Hubble Space Telescope image of the globular cluster Omega Centauri, a dense spherical cluster of hundreds of thousands of stars against black space. The central region glows with tightly packed blue, orange, and yellow stars. A red square frame near the center marks the location of the black hole candidate oMEGACat BH-2. Credit: NASA, ESA, and the Hubble SM4 ERO Team.
Omega Centauri, the largest globular cluster in the Milky Way, contains roughly 10 million stars packed into a sphere just 150 light-years across. It is visible to the naked eye from dark southern skies. The red square near the center marks the location where astronomers found the first stellar-mass black hole ever detected in the cluster. Credit: NASA, ESA, and the Hubble SM4 ERO Team; red box added by STScI.

Omega Centauri, the largest globular cluster in the Milky Way, is a dense ball of roughly 10 million stars about 17,000 light-years from Earth. For decades, astronomers have believed it should contain about 10,000 stellar-mass black holes, the collapsed remnants of massive stars that long ago exploded as supernovae and sank toward the cluster's center. Yet no one had ever found a single one. They were, as NASA put it, the cluster's missing black holes.

Now, a team of astronomers has finally found the first. Using over 20 years of archival images from the Hubble Space Telescope and new observations from the James Webb Space Telescope, researchers detected a black hole called oMEGACat BH-2 not by any light it emits, but by tracking the minute motion of a visible star orbiting an invisible companion. The star, a modest 0.78 solar masses, traces an orbit that takes 94 years to complete, making this the longest-period black hole binary system ever discovered. The findings were published July 13 in the Astrophysical Journal Letters.

"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. "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."

Two decades of data, one invisible object

Black holes do not emit light unless they are actively pulling in gas, which causes the gas to heat up and glow. Most of the black holes expected in Omega Centauri are dormant and dark, invisible to even the most powerful telescopes. The only way to find them is by their gravitational influence on nearby stars.

The team led by the University of Utah used hundreds of images taken by Hubble between 2002 and 2023, measuring the positions of individual stars in the cluster with extraordinary precision over two decades. They combined this with near-infrared data from JWST to improve the measurements further. By tracking how one particular star moved across the sky, they could calculate the mass of whatever was pulling on it.

The earlier data had suggested the invisible companion might be a neutron star, the dense collapsed core of a star that was not quite massive enough to form a black hole. But the expanded dataset told a different story. At 4.46 solar masses, the object was too heavy to be anything but a black hole. The visible star itself, at 0.78 solar masses, is less massive than the Sun, making the black hole nearly six times heavier than its companion.

Two panels showing the Hubble image of Omega Centauri with a marked star location and a close-up pullout showing the star tracked over 20-plus years of observations around the invisible black hole oMEGACat BH-2.
Left: Hubble image of Omega Centauri with a red square and pullout showing the location of the star orbiting the black hole oMEGACat BH-2. Right: The star's motion over 20-plus years of Hubble and Webb observations, traced around an invisible companion. Credit: NASA, ESA, STScI, University of Utah.

A black hole that was not born with its companion

The 94-year orbit is not just a record. It tells astronomers something important about how this binary system formed. If the star and black hole had started out together as a pair of massive stars, the shorter-lived star would have exploded as a supernova and collapsed into the black hole first. Any stars born together would have had roughly similar ages and masses. But the visible star at 0.78 solar masses is a lightweight, and the black hole is surprisingly modest compared to others formed from metal-poor environments.

The researchers calculated that the system was probably formed dynamically. The star and black hole likely did not begin together. Instead, they stumbled into each other in the dense interior of the globular cluster, captured by each other's gravity into a long, slow orbit. The arrangement is fragile. The team estimates that oMEGACat BH-2 will survive for less than a billion years before close encounters with other stars in the cluster tear the pair apart. That is a short lifespan in a cluster that is roughly 12 billion years old.

That short lifespan helps explain why no one had found a black hole in Omega Centauri before. Most of the black holes formed early in the cluster's history have already been ejected by gravitational interactions with other stars and black holes. The remaining ones, like oMEGACat BH-2, may be the stragglers that happened to find a companion to hold on to.

What the mass means

The black hole's mass of 4.46 solar masses is lower than expected for a black hole formed in a metal-poor environment like Omega Centauri. Stars in globular clusters formed early in the universe's history from gas that contained very few heavy elements. In theory, metal-poor stars should produce more massive black holes because there are fewer heavy elements in their cores to drive mass loss through stellar winds before the final collapse.

oMEGACat BH-2 does not fit that pattern. It is on the lower end of the stellar-mass black hole range, closer to the 5-solar-mass black holes found from gravitational wave detections than the 10-to-20-solar-mass black holes expected from metal-poor progenitors. That means either the formation models need adjustment, or this black hole started with a different evolutionary history than the standard picture predicts. The detection provides some data to those who model black hole formation in metal-poor environments, a puzzle that has been almost entirely theoretical until now.

The first of 10,000

The discovery of oMEGACat BH-2 is just the beginning. The Hubble and Webb dataset that enabled this detection covers thousands of stars across the cluster, and the same technique can be applied to find more dark companions. The team's approach, combining two decades of astrometric data with precise near-infrared photometry, has proven that dormant black holes in dense star clusters can be found even when they are not accreting gas.

Omega Centauri has long been suspected to be the stripped core of a dwarf galaxy that merged with the Milky Way billions of years ago, not a typical globular cluster. If that is true, its black hole population may be different from that of other clusters. The discovery of oMEGACat BH-2 gives astronomers their first real data point to test that idea.

"This detection is providing some data to those who do that kind of modeling," Seth said. For now, it is one black hole found out of an estimated 10,000. The other 9,999 are still waiting.

Sources

Related on Impossible Universe

Hero image: Hubble Space Telescope image of the globular cluster Omega Centauri, the largest such cluster in the Milky Way. Credit: NASA, ESA, and the Hubble SM4 ERO Team. Inline image: Hubble image of Omega Centauri with the black hole candidate oMEGACat BH-2 location marked, showing the visible star's motion over 20-plus years of observations. Credit: NASA, ESA, STScI, University of Utah. Research published in The Astrophysical Journal Letters (Whitaker et al., July 13, 2026).