First Stellar-Mass Black Hole Discovered in Omega Centauri by Astronomers

After decades of exploration, astronomers have finally identified the first of an estimated 10,000 stellar-mass black holes. Omega Centauri is the most massive star system in the Milky Way, boasting an impressive mass of 3.6 million solar masses.

Located about 18,000 light-years away in the constellation Centauri, Omega Centauri is a fascinating astronomical body.

Also referred to as NGC 5139, this globular cluster spans approximately 150 light-years in diameter and is around 12 billion years old.

As the largest among the 200 globular clusters circling the Milky Way, Omega Centauri houses over 10 million gravitationally bound stars.

It stands out as the brightest globular cluster in our galaxy, visible to the naked eye from the southern hemisphere with an apparent magnitude of 3.9.

Previous astronomical research indicated the presence of intermediate-mass black holes at the centers of star clusters, using NASA/ESA’s Hubble Space Telescope. However, models predict that Omega Centauri should contain approximately 10,000 smaller stellar-mass black holes.

This remarkable population of black holes had previously evaded detection through methods like radial velocity or observations of radio and X-ray emissions from matter falling into black holes.

The recent discovery employed a novel approach known as astrometry, which analyzes the tiny movements of stars over time.

By meticulously examining over two decades of archival data from Hubble, alongside recent findings from the NASA/ESA/CSA James Webb Space Telescope, University of Utah astronomer Matthew Whitaker and his team uncovered a star orbiting an invisible object so massive that it must be a black hole.

This black hole, named oMEGACat BH-2, marks the first detection of a stellar-mass black hole in Omega Centauri and showcases some intriguing characteristics.

oMEGACat BH-2 has a lower mass than anticipated, and along with its visible companion star, it possesses the longest orbital period of any known black hole binary system to date.

“Utilizing the Hubble and Webb data, we could observe the motion of the visible main-sequence stars within this binary system, located 18,000 light-years away in the dense surroundings of Omega Centauri,” Dr. Whitaker stated.

“The precision of these measurements is remarkable, achieving a fraction of a pixel with the Hubble and Webb detectors.”

“The discovery of this black hole would have been impossible without these two space telescopes.”

Thanks to the accurate data from Hubble and Webb, astronomers were able to analyze the star’s orbit over more than 20 years, focusing on its closest approach to the black hole’s companion star.

Using extensive data, they found that the visible star orbits oMEGACat BH-2 every 94 years, making it the longest-period black hole binary system known to date.

This system likely formed dynamically, indicating that the star and its black hole companion did not originate together but rather encountered each other within the cluster.

The researchers estimate that star systems like oMEGACat BH-2 may only persist for less than a billion years before being disrupted by collisions with nearby stars, much shorter than the cluster’s age.

“Understanding black hole populations within globular clusters is crucial, as there are ongoing uncertainties regarding their physics and formation,” declared Dr. Anil Seth from the University of Utah.

“Grasping the processes of black hole formation and subsequent binary star creation is vital for interpreting gravitational wave phenomena.”

“Environments like Omega Centauri are thought to be prime locations for binaries merging and generating these waves.”

For further insights on this discovery, refer to the research paper published in the Astrophysical Journal Letters.

_____

Whitaker et al. 2026. ω Centauri’s long-period stellar mass black hole binary. APJL 1006, L1; doi: 10.3847/2041-8213/ae7a5c