The field of astrophysics that describes systems of many point mass particles whose mutual gravitational interactions determine their orbits. These systems include star clusters, globular clusters, and galaxies. Stellar dynamics deals with systems in which each member contributes importantly to the overall gravitational field and is usually concerned with the statistical properties of many orbits. It can be compared to the kinetic theory of gases developed in the late 19th century. In contrast, celestial mechanics deals with systems where the gravitational force of a massive planet or star determines the orbits of its satellites.