Thousands of ancient stars sparkle in the heart of M13, one of the largest star clusters in the Milky Way galaxy. It contains perhaps 500,000 stars packed into a ball that's only a few dozen light-years across. The cluster's stars are all 10 billion years old or older. Under dark skies, M13 is just visible to the unaided eye in the constellation Hercules. [NASA/ESA/STCsI]