A cluster's masquerade

By Phil Plait | September 8, 2011 6:40 am

When I first saw this picture of NGC 2100, I thought it was a globular cluster. But I was wrong. That happens sometimes. Still, it’s worth it to see such a magnificent photo:

Yegads! What a shot! [Click to enstellarnate.]

Globular clusters are tightly packed collections of thousands of stars in a roughly spherical shape (hence their name), and are generally very old. But upon second glance, the stars of NGC 2100 in this image didn’t look quite right to me. There didn’t appear to be enough, for one thing, and though they’re highly concentrated in the center, the distribution around the core seemed off somehow.

Turns out that’s correct. NGC 2100 is an open cluster, a looser collection of stars. Unlike globulars, they tend to disperse after some tens or hundreds of millions of years, and so any we see are usually pretty young as these things go. NGC 2100 is only about 15 million years old — a baby! — so there you go (I’ll note though, that the NASA/IPAC Extragalactic Database lists it as a globular).

There are also several red supergiants in this cluster: massive stars near the ends of their lives, and ticking away the days until they explode as supernovae. If that happens, they’ll be visible to the unaided eye, though just barely! This cluster is not in our Milky Way, but instead in the satellite galaxy called the Large Magellanic Cloud, well over 150,000 light years away. NGC 2100 happens to lie near the extremely huge and luminous nebula called the Tarantula, where stars are forming at a furious rate. The nebulosity you can see in the picture above is from the Tarantula, located outside the field of view here to the upper right.

The picture itself is actually cool, too: it was taken using seven filters. Three are what are called broad band filters which mimic the way the eye sees light, and were used to create the true color nature of the picture. The other four are narrow band filters, which pick out the wavelengths of light emitted by warm gas — specifically nitrogen, oxygen, sulfur, and hydrogen. It’s interesting that the gas above the cluster is blue and below it is red; that may mean there is a change in the way the gas is heated across this picture. Does the cluster itself affect that? Probably not; the brightest stars in NGC 2100 are not the right kind to be able to excite gas. It’s more likely that this far from the hot stars in the center of the Tarantula is where their influence starts to wane. However, I bet that faint arc of gas above the cluster is from the winds of the stars pushing on the surrounding gas. It appears centered on the cluster, and the size is about right.

All in all, I love stuff like this: a gorgeous picture that is simply wondrous to behold on its own, but which also has significant scientific value. But beauty works that way, doesn’t it? Something pretty may catch our eye, but it needs more depth to hold our attention.

Image credit: ESO

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CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (12)

  1. Nigel Depledge
  2. Gary Ansorge

    My monkey mid brain is enthralled. Good thing it’s not in a bottle,,,(Ack! Hand stuck! Ack!)

    Great pic.

    Gary 7

  3. Jess Tauber

    I see the image of the Flying Spaghetti Monster in all His Noodliness

  4. Messier Tidy Upper

    @ ^ Jess Tauber : Ramen! :-)

    Great image, great write up. :-)

    I first saw this picture of NGC 2100, I thought it was a globular cluster. But I was wrong. That happens sometimes. Still, it’s worth it to see such a magnificent photo.

    Absolutely. If I hadn’t been told otherwise, I’d call it a globular too. ūüėČ

    Hard to believe it’s in another different galaxy rather than our Milky Way even one as close as the Large Magellanic Cloud.

  5. VinceRN

    Still looks like a globular to me, but I bow to superior knowledge. Beautiful as a globular or as an open cluster.

  6. Jes

    Just beautiful! It looks like someone threw a handful of glitter across the sky.

    Ot, but what happened to the Facebook share button? Edit: huh… It popped up after my comment, and the page looks different, too. Weird.

  7. briankb

    Is it a globular that ‘blew’ it’s globular apart?

  8. Crux Australis

    I had a trainee teacher in my class recently who showed the class a pic of M31, and told them that the reason it was slightly red is that it’s moving away from us! *facepalm* I quickly and politely corrected her, she accepted the critique and moved on. Other than that, she was pretty good.

    I wonder what she would have said about this photo before I corrected her; maybe that it was rotating at close to the speed of light?

    Still, a good chance to demonstrate to students that now all facts are above checking.

  9. andy

    This makes me wonder what a globular cluster would have looked like at the time they were young and still forming stars.

  10. Lavocat

    “My God, it’s full of stars!”

  11. Kim

    Just thought that now: astronomers use very narrow filters to catch H-alpha lines and such. Do you have to redshift the filters to catch these lines at far-away galaxies, or filters are not that much precise?

  12. Captn Tommy

    Does anyone else see the bright orange turtle swimming at the bottom of the picture???

    Proof of Disk world! Oh my! Tell the Republicans!



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