Omega Centauri (NGC 5139)
Distance: 15,000 Light Years

Right Ascension: 13 : 26.8 (hours : minutes)
Declination: -47 : 29 (degrees : minutes)


Text copyright Robert Gendler

Omega Centauri represents the most massive and brightest stellar systems of all known Milky Way globular clusters. Its 10 million stars make it similar in size to the smallest known galaxies. Its immense age of 16 billion years makes it one of the most ancient objects in the universe. Aside from its impressive size and brilliance Omega has attracted attention for the complexity of its stellar populations. Star populations within globular clusters are characterized by their metal content which is usually homogeneous among cluster members. Omega shows several distinct peaks in the metallicity of its stellar members. The bluer of its hydrogen fusing stars, which comprise 25% of the main sequence stars show an increased metallicity. These high metallicity stars have a surprising overabundance of helium of 50% compared to the lower metallicity group. The origin of these different populations has attracted different theories to try to explain the disparate stellar populations. One explanation given is that the helium rich stars were formed in a more recent epoch of star formation after the interstellar medium was helium enriched by several generations of supernovae. Alternatively differing populations of stars may be accounted for by mergers and captures of stars during the immense history of Omega. One fascinating theory is that Omega may be the remnant nucleus of an ancient satellite dwarf galaxy, which was tidally stripped and later merged with the halo of the Milky Way. The differing stellar populations may be the signature of past merging events in the remote history of the progenitor satellite dwarf galaxy.