Binaries in clusters act as energy sources and sinks. As sinks of gravitational potential energy, they can absorb more than one-half the total binding energy of a cluster. As corresponding sources of kinetic energy they heat the cluster.

How fast will binaries form? How important can they be? An analytic theory of these processes, even in the approximations for which it is available, is quite complex. N-body simulations are sometimes the only way to answer detailed questions, especially for a broad spectrum of masses. Nevertheless, many useful results are available, and this section describes some. Following discussions of how binaries form by interactions of single stars, we consider how they can affect a cluster's evolution. From a more general point of view, these can be regarded as examples of energy transfer between different levels of hierarchial structure in a cluster.

Formation by few-body interactions

Binaries may be born directly when stars or galaxies form. We do not really understand how this occurs, so all we can do is add such initial binaries into the cluster in an ad hoc way, and then examine their development. Even if there are no initial binaries, however, the fluctuating gravitational field may chance to bring a few stars so close together, with such low relative velocities, that they remain temporarily bound. Usually these subclusters lead a fleeting existence, for later fluctuations may inject enough kinetic energy to make them decay.