A photo from the Nasa/ESA Hubble Space Telescope released in December 2012 shows NGC 6388, a middle-aged globular cluster in the Milky Way. While the cluster formed in the distant past - like all globular clusters, it is more than 10bn years old - a study of the distribution of bright blue stars within it shows that it has aged at a moderate speed, and its heaviest stars are in the process of migrating to the centrePhotograph: F Ferraro/AFP/Getty Images