With an emphasis on numerical modeling, Physics of the Sun: A First Course presents a quantitative examination of the physical structure of the Sun and the conditions of its extended atmosphere. It gives step-by-step instructions for calculating the numerical values of various physical quantities. The text covers a wide range of topics on the Sun and stellar astrophysics, including the structure of the Sun, solar radiation, the solar atmosphere, and Sun-space interactions. It explores how the physical conditions in the visible surface of the Sun are determined by the opacity of the material in the atmosphere. It also presents the empirical properties of convection in the Sun and discusses how the physical parameters increase with depth through the convection zone. The author shows how certain types of "real stars" are actually polytropes and offers a simplified version of oscillation equations to highlight the properties of p- and g-modes in the Sun. He also focuses on the initial temperature rise into the chromosphere, why the temperature in the quiet corona has the value it does, and how the physics of magnetic fields help us to understand various striking phenomena that are observed on the Sun. This text enables a practical appreciation of the physical models of solar processes. Through the included numerical modeling problems, it encourages a firm grasp of the numerical values of actual physical parameters as a function of radial location in the Sun.