Omega Centauri was first discovered by Greek-Egyptian astronomer Ptolemy and recorded in his Almagest around 150 AD - but he listed it incorrectly as a star. It was not known then that the object was in fact a globular star cluster tightly packed with an estimated ten million stars. Omega Centauri was named by Johann Bayer in his 1603 star catalogue Uranometria, which is why it has the Greek letter nomenclature (like an individual star). It was subsequently noted by Edmond Halley in 1677, who correctly identified it as a globular star cluster, and so 'officially' discovered it. He described it in his notes as 'a luminous spot or patch in Centaurus' and tagged it Halley 180. Bayer's designation (original tag), however, has not been changed by the scientific community, so the spectacular globular cluster is still referred to as the enigmatic-sounding Omega Centauri.

Other Designations

• French astronomer Nicolas Louis de Lacaille (1713-62) collated the catalogue Coelum Australe Stelliferum (Catalogue of Nebulae of the Southern Sky) which described 42 nebulous regions among almost 10,000 southern stars. These deep-sky objects included the globular cluster Omega Centauri as Lac I.5.

• Swiss astronomer Philippe Loys de Chéseaux (1718-51) was the discoverer of the magnificent six-tailed comet C/1743 X1¹ which graced the skies between December 1743 and March 1744. De Chéseaux also catalogued 21 deep-sky objects, and Omega Centauri appeared as de Chéseaux 18 in his listing.

• Scotsman James Dunlop (1795-1848), working at the Brisbane observatory in Australia, put together a catalogue of over 300 verified deep-sky objects. Omega Centauri featured as Dunlop 440.

• The New General Catalogue (NGC) was compiled by John Louis Emil Dreyer, the director of the Armagh Observatory from 1882 - 1916. Omega Centauri is NGC 5139.

• Sir Patrick Moore included it in his catalogue for backyard astronomers (The Caldwell Catalogue) as Caldwell 80.

The cluster measures 150 light years² across and is approximately 16,000 light years away, orbiting our galaxy the Milky Way. Omega Centauri is different to most other star clusters in that its stars are different in age and chemical make-up to each other. It is generally accepted that Omega Centauri may be the remnant of a dwarf galaxy which passed too close to the Milky Way many millions of years ago. Such encounters are called 'cosmic trainwrecks', and over time, thanks to gravity, the galaxies merge, resulting in one larger galaxy. Omega Centauri has been studied by the Hubble Space Telescope (HST) and resulting data analysed by astronomers produced evidence of the presence of an intermediate-mass³  black hole.

Images of Omega Centauri

During a servicing mission in 2009 the Hubble Space Telescope was fitted with a WFC3 (Wide Field Camera 3). One of the most spectacular new images was that of Omega Centauri. The Astronomy Picture of the Day website provides a wealth of spectacular images and Omega Centauri features regularly.