A study of the H/R diagrams of various clusters will show a small sprinkle of stars blueward of the turnoff point. These "blue stragglers" are thought to be merged binary pairs. When one star reaches the giant phase, it expands enough to envelope its neighbor and the two coalesce into a single star that settles down to a larger and younger looking member of the cluster and thus will burn hotter and be bluer than the turnoff point. The very close binary pairs that are required for this are a product of the orbital mechanics that occur in these very dense clusters. When two close stars encounter a third star, one member is thrown off at high velocity and it takes away momentum that leaves the other two in a very close pair. Over time a lot of these interactions take place and a population of very close pairs builds up. http://en.wikipedia.org/wiki/Blue_straggler DT