At the heart of Omega Centauri, one of the most massive star clusters in the Milky Way, an invisible object has finally revealed its presence.
Thanks to more than twenty years of observations by Hubble, complemented by James Webb, astronomers have identified the first confirmed stellar-mass black hole in this extremely dense environment.
This globular cluster gathers about 10 million stars bound by gravity, nearly 18,000 light-years from Earth. Models predict it should also harbor nearly 10,000 small black holes. Until now, these remnants of dead stars remained nearly impossible to distinguish among this luminous crowd.
The new object, named oMEGACat BH-2, was not detected by X-rays or radio emissions. Researchers tracked the minuscule motion of a visible star. Its trajectory indicated it was orbiting a dark companion, massive enough to only be a stellar-mass black hole.
The measurements are based on images taken by Hubble between 2002 and 2023, then refined with James Webb's infrared capabilities. This combination allowed the position of the star to be tracked with sub-pixel precision. Astronomers thus reconstructed a significant portion of its orbit around the invisible object.
The black hole's mass reaches about 4.46 times that of the Sun. Its companion star is only 0.78 solar masses. A previous study had instead suggested a neutron star, but the new estimate rules out this possibility. The dark object is too heavy to belong to that category of stellar remnants.
Composite image of ω Cen made from HST images.
Left: position of the visible star relative to the cluster center (J. Anderson & RP van der Marel 2010 ; M. Häberle et al. 2024b ).
Right: close-up view of the visible star and its neighbors, notably a bright horizontal branch star located about 1″ to the northwest.
Image credit: ESA/Hubble, NASA, Maximilian Häberle (MPIA).
Omega Centauri contains few heavy elements, a characteristic that generally favors the formation of more massive black holes. oMEGACat BH-2 thus falls below the expected values.
The pair has another peculiarity: the star completes one revolution in about 94 years. This is the longest known orbital period for a binary system containing a black hole. The two objects were probably not born together but met through gravitational interactions within the cluster.
However, this association could be temporary on a cosmic scale. Simulations indicate that encounters with other stars will likely separate the pair in less than a billion years.