Spitzer Space Telescope caught this image of the cluster Omega Centauri, which I recently wrote about. Whether Omega Cen is a galaxy or a globular cluster doesn't change the fact that this is a striking image. It's actually a combo of Spitzer and ground-based images taken with a 4-meter telescope in Chile. Stars shown here as blue (more on that in a sec) are low-mass stars seen in both the 4-meter 'scope and Spitzer. Yellow and red stars were seen only by Spitzer. Since it looks in the infrared, that means these stars are probably more evolved. Why? Clusters (or galaxies) like Omega Cen form pretty much all at once, using up all their gas to make stars. Since high-mass stars age more rapidly, and clusters like this are old, all the more massive stars have already either blown up (if they were massive enough) leaving behind black holes or neutron stars, or have long since turned into red giants and blown off their outer layers. That leaves behind a very faint white dwarf. Omega Cen is very old, so only lower mass stars are still around, but even they have evolved into red giants. Those stars tend to emit more light in the infrared, both due to their low temperature as well as blowing off lots of dust, which enshrouds them and reddens their light. In fact, these images were taken to investigate how much dust these stars are generating. Fewer dusty stars were found than expected, which is interesting. Clusters like this tend to have lower metals (the term astronomers use for anything on the periodic table heavier than helium) because they formed early in the Universe, before massive stars could make those heavy elements and seed them into space. Could this lack of metals affect the dust formation? Images like this one can lead us to those answers. By the way, in Omega Cen the lowest mass stars are still going strong. Ironically, in this false-color image, those stars are colored blue (meaning visible light), but if you looked at them with your eye they'd look orange or red. False color images can be tricky, and in this case things are really topsy-turvy. But it sure is a pretty picture.