After decades of searching, astronomers have pinpointed the first of an estimated 10,000 stellar-mass black holes thought to lurk in Omega Centauri, the most massive stellar system in the Milky Way found so far, with a stellar mass of 3.6 million solar masses.
Whitaker et al. found Omega Centauri’s first stellar-mass black hole, which has a visible star companion that is shown in greater detail. Image credit: ESA / NASA / Maximilian Häberle, MPIA / Joseph DePasquale, STScI.
Omega Centauri is located approximately 18,000 light-years away in the constellation of Centaurus.
Also known as NGC 5139, this globular cluster is about 150 light-years in diameter and 12 billion years old.
Of the 200 or so globular clusters that orbit the Milky Way, Omega Centauri is the largest, containing over 10 million gravitationally bound stars.
It is also the brightest globular cluster in our Galaxy, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye.
Though the astronomical community previously found evidence with the NASA/ESA Hubble Space Telescope that an intermediate-mass black hole lurks at the cluster’s center, models suggest Omega Centauri should also contain about 10,000 smaller, stellar-mass black holes.
This notable population of black holes evaded detection in previous observational studies, which used the radial velocity method or looked for radio and X-ray emission from material falling onto black holes.
The new discovery features a different approach, known as astrometry, to measure very small movements of stars over time.
By sifting through more than 20 years of archival data from Hubble and pulling in recent data from the NASA/ESA/CSA James Webb Space Telescope to further refine their astrometric measurements, University of Utah astronomer Matthew Whitaker and his colleagues located a star orbiting an invisible object so hefty that it has to be a black hole.
Dubbed oMEGACat BH-2, it is the first stellar-mass black hole detected in Omega Centauri, and it has some surprising qualities.
The compact object has a lower-than-expected mass and, with its visible star companion, oMEGACat BH-2 has the longest orbital period of any black hole binary system known to date.
“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,” Dr. Whitaker said.
“The precision of these measurements is incredible, down to a fraction of a pixel on Hubble and Webb’s detectors.”
“It would not have been possible to find this black hole without these two space telescopes.”
Based on the precise data from Hubble and Webb, the astronomers could chart the star’s path over 20-plus years, during its closest approach to its black hole companion when it moved the fastest across the sky.
From the extensive data, they determined that the visible star orbits oMEGACat BH-2 once every 94 years, making it the longest-period black hole binary ever known.
The system was probably dynamically formed, meaning the star and its black hole companion did not start out together but rather found each other in this cluster.
The researchers calculated that a system like oMEGACat BH-2 will survive for less than a billion years before it is torn apart by encounters with nearby stars, a much shorter span than the age of the cluster.
“It’s important to understand black hole populations in globular clusters because there’s uncertainty about their physics and formation,” said Dr. Anil Seth, also from the University of Utah.
“More specifically, understanding the process of forming black holes and then dynamically forming binaries is vital, because it affects our ability to interpret and understand gravitational wave events.”
“Environments like Omega Centauri are the primary places where we think binaries are merging and creating these waves.”
The discovery is described in a paper in the Astrophysical Journal Letters.
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Matthew Whitaker et al. 2026. A Long Period Stellar-mass Black Hole Binary in ω Centauri. ApJL 1006, L1; doi: 10.3847/2041-8213/ae7a5c
