When you look at this picture of NGC 2100, you're forgiven if you think it shows a globular cluster. That's what I thought it was at first!
But it's not. Globulars are ancient ball-shaped collections of hundreds of thousands of stars bound by their own gravity. NGC 2100, though, is actually far smaller, and is called an open cluster. For one thing, NGC 2100 doesn't have as many stars as a globular does, and for another, it's quite young, only 15 million years old. Globulars are billions of years old!
This picture, taken with the European Southern Observatory's New Technology Telescope, is lovely. The cluster is sitting near or in a lot of gas; it's in the outskirts of the vast (and I mean vast) Tarantula nebula. The fact that the gas above it is blue and below it is red is telling us something. At the core of the Tarantula (to the upper right, off-screen) is another cluster of stars, but this cluster is younger, more vigorous, and has a large number of extremely massive and hot stars, luminous enough to cause oxygen in the gas to glow blue. However, farther away their light dims and cannot excite that gas. It glows red there due to warm hydrogen, which doesn't take as much energy to light up. Most likely NGC 2100 has little or no influence on this gas; the stars in it don't have the oomph needed to light it up. But note the arc of material just above the cluster; could that be gas being pushed by the combined winds of the stars in NGC 2100? It's the right size, and centered right on the cluster. Maybe!
But having a dense cluster like that sitting, from our view, right where the gas in the nebula changes degree makes for a fantastic picture, even if it did fool me into thinking it was something it wasn't.
Image credit: ESO