Although stars are formed in groups, these groups typically do not stay together for very long. Perturbations from other stars and gas clouds in their vicinity are often enough to break up the fragile gravitational hold they initial have over each other. Some of the more massive groups of newly formed stars, however, are sufficiently tightly bound to survive their environmental harassment. They form the so-called open clusters, where there name indicates that they have central densities that are typically less than what we see in globular clusters.
Fig. 1.4 shows one of the richest and densest open clusters, M67, as observed by the Anglo-Australian Observatory. Since this cluster is old enough to have lost its gas and dust, all stars are visible at normal optical wavelengths, at which this image is taken. In the central regions of this cluster, there are indications that some of the stars have undergone close encounters or even collisions. Consequently, this star cluster qualifies as a dense stellar system.
Open clusters typically have fewer members than globular clusters. Also, they are younger. Both facts makes it easier to simulate open clusters than globular clusters. On the other hand, the densest globular clusters show a higher frequency and a far richer variety of stellar collisions, making them a more interesting laboratory. In that sense, a dynamical simulation of an open cluster can be seen as providing preparatory steps toward the modeling of globular clusters, just as a study of the latter forms a stepping stone toward the investigation of galactic nuclei.
