‘Tiny’ black hole discovery changes our view of the Milky Way

1 Nov 2019

Image: Owen Humphreys/PA

Researchers may have missed the existence of ‘tiny’ black holes, including one with a mass more than three times that of our sun.

Astronomers may have discovered a new class of black holes, smaller than any previously identified in the universe. The gravitational vortexes are an important part of how astrophysicists make sense of the universe – so much so that scientists have been trying to build a census of all the black holes in the Milky Way.

But a study published in the Science journal indicates their search might have been missing an entire class of black holes. Lead author Todd Thompson, a professor of astronomy at Ohio State University, said: “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.”

Universal census

Using data from various observations and satellites, researchers found a giant red star that appeared to be orbiting something much smaller than the known black holes in the Milky Way. After careful analysis, Thompson realised the discovery was a low-mass black hole about 3.3 times the mass of the sun.

For years astronomers have been searching for black holes that have gravitational pulls so fierce that nothing can escape. They form when some stars die, shrink into themselves and explode. They have also been looking for neutron stars – small, dense stars that form when some stars die and collapse.

It had long been thought that known black holes were all between approximately five and 15 times the mass of the sun. Known neutron stars are generally no bigger than about 2.1 times the mass of the sun – if they were above 2.5 times the sun’s mass, they would collapse to a black hole.

But in 2017, a survey called the Laser Interferometer Gravitational-Wave Observatory (LIGO) saw two black holes – one about 31 times the mass of the sun, the other about 25 times the mass of the sun – merging together in a galaxy about 1.8m light years away.

A cosmic investigation

Black holes often exist in something called a binary system, meaning that two stars are close enough to one another to be locked together by gravity in a mutual orbit around one another. Thompson and other astrophysicists had long suspected they might come in sizes outside the known range, and he decided to see if he could solve that mystery.

He looked at data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), which collected light spectra – intensity of light as a function of wavelength – from around 100,000 stars across the Milky Way.

Thompson realised this could show whether a star was orbiting around another object, and began combing through the data, looking for stars that showed a change indicating they might be orbiting a black hole. He narrowed the APOGEE data to 200 stars and gave the data to a graduate research associate at Ohio State, Tharindu Jayasinghe.

They found a giant red star that appeared to be orbiting something likely to be much smaller than the known black holes in the Milky Way, but bigger than most known neutron stars. After more calculations and additional data from the Tillinghast Reflector Echelle Spectrograph and the Gaia satellite, they realised they had found a low-mass black hole.

Thompson said: “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.

“The masses of things tell us about their formation and evolution, and they tell us about their nature.”

– PA Media