Omega Centauri or NGC 5139 is a globular cluster of stars seen in the constellation of Centaurus, discovered by Edmond Halley in 1677. It orbits our galaxy, the Milky Way. One of the few that can be seen with the naked eye, it is both the brightest and the largest known globular cluster associated with the Milky Way. Omega Centauri is located about 18,300 light-years (5,600 pc) from Earth and contains several million Population II stars. The stars in its center are so crowded that they are believed to be only 0.1 light years away from each other. It is about 12 billion years old.

Though it is not a star, Omega Centauri was given a Bayer designation. Unlike other globular clusters, it contains several generations of stars. It has been speculated that Omega Centauri may be the core of a dwarf galaxy several hundred times its present size which was ripped apart and absorbed by our Milky Way galaxy. Omega Centauri's chemistry and motion in the galaxy are also consistent with this picture.

Like Mayall II, Omega Centauri has a range of metallicities and stellar ages which hints that it did not all form at once (as globular clusters are thought to form) and may in fact be the remains of the core of a smaller galaxy long since captured into the Milky Way.