Astronomers Spot Rare, Mid-Sized Black Hole in Our Galaxy

While all black holes are massive from a human perspective, they actually come in a wide range of sizes. But these sizes usually fall at either end of a spectrum—either relatively small or absolutely enormous. Now, researchers have detected a rare, medium-sized black hole in the nearby star cluster Omega Centauri, according to a new paper published Wednesday in the journal Nature.

“This is exciting, because there are only very few other black holes known with a similar mass,” Nadine Neumayer, a co-author of the study and an astronomer at the Max Planck Institute for Astronomy in Germany, says in a statement from NASA.

Astronomers have located many smaller black holes—stellar remnants ranging from 5 to 150 times the mass of our sun. They’ve also spotted supermassive black holes at the center of galaxies with masses greater than 100,000 times that of the sun. But only a few black hole candidates have been found within these extremes, containing between 150 and 100,000 solar masses.

“People have wondered, is it difficult to find them because they are just not there, or because it’s difficult to detect them?” study lead author Maximilian Häberle, an astronomer at the Max Planck Institute for Astronomy, says to NPR’s Nell Greenfieldboyce.

“There has been a long debate whether intermediate-mass black holes exist in general, and specifically in Omega Centauri,” Häberle adds to Reuters’ Will Dunham. “And our detection might help to resolve that debate.”

Bigger than black holes created by collapsed stars but smaller than supermassive black holes at the cores of galaxies, "mid-sized" black holes are elusive.

By analyzing 500 Hubble images from two decades, astronomers found compelling evidence for one: https://t.co/BKn1m4M6p3 pic.twitter.com/LC9AQCWQpx

— Hubble (@NASAHubble) July 10, 2024

Omega Centauri, a cluster of ten billion stars held together by gravity, is almost as massive as a small galaxy. It’s located 16,000 light-years from Earth and is the most massive star cluster in the Milky Way—making it an ideal spot to hunt for black holes. Several studies have previously detected what possibly could be a medium-sized black hole in the star cluster, but some astronomers have questioned those findings.

For instance, observations that appear to suggest an intermediate-mass black hole could actually indicate a cluster of smaller, stellar-mass black holes. Additionally, nobody had detected stars moving quickly enough to point to the presence of a medium-sized black hole.

In the new study, researchers measured the velocities of 1.4 million stars in Omega Centauri based on Hubble Space Telescope images of the cluster. Much of the data was collected to calibrate Hubble’s instruments, but it ended up being useful for this particular project.

“What’s spectacular here,” Simon Portegies Zwart, an astronomer at the Leiden Observatory in the Netherlands who did not contribute to the findings, tells Sky & Telescope’s Govert Schilling, “is that they have measured velocities of individual stars.”

“We discovered seven stars that should not be there,” Häberle says in NASA’s statement. “They are moving so fast that they would escape the cluster and never come back.”

The fact that these seven stars are not escaping into space suggests they are gravitationally bound by a black hole at least 8,200 times the mass of the sun, which would make it a medium-sized black hole. If confirmed, the black hole would lie 17,700 light-years from Earth—closer than the supermassive black hole at the center of the Milky Way.

“This is really exciting, right? This is only the second black hole where you can see individual stars whizzing around the black hole,” Jenny Greene, an astrophysicist at Princeton University who did not contribute to the findings, tells NPR.

Studying mid-sized black holes could help researchers better understand the evolution of the enormous structures in general. Researchers have previously suggested that supermassive black holes could grow from merging medium-sized black holes.

Mid-sized black holes are “like a missing link that is needed to explain the existence of the supermassive black holes,” Eva Noyola, an astronomer who was not involved in the research, tells Science News’ Lisa Grossman. “If it’s proven that [medium-sized black holes] happen in dense stellar clusters, you have a solution there that’s pretty elegant and simple.”

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Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.