Omega Centauri or NGC 5139 is a globular cluster of stars orbiting our galaxy, the Milky Way, discovered by Edmond Halley in 1677. It is the largest globular cluster associated with the Milky Way that has been discovered to date, and is one of the few that can be seen with the naked eye. Omega Centauri is located about 18,300 light years (5,600 pc) from Earth and contains several million Population II stars. It is about 12 billion years old.

Omega Centauri's center is located at right ascension, declination, with an apparent magnitude of 3.68, and covering 36′.3 of sky. It is in the constellation Centaurus, and was given a Bayer designation despite the fact that it is a cluster and not an individual star.

With more observations, it has been found that Omega Centauri is not like other globular clusters, in that it has several generations of stars. Laura Stanford, a graduate student in the Australian National University’s Research School of Astronomy and Astrophysics who conducted a study of Omega Centauri's stars, speculates that it is actually the core of a dwarf galaxy several hundred times its present size, which was ripped apart and absorbed by our Milky Way galaxy. Theoretical modeling by Dr Glenn van de Ven at Leiden University agrees with her suspicions.