The Center of the Milky Way May Be Chock-Full of Black Holes

Researchers spotted about a dozen black holes lurking at our galactic center—and there may be up to 10,000 more

Black Hole Sun
Columbia University

At the center of our Milky Way galaxy, sits a supermassive black hole called Sagittarius A*, which has a mass that is 4 million times that of our sun. Scientists have long thought it unlikely that this black hole is there alone. But they didn't yet have evidence that these other bodies existed, until now.

As Nell Greenfieldboyce reports for NPR, researchers detected evidence of about a dozen black holes swirling around the galactic center, estimating there could be up to 10,000 isolated black holes in the region.

Black holes emit no visible light or radiation, making them extremely difficult to directly observe using our current technology. But scientists can study how they interact with bodies nearby. For the latest find, researchers picked up the signal from a star orbiting the black hole, which causes the system to emit X-ray radiation.

For the latest study, a team headed by the Columbia University astrophysics lab picked through 12 years of data collected by NASA’s Chandra X-Ray Observatory on a search for the telltale radiation within 13 light-years of the galactic center. As Phil Charles, an astrophysicist at the University of Southampton who was not involved in the study, explains for The Conversation, the team found about 100 X-ray sources in that search area, including 26 signals within three light-years of Sagitarrius A*.

Another common object, called a millisecond pulsar, also emits a similar radiation signal as the binary systems. The researchers estimate that about half the signals they detected were from pulsars, leaving 13 X-ray signals coming from black holes with companion stars. They published their results in the journal Nature.

“It’s the confirmation of several theories that predicted this ought to be the case,” says Chuck Hailey, a Columbia astrophysicist and first author of the study, reports ​Gizmodo's Ryan F. Mandelbaum. “But it’s strange to have had this many and not really see them.”

According to a press release, the researchers estimate that based on the spatial distribution of the black holes they found, there are probably 300 to 500 black hole binaries in the immediate region around Sagitarius A* and about 10,000 isolated black holes not orbited by stars.

As Sarah Gibbens at National Geographic reports, those estimates are based on the theory that about 1 in 20 black holes will attract a low-mass star. Even if that calculation is slightly off, Gibbens reports, there are still likely way more black holes than the 60 or so scientists have yet detected.

“Let's suppose the theory is wrong, even by a factor of two or three,” Hailey says. “We'll take it. If there’s even a thousand of them there, that’s spectacular, compared to not having found any.”

While the discovery confirms what physicists already theorized, it also opens up new areas of research. For instance, understanding the density and distribution of black holes in the galactic center could help researchers understand when and how often gravitational waves—the massive energy bursts produced when two black holes collide—are created.

First detected in 2015, gravitational waves hold promise to help astronomers probe objects and regions of the universe that our eyes and telescopes cannot see. “It strengthens the arguments towards having these black hole populations as potential gravitational wave source,” Imre Bartos, a physicist at the University of Florida, tells Mandelbaum. “This is an exciting confirmation that fits with the rest of the picture as we understand it now.”

While 10,000 black holes sounds like a lot, it's a tiny fraction of the total number of black holes scientists estimate the Milky Way contains. Based on current calculations, there are 100 million stellar-mass black holes in our galaxy alone. And here's something crazy to consider: One new black hole likely forms somewhere in the universe every single second.

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