The astronomers say the black hole suggests Omega Centauri is a dwarf galaxy, not a globular cluster.

First identified as a single star nearly 2,000 years ago, Omega Centauri was later reclassified as a nebula, and more recently as a globular cluster. It's visible to the unaided eye to those in the southern hemisphere.

The discovery was led by Eva Noyola of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, using images taken by the Hubble Space Telescope, in addition to spectrographic data from the Gemini South telescope in Chile.

Noyola and her colleagues, including astronomer Karl Gebhardt of the University of Texas at Austin, found the high velocities of stars in the cluster's center could be accounted for only by the presence of an invisible object in the center of the cluster with a mass 40,000 times that of the Earth's sun.

"Finding a black hole at the heart of Omega Centauri could have profound implications for our understanding of its past interaction with the Milky Way," said Noyola.