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.

Omega Centauri - NGC 5139 (credit:-  ESO)

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.

Finder Chart for Omega Centauri - NGC 5139 (credit:- freestarcharts)

Finder Chart for Omega Centauri - NGC 5139 - pdf format (credit:- freestarcharts)

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

NGC5139
Caldwell80
NameOmega Centauri
Object TypeGlobular Cluster
ConstellationCentaurus
Distance (light-years)15,800
Apparent Mag.3.7
RA (J2000)13h 26m 46s
DEC (J2000)-47d 28m 37s
Apparent Size (arc mins)36
Radius (light-years)85
Age (years)12 Billion
Number of Stars10 Million
Notable FeaturePossible core remnant of a disrupted dwarf galaxy. Naked eye, largest and brightest Milky Way globular.

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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