Updated

Updated · ESA/Hubble · Jul 13

Hubble, Webb Detect 4.46-Solar-Mass Black Hole in Omega Centauri as 10,000 More Are Expected

Updated

Updated · ESA/Hubble · Jul 13

Hubble, Webb Detect 4.46-Solar-Mass Black Hole in Omega Centauri as 10,000 More Are Expected

Q: Why did finding a black hole in our galaxy's largest star cluster require two decades of data from Hubble and Webb?

It took two decades because the black hole was not visible on its own and could only be identified by measuring the tiny, slow wobble of a companion star in an exceptionally crowded cluster. In Omega Centauri, about 18,000 light-years away and packed with roughly 10 million stars, that motion is extremely hard to isolate and requires astrometry precise to a fraction of a pixel. The key complication was the binary’s unusually long 94-year orbit, the longest known for a black hole binary. Astronomers needed a long time baseline to trace enough of the star’s path to show it was circling a dark object and to calculate that object’s mass. Hubble’s archive, spanning 2002 to 2023, provided that baseline. The timing also helped because those years captured the star near the part of its orbit where it moved fastest. Webb was needed because it sharpened the measurement. Its recent near-infrared observations improved the positional precision enough to tighten the orbit and mass estimate, showing the unseen companion was about 4.46 times the Sun’s mass. That ruled out a neutron star, which would be expected below about 3 solar masses, and confirmed a black hole. Earlier methods had missed such objects because this system is not actively feeding, so it does not stand out in X-rays or radio. The discovery depended on combining Hubble’s long historical record with Webb’s higher-precision follow-up.

3 articles · Updated · ESA/Hubble · Jul 13

Astronomers identified oMEGACat BH-2 as the first stellar-mass black hole found in Omega Centauri, a globular cluster about 18,000 light-years away with roughly 10 million stars.
More than 20 years of Hubble astrometry, refined with Webb data, tracked a visible 0.78-solar-mass star orbiting an unseen companion massive enough to rule out a neutron star.
The dark object weighs 4.46 solar masses—lower than expected for Omega Centauri’s metal-poor environment—and the binary completes one orbit every 94 years, the longest known for any black hole binary.
Researchers say the pair was likely formed dynamically inside the cluster and may survive less than 1 billion years, far shorter than Omega Centauri’s roughly 12 billion-year age.
The detection addresses a long-standing puzzle because models suggest the cluster should host about 10,000 stellar-mass black holes, and it could sharpen theories tied to black hole formation and gravitational-wave sources.

Sources

ESA/Hubble1d ago

Hubble, Webb Detect First Stellar-Mass Black Hole oMEGACat BH-2 in Omega Centauri

Phys.org1d ago

Hubble discovers first of star cluster's missing black holes

X16h ago

The Ghost in the Star Cluster: Hubble and Webb Finally Catch Omega Centauri's First Stellar-Mass Black Hole For decades, Omega Centauri has been one of astronomy's greatest teases. This colossal globular cluster —

10 Sources

This new black hole is surprisingly small. Why does its discovery challenge our theories of how massive stars die?

Why did finding a black hole in our galaxy's largest star cluster require two decades of data from Hubble and Webb?

How can a black hole's 94-year orbital 'slow dance' help astronomers decode violent cosmic mergers and gravitational waves?

First Confirmed Stellar-Mass Black Hole in Omega Centauri Reveals Surprising Formation and Hints at Hidden Black Hole Population

Overview

In 2024, astronomers announced the discovery of oMEGACat BH-2, the first confirmed stellar-mass black hole in Omega Centauri, the largest globular cluster in the Milky Way. This finding surprised scientists because oMEGACat BH-2 is lighter than computer models predicted for black holes formed from ancient, metal-poor stars. Previously, theories suggested that such stars should create heavier black holes since they retain more material before collapsing. The unexpected mass of oMEGACat BH-2 challenges these ideas, prompting a rethinking of how black holes form and evolve in old, metal-poor environments.

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