Abstract

In 1939, P. Th. Oosterhoff investigated the properties of RR Lyrae stars in five of the globular clusters of the Milky Way. He discovered that these clusters divided into two groups, now known as Oosterhoff groups I and II, on the basis of the properties of their RR Lyrae stars. Subsequent studies of RR Lyrae variables in additional globular clusters found that most Milky Way globular clusters that contain significant numbers of RR Lyrae stars fall into one or another of the two Oosterhoff groups. Moreover, globular clusters of Oosterhoff group I tend to be more metal-rich than those of Oosterhoff group II. However, the dwarf galaxies surrounding the Milky Way, and their globular clusters, do not exhibit the Oosterhoff dichotomy. Moreover, the bulge globular clusters NGC 6388 and NGC 6441 cannot be easily assigned to one of the traditional Oosterhoff groups. We will discuss the implications of the Oosterhoff dichotomy and the Oosterhoff gap for stellar evolution and for the formation and evolution of the Galactic halo. This work has been supported in part by the National Science Foundation.