Astronomers have achieved a major breakthrough in the hunt for thousands of missing black holes that have puzzled scientists for decades. By combining more than 20 years of observations from NASA's Hubble Space Telescope and the James Webb Space Telescope, researchers have identified a stellar-mass black hole in the vast Omega Centauri star cluster for the first time.
First Stellar-Mass Black Hole Confirmed in Omega Centauri
Omega Centauri, one of the largest globular clusters in the Milky Way, contains about 10 million stars held together by gravity. Scientists long predicted it should contain thousands of stellar-mass black holes left behind by exploding massive stars. However, previous searches using radial velocity measurements or detecting X-rays and radio signals found little evidence of this hidden population.
The new study employed a technique called astrometry, which tracks tiny changes in star positions over time. By analyzing decades of Hubble data and incorporating Webb's infrared observations, researchers identified a star orbiting an unseen object with enough mass to confirm it as a black hole.
oMEGACat BH-2: A Unique Black Hole Binary
Named oMEGACat BH-2, the object is the first confirmed stellar-mass black hole in Omega Centauri. It has unusual characteristics: its mass is lower than expected, and together with its visible companion star, it forms the longest-period black hole binary system known. The visible companion star has a mass of about 0.78 solar masses, while the black hole weighs approximately 4.46 solar masses—too heavy to be a neutron star.
“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, lead author of the paper. “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.”
Surprising Mass and Formation Implications
The discovery overturns an earlier suggestion that the system contained a neutron star. By extending previous Hubble research with additional astrometric measurements from 2002 to 2023 and incorporating Webb’s near-infrared data, the team calculated the invisible object's mass more accurately.
“While we already knew that the star was 0.78 solar masses, we can now calculate the black hole’s mass, which is 4.46 solar masses and therefore too heavy to be a neutron star,” said Anil Seth of the University of Utah, a coauthor of the study. “However, its mass is much lower than would be expected in a metal-poor environment like Omega Centauri. This is surprising and exciting. We now know that a metal-poor star is able to form a black hole like this, and we need to figure out how that happens. This detection is providing some data to those who do that kind of modelling.”
Long-Period Binary and Future Prospects
The detailed observations allowed researchers to trace the star's orbit around its black hole companion for more than two decades. The star completes one orbit every 94 years, making oMEGACat BH-2 the longest-period black hole binary system discovered so far. Scientists believe the pair may not have formed together but became linked through interactions within the crowded cluster. Their calculations suggest the system will likely survive for less than one billion years before being disrupted by encounters with nearby stars—a relatively short period compared with Omega Centauri’s estimated age of about 12 billion years.
“It’s important to understand black hole populations in globular clusters because there’s uncertainty about their physics and formation,” said Seth. “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.”
Researchers say the discovery marks the beginning of a wider search for similar hidden black hole populations in globular clusters. “With Hubble and Webb, we can continue to look at Omega Centauri and expand our search for similar systems within other clusters,” said Whitaker. “We’re also very excited for the launch of NASA’s Nancy Grace Roman Space Telescope because it will image the crowded galactic bulge, including the galactic center, very regularly with Hubble-like resolution and with a much wider field of view. We’re hoping we’ll be able to find black hole binary systems like this one because of the regular cadence of Roman’s observations.”
The findings were published on July 15, 2026, in The Astrophysical Journal Letters.
