M13, also known as the Great Globular Cluster in Hercules, is a beautiful cluster of more than 100,000 stars concentrated into a region of space about 150 light years across. It lies about 25,100 light years away in the constellation of Hercules and appears roughly half the apparent size of the full moon. At magnitude 5.8, it is just visible to the unaided eye on a dark night. M13 was discovered by Edmund Halley in 1714.