Omega Centauri (ω Cen or NGC 5139) is a globular cluster in the constellation of Centaurus. It is 15,800 light-years (4,850 pc) away, and is the largest globular cluster in the Milky Way. Its diameter is about 150 light-years. It contains about 10 million stars with a total mass equivalent to 4 million solar masses.
In 150 A.D. Greco-Roman writer and astronomer Ptolemy catalogued this object in his Almagest. Using a telescope from the South Atlantic island of Saint Helena, English astronomer Edmond Halley rediscovered this object in 1677, listing it as a non-stellar object. In 1715, it was published by Halley among his list of six "luminous spots or patches" in the Philosophical Transactions of the Royal Society.
Omega Centauri is one of the few globular clusters visible to the naked eye. It appears almost as large as the full Moon when seen from a dark, rural area. It is the brightest, largest and, at 4 million solar masses, the most massive known globular cluster in the Milky Way. Of all the globular clusters in the Local Group of galaxies, only Mayall II (orbiting the Andromeda Galaxy) is brighter and more massive. Orbiting through the Milky Way, Omega Centauri contains several million Population II stars and is about 12 billion years old.
The stars in the core of Omega Centauri are so crowded that they are on average only 0.1 light years away from each other.
- "Omega Centauri: The largest globular cluster". Universe for Facts. Retrieved 21 December 2014.
- "APOD: 2010 March 31 - Millions of Stars in Omega Centauri". apod.nasa.gov.
- Black hole found in Omega Centauri. ESA.
- Frommert, Hartmut; Kronberg, Christine (March 22, 1998), "NGC 5139", The Munich Astro Archive. 
- "Peering into the core of a globular cluster", Hubble Site news Center.