Astronomers using archival Hubble data and recent James Webb observations identified the first stellar‑mass black hole in the globular cluster Omega Centauri, according to a study published Monday in The Astrophysical Journal Letters.
Omega Centauri contains roughly 10 million gravitationally bound stars and models predict about 10,000 smaller stellar‑mass black holes should be present, yet none had been observed. Earlier Hubble results suggested an intermediate‑mass black hole at the cluster’s center, but the expected population of stellar‑mass black holes remained undetected because previous searches relied on radial velocity or radio and X‑ray signatures.
The team applied astrometry, measuring minute changes in star positions over more than 20 years of Hubble archival data and supplemental near‑infrared observations from Webb. This approach revealed a main‑sequence star orbiting an unseen companion with a mass of 4.46 solar masses, too heavy to be a neutron star. The binary, designated oMEGACat BH-2, completes an orbit roughly every 94 years, the longest period known for a black‑hole binary system.
Matthew Whitaker of the University of Utah, lead author, said the star’s motion was tracked with a precision down to a fraction of a pixel on Hubble and Webb detectors. Anil Seth, a coauthor, noted that the black hole’s mass is lower than expected for a metal‑poor environment, indicating that such black holes can form in globular clusters.
The long orbital period suggests the system formed dynamically, with the star and black hole meeting within the dense cluster rather than originating together. Simulations indicate such a binary will be disrupted by encounters with nearby stars in less than a billion years, shorter than the cluster’s estimated age of 12 billion years.
The finding refines an earlier interpretation that proposed a neutron star in the same system. By constraining the companion’s mass, the study supports the presence of stellar‑mass black holes in metal‑poor settings and will inform models of black‑hole formation and binary merger rates relevant to gravitational‑wave astronomy.
Researchers plan to continue observing Omega Centauri with Hubble and Webb and to use NASA’s upcoming Nancy Grace Roman Space Telescope, which will provide regular, high‑resolution imaging of crowded regions, to search for additional black‑hole binaries.

