Messier 15, or M 15 among friends, is a globular cluster located 10 kiloparsecs or so from us, one of the 150 or so globular cluster associate with the Milky Way galaxy.
In addition to being very pretty, it's very old, about 12 billion years, nearly as old as the Universe itself (13.6 billion, or so). All the 100,000 stars in it are thought to have formed at about the same time from a single large gas cloud. Blue stars, you may recall, are all very young - they burn themselves out quickly. So how does M 15 have some blues? It's thought that they may have formed from collisions or mergers of smaller stars, perhaps those that formed as close binaries.
M 15 is also thought to harbor an intermediate mass black hole (IMBH), a black hole larger than those formed in Supernovae of a few solar masses, but smaller than the million and billion solar mass monster than inhabit the center of galaxies - perhaps a few thousand solar masses.
It also has a planetary nebula, the blue patch above and to the left of center, the only one known in a globular cluster.