The First of Omega Centauri Star Cluster’s Missing Black Holes Found | Hubble

The First of Omega Centauri Star Cluster’s Missing Black Holes Found | Hubble

Hubble image of globular star cluster Omega Centauri. It looks like a highly dense field of stars. There are stars that appear a bit larger and brighter than others with the majority of stars appearing blue, orange, and yellow. They are scattered mostly uniformly, like grains of sand. Toward the center they gradually become closer, creating a more luminous area at the globular star cluster’s core. A small red square frame is near the center. It connects to a square pullout in the top right corner. It shows the outlined area in greater detail. Among the blue- and orange-colored stars is a small blue-white dot that is highlighted by a small red circle.

A globular cluster, appearing as a highly dense and numerous collection of shining stars. A number appear a bit larger and brighter than others with the majority of stars appearing blue and orange. They are scattered mostly uniformly, but in the center they crowd together more and more densely, and merge into a stronger glow at the cluster’s core.

The massive globular star cluster Omega Centauri has puzzled astronomers for decades. It should be filled with black holes left behind by exploding stars, yet evidence for them is scarce. Now, astronomers using archival data from the NASA/European Space Agency Hubble Space Telescope and supportive observations from the NASA/European Space Agency/Canadian Space Agency James Webb Space Telescope have finally located their first stellar-mass black hole in this cluster. Discovering the first of this missing black hole population will help refine current theories on black hole formation within environments, such as Omega Centauri.

Omega Centauri is visible from Earth with the naked eye and is one of the favorite celestial objects for stargazers in the southern hemisphere. Although the cluster is 17,700 light-years away, lying just above the plane of the Milky Way, it appears almost as large as the full Moon when seen from a dark rural area. The exact classification of Omega Centauri has evolved through time, as our ability to study it has improved. It was first listed in Ptolemy's catalogue nearly two thousand years ago as a single star. Edmond Halley reported it as a nebula in 1677, and in the 1830s the English astronomer John Herschel was the first to recognise it as a globular cluster. Omega Centauri consists of roughly 10 million stars that are gravitationally bound.

Though the astronomy community has previously found evidence with Hubble that an intermediate-mass black hole lurks at its center, models suggest that this star cluster should contain about 10,000 smaller, stellar-mass black holes. This notable population of black holes has evaded detection in previous studies, which used the radial velocity method [1] or looked for radio and X-ray emission from material falling onto the black holes.

A new discovery features a another approach, known as astrometry [2], to measure the very small movements of stars over time. By sifting through more than 20 years of Hubble archival data and pulling in recent Webb data to further refine the astrometric measurements, the team 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 within Omega Centauri, and it has surprising qualities. oMEGACat BH-2 has a lower-than-expected mass and, with its visible star companion, the black hole-star duo has the longest orbital period of any black hole binary system known to date.

The team’s findings were published in The Astrophysical Journal Letters.

“With the Hubble and Webb data, we were able to see the motion of the visible main sequence star [3] that is part of this binary, which is about 18,000 light-years away in the dense environment of Omega Centauri,” said the paper’s lead author Matthew Whitaker of the University of Utah, Salt Lake City, in the United States. “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.”

The team’s findings refine a past study by a different group of scientists suggesting that this binary system included a neutron star. By expanding the Hubble data analyzed so that it included astrometric measurements from 2002 to 2023, and pulling in Webb near-infrared data to improve precision, the University of Utah-led team was able to better constrain the mass of the visible star’s dark companion, ruling out the neutron star possibility.

“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. However, its mass is actually much lower than would be expected in a metal-poor environment like Omega Centauri. This is surprising and exciting,” said Anil Seth of the University of Utah, a coauthor of the study. “We now know that a metal-poor star should be 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 modeling.”

Long time coming
Based on the precise data from Hubble and Webb, the team could chart the star’s path over 20-plus years. Fortunately, this was during its closest approach to its black hole companion when it moved the fastest across the sky. From the extensive data, the team determined that the visible star orbits oMEGACat BH-2 once every 94 years, making it the longest period black hole binary ever known.

Its long orbital period also gives a clue to the origin of this binary system. It 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, much shorter than the age of the cluster (approximately 12 billion years old).

“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.”

The team’s discovery of stellar-mass black hole oMEGACat BH-2 with the Hubble-Webb dataset is just the start of finding these evasive black hole populations in globular star clusters.

“This new discovery highlights the immense legacy value of the Hubble Space Telescope archive” said Maximilian Häberle, postdoctoral fellow at the European Southern Observatory, who led the data reduction for the Hubble and Webb data. “It marks the second breakthrough from our oMEGACat astrometric re-analysis, following the confirmation of the intermediate-mass black hole in Omega Centauri."

Notes

[1] The component in the velocity of an object's motion that is moving away or toward an observer. By observing Doppler shifts in spectral lines, astronomers can derive the radial velocity and determine how fast objects are moving away from or toward us. Measuring such shifts in the light of a star can reveal the presence of exoplanets and brown dwarfs orbiting them.

[2] Astrometry measures the precise locations and movements of stars over time. The orbit of a planet can cause a star to wobble around in space in relation to nearby stars in the sky.

[3] A normal star forms from a clump of dust and gas in a stellar nursery. Over hundreds of thousands of years, the clump gains mass, starts to spin, and heats up. When the clump's core heats up to millions of degrees, nuclear fusion starts. This process occurs when two protons, the nuclei of hydrogen atoms, merge to form one helium nucleus. Fusion releases energy that heats the star, creating pressure that pushes against the force of its gravity. A star is born. Scientists call a star that is fusing hydrogen to helium in its core a main sequence star. Main sequence stars make up around 90% of the universe’s stellar population. They range in luminosity, color, and size—from a tenth to 200 times the Sun’s mass—and live for millions to billions of years.

Credit: ESA/Hubble & NASA, M. Häberle (MPIA)
Date: July 13, 2026

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