Astronomers Confirm Long-Period Stellar-Mass Black Hole in Omega Centauri

NASA said Tuesday that astronomers have found the first confirmed stellar-mass black hole in Omega Centauri, using more than two decades of Hubble Space Telescope data sharpened by newer observations from the James Webb Space Telescope. The object sits in the Milky Way’s largest globular cluster, a dense, ancient ball of stars that astronomers have long expected could hold black holes of this kind.

What makes the result unusual is how it was found. Instead of spotting X-rays or radio waves from matter falling into a black hole, the team tracked the tiny back-and-forth motion of a visible star over 23 years and used that wobble to infer the gravity of an unseen companion. The authors conclude the dark object is too massive to be a neutron star, making this a rare astrometric detection of a non-accreting black hole — one that is effectively dark except for its gravitational pull.

The paper reports that the unseen companion has a mass of 4.46 times that of the sun, with an uncertainty of plus 1.22 and minus 1.01 solar masses. Its visible partner is a main-sequence turnoff star — a star near the point where it is exhausting hydrogen in its core — with an assumed mass of about 0.78 solar masses. The binary system is in Omega Centauri, about 18,000 light-years away, and appears to orbit every 94 years, though the estimate carries large uncertainty. The orbit is also highly elongated, with an eccentricity of 0.72.

The finding, announced by NASA on July 13, is notable in part because Omega Centauri, also known as NGC 5139, has long been considered a promising place to retain stellar-mass black holes because of its size and internal dynamics. Even so, such objects are difficult to pin down if they are not actively feeding on nearby gas. This same system had been suggested in a 2024 study to host a neutron star, but the new Hubble-plus-Webb analysis reclassifies it as a black hole. The paper, “A Long Period Stellar-Mass Black Hole Binary in ω Centauri,” was accepted for publication in The Astrophysical Journal Letters on June 6 and is available on arXiv.

“With Hubble and Webb data, we were able to see the motion of the visible main sequence star that is part of this binary, which is about 18,000 light-years away in the dense environment of Omega Centauri,” said Matthew Whitaker of the University of Utah, the paper’s lead author.

The team says the system is now the longest-period black-hole binary known, and it stands out for another reason: dynamically confirmed stellar-mass black holes in Galactic star clusters are extremely rare. According to the paper, only two had previously been found in such clusters, both in NGC 3201. The authors say the Omega Centauri result should help researchers test ideas about how black holes form in metal-poor environments and how they are paired into binaries through close encounters inside dense clusters — a process that also matters for models used in gravitational-wave astronomy.