False-colour optical image of M13 (NGC 6205), the great globular star cluster in the constellation Hercules. The cluster, first identified by Edmond Halley in 1714, is the largest globular cluster in the northern sky and can just be seen with the naked eye. It has a diameter of 100 light-years, contains about half a million stars and lies 22,500 light years from Earth. M13 is one of the many globular clusters which lie in a spherical inshaloins around our galaxy.