All stars seem to form from large molecular clouds, and each such formation event seems to produce many stars - hundreds or thousands. Many of these clouds quickly disperse, but so-called open clusters can endure for billions of years. They do exhibit a peculiar behavior though. Instead of the expected behavior, called dynamical relaxation, in which the heaviest stars sink gradually toward the center while the lighter ones expand they seem to show a very gradual uniform expansion. It appears that this is due to interactions of other stars with binaries pairs of stars. (A SciAm article by Steven W. Stahler)
What drives the uniform expansion of open clusters? Converse and I demonstrated that the key is binary stars: pairs of close, orbiting companions that are quite common in stellar groups. Simulations performed by Douglas Heggie, now at the University of Edinburgh in Scotland, showed in the mid-1970s that when a third star approaches such a pair, the three engage in a complicated dance, after which the lightest of the three is usually ejected at high speed. The ejected star soon encounters other members and shares its energy with them, increasing those stars’ orbital velocities and e!ectively “heating up” the cluster. In our N-body simulations, it was the energy from these binary encounters that caused the open cluster to expand— albeit so slowly that the expansion could easily go unnoticed by astronomers.
The article is interesting throughout, with the usual good Scientific American figures and charts.