Providing an introductory but quantitative discussion of solar phenomena and relevant astrophysical processes, this textbook describes the photosphere, the sun's internal structure, sunspots, the chromosphere and corona, the dynamics of the solar magnetic field, and the solar wind. Originally published in 1990, the second edition deletes obsolete material and adds new developments in neutrino and helioseismic measurements, solar convection simulations, and data from space missions.