<P> The different stages of core - collapse may be divided into three phases . During a globular cluster's adolescence, the process of core - collapse begins with stars near the core . However, the interactions between binary star systems prevents further collapse as the cluster approaches middle age . Finally, the central binaries are either disrupted or ejected, resulting in a tighter concentration at the core . </P> <P> The interaction of stars in the collapsed core region causes tight binary systems to form . As other stars interact with these tight binaries, they increase the energy at the core, which causes the cluster to re-expand . As the mean time for a core collapse is typically less than the age of the galaxy, many of a galaxy's globular clusters may have passed through a core collapse stage, then re-expanded . </P> <P> The Hubble Space Telescope has been used to provide convincing observational evidence of this stellar mass - sorting process in globular clusters . Heavier stars slow down and crowd at the cluster's core, while lighter stars pick up speed and tend to spend more time at the cluster's periphery . The globular star cluster 47 Tucanae, which is made up of about 1 million stars, is one of the densest globular clusters in the Southern Hemisphere . This cluster was subjected to an intensive photographic survey, which allowed astronomers to track the motion of its stars . Precise velocities were obtained for nearly 15,000 stars in this cluster . </P> <P> A 2008 study by John Fregeau of 13 globular clusters in the Milky Way shows that three of them have an unusually large number of X-ray sources, or X-ray binaries, suggesting the clusters are middle - aged . Previously, these globular clusters had been classified as being in old age because they had very tight concentrations of stars in their centers, another test of age used by astronomers . The implication is that most globular clusters, including the other ten studied by Fregeau, are not in middle age as previously thought, but are actually in' adolescence' . </P>

Some globular clusters have three main sequence tracks. this indicates