The binary star population of star clusters

Most open clusters contain a substantial number of binary stars. In some of the old clusters (notably M67) the binary star frequency is indistinguishable from the field binary star population. There are, nevertheless, some young and old clusters with relatively few binaries. This raises the question of whether they were formed as bound systems while the cluster itself was forming; whether they were created later in three-body encounters; or whether the environment-of some clusters leads to the destruction of nearly equal-mass binaries. Most open clusters also contain substantial numbers of blue stragglers, which mag; or may not be binaries, and some old clusters contain a number of W UMa stars. In contrast, the globulars contain relatively few binaries, most of them being tightly-bound systems located near the cores of the clusters. The rich variety of binary star phenomena in star dusters makes then natural laboratories for studying the formation and evolution of binary stars.