ABSTRACT
This chapter presents purely physical topics selected for study of the interstellar medium. They concern the statistical distribution of atoms and photons, basic thermodynamic relations and the radiation of blackbodies. For a large number of atoms in thermodynamic equilibrium, their distribution in energy space is just a function of temperature and described by the Boltzmann equation. The chapter outlines the derivation of this fundamental statistical relation and append a few loosely connected items. The probability that an arbitrary gas atom has a velocity in a certain range follows as a corollary from the equilibrium energy distribution. In deriving the energy distribution for an ensemble of particles after Boltzmann, we divided the energy or momentum space into arbitrarily small cells. In quantum statistics, one divides the phase space, which consists of coordinates and momenta, into cells.
