“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,” said Todd Thompson, lead study author and professor of astronomy at The Ohio State University.
Black holes can form after stars die, collapse and explode, creating gravitational whirls so strong that not even light can escape them. They can be found at the center of galaxies, acting like a supermassive engine.
But that’s not always the case. When some stars die and collapse, they form neutron stars, which are small and incredibly dense.
These are both important to study because astronomers can learn about the evolution of stars and black hole formation.
But if astronomers only focus on supermassive black holes, like the one at the center of our galaxy, and neutron stars, they’re missing whole demographics of the population, the researchers said. They compared it to census takers only including people of a specific height.
“People are trying to understand supernova explosions, how supermassive black stars explode, how the elements were formed in supermassive stars,” Thompson said. “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.”
Black holes are often found in binary systems, where two stars orbit each other — like the twin suns we see on Tatooine in the “Star Wars” films.
If one star dies, the other still orbits the space where it existed, even if that space is now filled by a black hole or neutron star.
Until this study, astronomers found black holes that clocked in between five and 15 times the mass of our sun, while neutron stars are only about two times the mass of the sun. But a neutron star could collapse and become a black hole if it reached more than 2.5 times the mass of the sun. Then, that black hole would grow.
Things changed after the gravitational wave observatory called LIGO spotted two black holes merging into one in a galaxy 1.8 million light-years away. The two monstrous black holes were 31 times the mass of the sun and 25 times its mass, respectively.
“Immediately, everyone was like ‘wow,’ because it was such a spectacular thing,” Thompson said. “Not only because it proved that LIGO worked, but because the masses were huge. Black holes that size are a big deal — we hadn’t seen them before.”
This expanded the known range for black holes, suggesting they could be larger. But what about those that were smaller, existing between the boundary of neutron stars and black holes?
Thompson and his colleagues turned to APOGEE data. It’s the Apache Point Observatory Galactic Evolution Experiment, which has observed light from 100,000 stars in the Milky Way.
They looked to see if shifts in the wavelengths of the star suggested a fellow companion that was otherwise invisible. If they showed a change in wavelength, going from blue to red, it might suggest that the companion had turned into a black hole.
While narrowing down the list of stars to the most likely candidates, Thompson and his colleagues found a giant red star orbiting something smaller than the smallest known black hole but larger than any known neutron star.
The potential low-mass black hole is thought to be 3.3 times the mass of our sun. Previously, the smallest black hole discovered was 3.8 times the mass of our sun.
“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,” Thompson said. “The masses of things tell us about their formation and evolution, and they tell us about their nature.”