In stellar dynamics we do not really study stellar systems like globular clusters and galaxies. That is the job of astronomers. What we do is study models of these systems. Just as in many branches of applied mathematics, a model is nothing other than a mathematical structure into which we try to incorporate our knowledge of the system at hand. Sometimes “knowledge” is not the right word: it may be nothing better than a hunch about how things might work. Often, however, our knowledge will include physical laws, especially the ones we think are relevant. So far in this book, for example, we have implicitly thrown out almost everything we know about stellar systems except the gravitational dynamics.