ABSTRACT
Most stars are observed to lie in clusters, that is, a collection of stars gravitationally bound to one another. We observe a wide array of clusters, from binary systems to globular clusters, composed of hundreds of thousands of stars. In a binary system, the two stars move in elliptical orbits with respect to their common center of mass. Viewed from each star, the other star would also appear to move in an elliptical orbit. The binary star systems are particularly important because they allow determination of the masses of the binary partners. One observes a wide range of binary systems with periods ranging from a few days to hundreds or thousands of years. In some cases the two stars nearly touch one another, whereas in other systems they might be separated by a distance greater than 100 AU.
