This chapter describes the Earth’s upper atmosphere including the ionosphere and its effect on radio propagation. The density and thermal structure of the neutral thermosphere is described with respect to energy deposition as a function of altitude and hydrostatic equilibrium determining the scale height. The structure of the partially ionized ionosphere is described as well as the physical process responsible for the aurorae. Supplements give more details on photochemistry that give rise to excited atoms and molecules that emit auroral photons.

The model photosphere is the core of the theoretical side of our studies.It is a numerical theory from which we compute the theoretical spectrum to be compared to the observations.Our model photospheres are based on hydrostatic equilibrium.To compute such models, we need the temperature distribution, a value for the surface gravity, and the chemical abundances of the electron donors.This chapter reveals how this process is done.

We have seen how a star’s color or peak wavelength indicates its characteristic temperature near the stellar surface. But what about the temperature in the star’s deep interior? Intuitively, we expect this to be much higher than at the surface, but under what conditions does it become hot enough to allow for nuclear fusion to power the star’s luminosity? And how does it scale quantitatively with the overall stellar properties, such as mass, radius, and luminosity? To answer these questions, we identify two distinct considerations.

As one of the core chapters, the general properties of compact stars are discussed. Spheres of fluid in hydrostatic equilibrium are studied within general relativity. The concept of the mass–radius relation is introduced for the classic case of a gas of noninteracting neutrons. Landau‘s argument for a maximum mass of neutron stars and white dwarfs is delineated. Thereby, the Landau mass and radius is defined for studying scaling solutions of the Tolman–Oppenheimer–Volkoff equation. The power of scaling arguments is demonstrated for the case of a free Fermi gas with arbitrary particle mass, a relativistic gas of fermions with a vacuum term, and the limiting equation of state from causality. The concept of selfbound stars is put forward, giving rise to limits on the compactness and the maximum density achievable for compact stars in general. Generic interactions between fermions are studied and their implications for compact star properties are derived. The general properties of compact stars made of bosons with and without interactions are also investigated.