Many astrophysical bodies are close to thermodynamic equilibrium.
A proper understanding of them requires that we understand the basic thermodynamic processes that control their properties. We begin with a review of classical and quantum mechanical statistical mechanics. Let us consider a simple thermodynamic system consisting of two subsystems in physical contact as shown below. Each subsystem is characterized by the thermodynamic variables of energy, volume and Entropy. The system is constrained by the conservation of total energy and volume.