Hercules Cluster (M13)
Every known regular galaxy has been found to contain several hundred of these mysterious dense collections of stars, known as globular clusters. Our Milky Way contains approximately 150 such clusters, the origins of which are poorly understood. They exist in the halo region of a galaxy - the half-spherical areas above and below the galactic plane - and are ancient relics from the formation of the galaxy. Many clusters exhibit bi-modal populations of stars including the Hercules cluster, which displays both red-yellow and blue-white stars numbering in the hundreds of thousands. Some clusters contain several million stars.