The Milky Way galaxy houses 100 million black holes

The Milky Way has a lot of black holes — about 100 million of them, a new study suggests.

But there’s no reason to fear. “It may sound like a big number, but by astronomical standards, it’s a pretty small number,” says Daniel Holz. He is a physicist at the University of Chicago in Illinois. For comparison, our galaxy has a thousand times more stars.

Scientists from the University of California, Irvine calculated the Milky Way’s black hole population as part of a new census. The team estimated the number of black holes that have masses tens of times that of the sun. These are called stellar-mass black holes. They form when massive stars collapse. The census estimates the numbers of these black holes in galaxies big and small. The results will be published in Monthly Notices of the Royal Astronomical Society.

Galaxies are enormous communities of stars. To estimate the number of black holes, the researchers looked at the properties of stars and galaxies. A star’s size and what it is made of determine whether the star can form a black hole. Those factors also determine how big the black hole will be. A galaxy’s size is important, too. From its size, scientists can estimate the number and properties of stars inside it. That allows researchers to determine the number of black holes and their sizes.

Based on the properties of the Milky Way’s stars and our galaxy’s size, the researchers estimate that our galaxy has 100 million stellar-mass black holes. About 90 million have less than 30 times the mass of the sun. About 10 million are biggies, with 30 times the mass of the sun or more.

Stellar-mass black holes are a target of LIGO, the Advanced Laser Interferometer Gravitational-Wave Observatory. This facility has detected three sets of gravitational waves from colliding black holes. Gravitational waves are ripples in the fabric of space. LIGO made its first detection of these ripples in 2016. The ripples appeared to come from the collision of two black holes that were about 30 times the mass of the sun. At the time, some physicists thought that mass of the black holes seemed surprisingly large. The stellar-mass black holes that scientists previously knew about were smaller than that. Some scientists started to propose exotic origins for LIGO’s black holes. Maybe those black holes didn’t form from collapsing stars. Maybe they formed during the universe’s infancy instead.

The new result contradicts that idea. In the Milky Way alone, there are about 10 million black holes with masses 30 times the sun’s mass or more. That means there should be a lot of them in other galaxies, too. “You don’t have to do anything particularly odd or unusual in order to explain the LIGO signal,” says James Bullock. He is a physicist at UC Irvine and a coauthor of the study.

Richard O’Shaughnessy is an astrophysicist at the Rochester Institute of Technology in New York. The abundance of bigger stellar-mass black holes doesn’t surprise him, he says. But, the new work may come to satisfy researchers who thought LIGO’s large black holes were an oddity, he says. Hopefully, he says, it will make even the skeptics recognize that it’s logical to see larger stellar-mass black holes merging.