April 8, 2008 -- Omega Centauri is one odd bird. Sparkling almost as big as the full moon in Earth's southern hemisphere, the object has been classified as a globular star cluster for more than a century. But it had some troubling peculiarities.
First of all, the cluster, located about 17,000 light-years from Earth, is huge -- nearly 10 times larger than other globular clusters, which typically have about one million stars tightly bound by gravity.
Also, most globular clusters are like retirement communities for senior stars. Omega Centauri has a more integrated population with old, middle-aged and young stars. It rotates faster and has a flatter shape.
Astronomers may have figured out the reason why: Omega Centauri has a black hole in its heart, a finding that may have revealed the globular cluster's true identity as a dwarf galaxy that was robbed of outlying stars by our very own Milky Way.
Writing in this week's Astrophysical Journal, scientists explain how pictures taken with the Hubble Space Telescope and spectral data from the ground-based Gemini Observatory show stars at the center of Omega Centauri moving much faster than expected, considering the cluster's size.
The most logical explanation, they say, is a medium-sized black hole that is about 40,000 times bigger than our sun. Only one other black hole of this size has been found, said astronomer Karl Gebhardr with the University of Texas at Austin.
"This result shows that there is a continuous range of masses for black holes, from supermassive, to intermediate-mass, to small stellar-mass types, said astronomer Eva Noyola with the Max-Planck Institute in Germany.
Scientists theorize that intermediate-mass black holes may be seeds for supermassive black holes, which exist in the centers of many galaxies, including the Milky Way. "We may be on the verge of uncovering one possible mechanism for the formation of supermassive black holes," Noyola said.
Scientists next plan to use the European Southern Observatory's Very Large Telescope in Chile for followup observations.
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