Omega Centauri (ω Cen) or NGC 5139, is a globular cluster in the constellation of Centaurus, discovered by Edmond Halley in 1677 who listed it as a nebula. Omega Centauri had been listed in Ptolemy's catalog 2000 years ago as a star on the horse's back "Quae est in principio scapulae", though the name "Omega Centauri" is a later Bayer star designation, even though the object was later identified as a star cluster. Lacaille also included it in his catalog as a nebula. It was first recognized as a globular cluster by James Dunlop in 1826.

Orbiting the Milky Way, it is both the brightest and the largest known globular cluster associated with our galaxy. Of all the globular clusters in the Local Group of galaxies, only Mayall II in the Andromeda Galaxy is brighter and more massive. ω Centauri is so different from other galactic globular clusters that it is thought to be of different origin.

It is located about 15,800 light-years (4,850 pc) from Earth and contains several million Population II stars. The stars in its center are so crowded that they are estimated to average only 0.1 light years away from each other. It is about 12 billion years old.

Omega Centauri is one of the few globular clusters visible to the naked eye and appears about as large as the full Moon. (Wikipedia)