#6: M13 Hercules Cluster, by hgg

Located 25,000 light-years away, the M13 globular cluster looks like a beehive. In fact it's a tight collection of several hundred thousand stars, most of which are packed into a ball just a few light-years across (though the whole object is over 100 light-years in diameter).

It's easily visible using binoculars, but only in a large telescope does its true splendor reveal itself. Over 150 such clusters orbit our Milky Way galaxy, but M13 is one of the best known and best observed.

This image clearly shows stars nearly all the way into the core, and you can see that many of the stars are red, indicating their great age: The cluster is over 10 billion years old.

Image source