As groups of gravitating objects condense together they become more and more detached from their surroundings. Growing inhomogeneity breaks the local translational and rotational symmetry more and more strongly. Eventually systems become isolated – no longer influenced significantly by the surrounding distribution of galaxies or stars.

In Part III we consider how systems become isolated, and their resulting properties. These are spherical systems so they retain as much local symmetry as possible. Their study makes it possible to understand the basic physical processes of finite clusters. Complexities of rotation and overall asymmetry will be introduced in Part IV.

The astronomical systems which can be represented approximately as spherical clusters range from globular clusters and galactic nuclei, on a scale of parsecs, to clusters of galaxies, on a scale of megaparsecs. After setting up a general theoretical framework to describe these systems, we briefly review some of their observed dynamical properties and then go on to discuss their internal interactions and evolution in more detail.