up with a new way to search for black holes,” said Professor Todd Thompson at The Ohio State University, lead author of the study, “but we’ve also potentially identified one of the first of a new class of low-mass black holes that astronomers hadn’t previously known about.
“If we could reveal a new population of black holes, it would tell us more about which stars explode, which don’t, which form black holes, which form neutron stars. It opens up a new area of study.”