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.