Omega Centauri (NGC 5139) is the brightest and most spectacular globular cluster in the sky. Located in Centaurus and visible to the naked eye (mag. +3.7) it was believed by early astronomers to be star. In AD 140, Greek mathematician and astronomer Ptolemy included it in the "Almagest" his star catalogue. Using Ptolemy's data, Johann Bayer a German lawyer and uranographer (celestial cartographer) designated the "star" as Omega Centauri in his publication Uranometria (1603). It was Edmond Halley, in 1677, who first identified its non-stellar nature.
The cluster is best seen from southern locations during the months of March, April and May. From latitudes north of 42N it never rises above the horizon and can't be seen at all. It's listed as number 80 in the Caldwell catalogue.
The globular is positioned close to the middle of Centaurus. Located 5 degrees east of Omega is zeta Centauri (ζ Cen - mag. +2.6). About 15 degrees southeast of the globular are first magnitude stars Alpha and Beta Centauri and about the same distance on the southwestern side is the famous constellation of Crux, the Southern Cross.
Omega Centauri lies 15,800 light-years distance and spans 36 arc minutes of apparent sky, larger than the apparent diameter of the full Moon. It's estimated to contain 10 Million stars and with a spatial diameter of 170 light-years is also the Milky Way's largest globular. It has been speculated that Omega Centauri may be the core of a dwarf galaxy that was disrupted and absorbed by the Milky Way. Kapteyn's Star, located only 13 light-years distant, is thought to originate from Omega Centauri.
To the naked eye, Omega appears as a hazy star or as a comet without a tail. When seen through binoculars its non-stellar nature is obvious, appearing as a large fuzzy disk. For comparison, Omega Centauri is about 50% larger than the most prominent globular cluster in the Northern Hemisphere, M13 in Hercules. A 150mm (6-inch) scope under dark skies resolves the cluster into a number of stellar points. The best view of Omega comes with 250mm (10-inch) or larger scopes. At low powers through such instruments, words don't do it justice. Omega looks sensational, an enormous object that fills or overflows the field of view with thousands of stars visible extending out from the dense core. A celestial wonder!
NGC 5139 Data Table
|Object Type||Globular Cluster|
|RA (J2000)||13h 26m 46s|
|DEC (J2000)||-47d 28m 37s|
|Apparent Size (arc mins)||36|
|Age (years)||12 Billion|
|Number of Stars||10 Million|
|Notable Feature||Possible core remnant of a disrupted dwarf galaxy. Naked eye, largest and brightest Milky Way globular.|