Hydrostatics and Hydrodynamics
Hydrodynamics deals with how fluids (gases or liquids)
move and respond in various situations. Since stars and nebulae may be idealized as
fluids, the theory of hydrodynamics is central to their description. Hydrostatics
(left panel) may be viewed as a special case of hydrodynamics that is valid when the
fluid changes very slowly.
When stars are in equilibrium, they are well
described by the simpler equations of hydrostatics. When they are not in equilibrium, they must
be described by the more complex equations of hydrodynamics.