The immense Omega Centauri is widely regarded as the most impressive globular cluster in the sky. Its deep southern location makes this object challenging to spot — but it can be done, as Italian observer Giuseppe Pappa explains.
Around mid-May to early June, the approach of northern summer means the nights become short. But the evening sky continues to show a lot of interesting deep-sky objects of all kinds. For those who live around mid-latitudes (35° to 40° north), the Big Dipper is high in the north after sunset, while Arcturus and Spica shine toward south. This year bright Jupiter is also passing through Leo, and Mars, near opposition, is easy to spot in the east.
Among the various objects in view at this time, one has a particular appeal for me because it is the biggest globular cluster visible in the sky: Omega (ω) Centauri or NGC 5139. Having a mass of perhaps 5 million suns, Omega Cen is 10 times more massive than a typical globular cluster — and it’s 230 light-years across.
This object got the name “Omega” inadvertently. It was first mentioned in AD 150 by Ptolemy, who referred to it as a star on a horse’s back. Much later, Johannes Bayer used Greek letters to designate stars in his famous Uranometria, published in 1603, and he assigned Omega to this “star” in Centaurus. When Edmond Halley rediscovered this object in 1677, he described it as the nonstellar telescopic object we now know it to be.
Several studies reveal that Omega Centauri has different stellar populations that formed at varying periods of time, unlike the situation with other globulars. So some astronomers suspect that this object is a remnant of a small dwarf galaxy that merged with the Milky Way.
Omega Cen has a declination of –47½°, and it’s easily seen by eye as a fuzzy star from the Southern Hemisphere. But here in the north, it only barely clears the southern horizon, if at all, and can only be seen at certain times of the year. The most convenient time to see it in the evening sky is from May to June around 10 or 11 p.m. (That corresponds to about 1 a.m. in mid-April and 3 a.m. in mid-March.)
You have a chance if your latitude is no farther north than 40°. Depending where you live, Omega Centauri culminates over the southern horizon with an altitude of 5° to 10°. So, to see it, you’ll need a very clear and dark sky without haze or much light pollution.
Start your search for this spectacular object with Spica, the brightest star in Virgo. Spica and Omega Centauri have nearly identical right ascensions, so they transit the north-south meridian in unison — except that Omega Cen is about 35° below Spica.
I took the pictures shown here last year from Sicily. The one above gives a good impression of what you might see by eye. With a good eyesight and careful attention, you can spot what appears to be an object that’s not starlike but a small fuzzy ball (which, in reality, contains billions of stars).
The second image, taken with a DSLR camera and a 300-mm lens, gives a decent impression of how Omega Centauri looks when as seen with binoculars. As you can see it is visible despite the skyglow from a city. With typical 10×50 binoculars it becomes a fantastic sight, with a lot of stars concentrated in very little area. With a telescope you can resolve more and more stars.
So try to find Omega Centauri yourself — and maybe it will become one of your favorite targets too.