Omega Centauri Come On Down

Back here I covered Duncan Forbes efforts to engage the public in the question of what a galaxy actually is, and how one distinguishes it from a globular star cluster.

Voting is still open, but enough people have taken part – 1600 as of a couple of weeks ago  – that Forbes felt comfortable to release the results. So far 68% have considered the presence of complex stellar populations to be the best test of galactic status.

I find this interesting for three reasons. Firstly it’s rather overwhelming. With five definitions in the running for one to get more than twice the other four combined is remarkable.

Secondly, it’s not the answer I would have thought would have won. If you walked down the street and asked people what made something a galaxy rather than a star cluster my guess is that most of those who had a some idea of the terms would say size. Perhaps I am wrong on that, but I think this suggests either that those who took part were atypical (which is likely) or that reading the paper made a difference. I find the latter particularly intriguing. I’d love to have seen a before and after poll, where people were asked their initial reactions, and then what they thought after reading Forbes’ paper. However, this might bias things, with some people not wanting to change their minds.

Finally, I note Forbes’ comment, “Since it was discovered, some people have referred to Omega Centauri as a massive star cluster, however others regard it as a galaxy. According to the popular definition, it has the status of a galaxy.” I always heard Omega Centauri described in astronomical circles as the best globular cluster in the sky. It wasn’t until I wrote an article for Australasian Science in which a scientist proposed it was the core of an ancient dwarf galaxy that I heard anything different. Since I’ve since covered theories that a quarter, or even all, globular clusters were once dwarf galaxy cores, so my impression is that the idea that Omega Centauri being upgraded while most other globulars are left behind is a rare one.

Nevertheless, I rather like it. Most books on naked eye astronomy will tell you it is only possible to see three galaxies other than our own without a telescope – Andromeda and the Large and Small Magellanic Clouds. The former is hard to see from most of the Southern Hemisphere, while the latter two are impossible from most of the north. Adding another southern hemisphere object further emphasizes the superiority of our skies.

So, feel free to go and mess with the figures if you like, but until then I’m going to celebrate with this image revealing just how diverse Omega Centauri’s stellar population really is. And I’ll be seriously chuffed if at my next astronomy night anyone mentions this post and asks to see the newly admitted member of the galactic club.

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About Stephen Luntz

I am a science journalist, specialising in Australian and New Zealand research across all fields of science. My book, Forensics, Fossils and Fruitbats: A Field Guide to Australian Scientists is out now through CSIRO Publishing. I am also a professional returning officer for non-government organisations. I'm very politically active, but generally try to restrict this blog to scientific matters.
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