We begin our bucket list for backyard stargazers with the dazzling star cluster Omega Centauri. Found in the deep southern sky, this globular cluster is an easy target for southern-hemisphere observers from March through October. But in May and early June, some northern stargazers get their best chance to spot the cluster as it peeks just above the southern horizon.
And it’s well worth a look. So breathtaking is this swirling mass of stars in a small telescope that astronomy writer Stephen James O’Meara says “observing Omega Centauri is like peering into the working mind of the Creator.”
Omega Centauri is the 800-pound gorilla of our galaxy’s globular clusters.
Hubble Space Telescope image of the core of Omega Centauri (credit: NASA)
Like other “globs”, Omega Centauri is a gravitationally-stable sphere of 12-billion-year-old stars that formed not long after the universe itself.
And like other globs, it orbits the center of our galaxy in a halo of some 180 similar ancient star clusters.
But Omega Centauri is far bigger than the rest. It packs 5-10 million stars into a diameter of 150 light years, a density some 10,000 times greater than we see in our own night sky. By some estimates, it’s at least 5-10x more massive than any other globular cluster in the Milky Way.
Omega Centauri is exceptional in another way. It seems to have formed more slowly than other globs, with two episodes of star formation over two billion years. This suggests it formed by a different mechanism than other globular clusters. Some astronomers speculate it may be the remains of a separate dwarf galaxy absorbed by the Milky Way billions of years ago.
The cluster is one of the few of its kind visible to the unaided eye. Which brings us to its name. Renaissance astronomers cataloged the cluster as a star, and at the time, stars were labeled roughly in order of brightness with Greek letters Alpha to Omega. Since the massive cluster looked like a dim star, it was listed as the star Omega Centauri. Not until John Herschel turned a large telescope on the “star” was its true appearance revealed.
The star cluster Omega (ω) Centauri, upper left, about 13 degrees northeast of the star Gacrux at the top of the Southern Cross (click to enlarge)
And what an appearance! Even in binoculars, the cluster is magnificent. Its misty glow spans a nearly a full degree of sky, twice the span of the full Moon.
Turn a 3 or 4-inch telescope on this cluster and it becomes a shimmering ball of stars, glowing like a frosted light bulb against a rich background of closer-by stars. Even in a small scope, individual stars are visible around the edge; a slightly larger scope resolves the 12-billion-year-old stars right to the core. When you have this object in your sights, look carefully and don’t rush. Examine the color and pattern of the stars, and enjoy watching for shapes and streams and gaps in the rich stellar tapestry of this magnificent cluster. Use low magnification to make sure the cluster fits in your field of view.
To see Omega Centauri at all, you need to be south of 43ºN latitude, roughly. It’s easy to see south of 30ºN latitude. Ideally, you can venture south of the equator, where the cluster is high in the sky and well placed for viewing from March to October. But a diligent few have seen it from as far north as Point Pelee, the southernmost point in Canada. From there, the cluster appears to skim the surface of Lake Erie for a few clear spring nights.
In southern latitudes, you can find Omega Centauri about 13 degrees– a little more than the width of your fist held at arms length– northeast of gamma Crucis, the top star of Crux, the Southern Cross. The cluster lies some 16,000 light years from Earth.
Find it, enjoy it, and tick one object off your celestial “bucket list”.