Physical Properties

At a distance 15,600 light years, Omega Centauri is one of the nearest globular clusters to the Solar System. Its visual size of about 36' corresponds to a true diameter of 175 light years. Its photographic size is 65', implying a diameter over 300 light years. As in all globular clusters, the stellar density increases rapidly toward the interior. The average distance between stars at its center is only about 0.1 light years.

Containing several million stars, and roughly 5 million solar masses, Omega Centauri is about 10 times as massive as a typical big globular, and about as massive as the smallest of whole galaxies. It is the brightest and most massive globular orbiting the Milky Way, and of all the globular clusters in the Local Group, only Mayall II (G1) in the Andromeda Galaxy (M 31) is more massive and luminous.

Origin and Evolution

Omega Centauri is so different from the MIlky Way's other globulars that it is thought to have a different origin. It is about 12 billion years old, but as suggested by a 1999 study, the stars of Omega Centauri formed over a 2-billion-year period rather than all at once. The team that carried out this work speculated that Omega Centauri is the remaining core of a small galaxy that merged with the Milky Way long ago.

NGC 5139 imaged by the Hubble Space Telescope.

This suspicion was confirmed in 2008, when a team of astronomers working with the Hubble Space Telescope and Gemini Observatory reported that Omega Centauri harbors an intermediate-sized black hole at its center. The motions of the stars at the center of Omega Centauri are much faster than expected from their mass alone. From this, the mass of this black hole was deduced to be about 40,000 Suns.

In 2009, astronomers announced that Kapteyn's Star, the 25th nearest star system at only 13 light years away, may have originated from Omega Centauri. Along with 16 other stars in the same moving group, all of which orbit the galaxy backwards and are very old, it shares the same elemental abundances as Omega Centauri's stars. If Omega Centauri is the left-over nucleus of a dwarf galaxy that merged with the Milky Way, some stars were flung towards us during the merger, including Kapteyn's.