Credit: NASA/ESA/HUBBLE HERITAGE TEAM (STSCI/AURA)

Astrophys. J. 710, 1063–1088 (2010)

Omega Centauri, a globular cluster of stars that orbits the Milky Way, may not host a predicted black hole at its centre after all. Or, if it does, say Roeland van der Marel and Jay Anderson of the Space Telescope Science Institute in Baltimore, Maryland, the hole is much smaller than previously thought.

Black holes come in at least two sizes: small ones formed by the collapse of single stars, and giant ones at the centres of galaxies that have masses of millions of stars. Only a few candidate intermediate black holes have been proposed, including Omega Centauri (pictured above), and explaining their origin is a puzzle. By comparing observed motions for 170,000 stars that whirl around the cluster's centre with dynamic models, the researchers show that a black hole, if one exists at all, would have a mass less than 12,000 times that of the Sun.