Oh, do I love me some globular clusters! These are compact balls of hundreds of thousands of stars, bound by their own gravity, orbiting each other every which way like bees circling a hive. Even through a small telescope they're magnificent. Of course, when you point Hubble at one, it's spectacular
M15 was always one of my favorites as a young man, standing with my 'scope at the end of my driveway in the late summer and early fall when Pegasus and the cluster were high in the sky. Bright, large, and easy to find, it was so pretty that I would stare at it and wonder what it would be like to be on a planet inside there, with hundreds of stars blazing away as bright as Venus...
What I didn't know then is that one particular star was different. Just below and to the left of the cluster center is a vividly blue star (see the super-duper high-res embiggened version to get a better look). It turns out that's a star like the Sun, but it's dying. It's shedding material at a furious rate, and lighting it up like a neon sign. Although it has nothing to do with planets, this is called a planetary nebula. While globular clusters are billions of years old, the planetary nebula phase of a star's life only lasts for a few thousand years, so it's exceedingly rare to find them in globulars. And even though it's marking the end of a star's existence, that flash of cerulean blue makes this cluster just that much more exquisite.
Image credit: ESA/Hubble & NASA