Some Basic Physics

Abstract

Before discussing the structure and history of our nearest star, the Sun, we will review the underlying basic physics. This will help us to better appreciate the subject. We start with a description of the fundamental structure of matter in order to focus on the constituent particles and working forces which are present at very different spatial scales. Despite their apparent complexity, only a limited number of elementary particles and fundamental forces are needed to explain the Sun as well as the rest of the Universe. Then, because the Sun is a huge mass of gas which can be modeled using simple physical laws that apply to a perfect gas in thermodynamic equilibrium, we introduce the equation of state, relating gas temperature, density and pressure, and the Maxwell distribution, which determines the number of gas particles (or molecules) having kinetic energies in a specific range as function of the gas temperature. Actually, solar gas molecules have internal degrees of freedom, that is, they not only exchange kinetic energy through elastic collisions, but also undergo excitation and ionization processes, where further energy exchanges occur. These two types of processes can also be treated in equilibrium conditions, leading us to the Boltzmann and Saha distributions. The Boltzmann equation determines the ratio of the numbers of particles occupying two subsequent excitation states as a function of the excitation energy difference and gas temperature. The Saha equation gives the ratio of the numbers of particles in two subsequent ionization states as a function of the ionization energy, gas temperature and electron density.