Globular clusters are magnificent balls of millions of stars packed into a relatively small, roughly spherical volume. And Omega Centauri is the grand daddy of them all, so huge and relatively  close that it can be seen with the naked eye.

Omega Cen is also very old, and a lot of the stars in it have aged to the point where they have become red giants. This image is a combination of visible light taken with a 4-meter telescope in Chile (colored blue) combined with images from Spitzer (green and red).

Stars like the Sun - still happily churning away, fusing hydrogen into helium in their cores - appear blue, but the red and yellow stars are older, and have become red giants. These stars are well on their way to dying, as our own Sun will... in another 6 billion years or so.

Original blog post
Original press release
Image credit: NASA/JPL-Caltech/M.Boyer (University of Minnesota)