In the constellation Centaurus
40,000 times the mass of the Sun
Diameter 150,000 miles (240,000 km), about two-thirds of the distance from Earth to the Moon
For almost two centuries, astronomers have classified Omega Centauri as a globular cluster -- a dense collection of stars packed into a spherical region of space only a few dozen light-years across.
Yet in more recent years, they have started having doubts, because Omega Centauri is unlike any other globular cluster in the Milky Way galaxy.
For one thing, it is by far the largest and most massive globular in the galaxy -- about 10 million times the mass of the Sun, compared to a few hundred thousand solar masses for most other globulars. For another, it is more flattened than any other globular cluster. And while other globulars contain only very old stars, Omega Centauri has a mixed population that includes many younger stars.
All of this evidence has suggested that Omega Centauri may really be a small galaxy that was captured by the Milky Way. And a finding from 2008 appears to seal the deal: a black hole about 40,000 times as massive as the Sun appears to inhabit its core.
Astronomers used Hubble Space Telescope and the Gemini South Telescope in Chile to measure the motions of stars near Omega Centauri's densely packed center. They found that the stars are accelerated to high speeds as they orbit the core. The speeds cannot be accounted for by the mass of all the visible stars in the cluster, so some unseen mass must be causing them to move faster. An analysis of the orbits suggests that the source of this acceleration is an intermediate-mass black hole -- one of only a handful discovered to date.
Supermassive black holes inhabit the cores of most big galaxies, including the Milky Way. There is a relationship between the mass of the black hole and its surrounding galaxy. Although the black hole in the center of Omega Centauri is smaller, it seems to follow the same relationship, providing more evidence that Omega Centauri was once an independent galaxy.
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This document was last modified: November 11, 2011.