Both neutron stars and black holes are formed when a star dies. The explosive deaths of larger stars lead to black holes, while smaller stars (like our sun) create a small, dense object called a neutron star.

The biggest neutron star that has been discovered is 2.1 times the mass of our sun. In comparison, the smallest black hole known is about five to six times the mass of our sun. The object discovered by the Ohio State team is only 3.3 times the mass of our sun, leading them to believe it is a black hole rather than a neutron star.

This is because the object's lower mass boundary — the lowest mass this object could be — is 2.6 times the mass of our sun, which is what astronomers think is the upper limit for how massive neutron stars can theoretically get. Any more massive than that, and the neutron star would collapse into a black hole.

Typically, astronomers search for black holes in our own galaxy by measuring X-rays that are emitted when the black holes siphon material from nearby stars. When they look for them further away, they look for gravitational waves caused by the merging of two black holes or from a collision of neutron stars. The latest research, though, found the new object in a completely different way.

The researchers went looking for relatively low-mass black holes that don't emit the tell-tale X-ray signals of other black holes by focusing on binary systems. They looked for evidence of the black holes by combing through data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), which has information on the light spectrum from over 100 000 stars in our galaxy. 

They analysed the changing light spectra from each of those stars. If the researchers noticed any changes such as a shift toward bluer wavelengths or a shift to redder wavelengths, they flagged the star as possibly orbiting an unseen companion. They then looked at changed in brightness of all of the flagged stars using data from the All-Sky Automated Survey for Supernovae (ASAS-SN).

What they discovered through this analysis was a massive dark object locked in a gravitational embrace with a rapidly rotating giant star about 10 000 light-years away, near the constellation Auriga.

“We’re showing this hint that there is another population out there that we have yet to really probe in the search for black holes. People are trying to understand supernova explosions, how supermassive black stars explode, how the elements were formed in supermassive stars. So 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,” The study’s lead author Todd Thompson, professor of astronomy at The Ohio State University, said in a statement.

“What we’ve done here is come up with a new way to search for black holes, 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.”