A cluster's masquerade

Contributed by
Sep 8, 2011
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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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