Part I - Idealized homogeneous systems – basic ideas and gentle relaxation

Summary

In reality we apprehend nothing exactly, but only as it changes according to … the things that impinge on or offer resistance to it.

Democritus

To understand some of the most important properties of gravitating systems we temporarily put aside the effects of their density and velocity gradients, their components of different masses and sizes, and any external forces which may act upon them. Imagine an idealized, isolated, homogeneous gravitating system of particles. Usually in discussing the physics of these systems we will call their particles stars for brevity, although when discussing many astronomical contexts they will often be galaxies, or even clusters of galaxies. These idealized systems will sometimes be finite, sometimes infinite. In later sections we will find that both sizes of homogeneous clusters turn out to be unstable. But never mind that for now; there are more basic properties to consider.