This undergraduate-level textbook addresses key physics concepts relevant to stellar observations, including radiation, electromagnetic spectrum, photometry, continuous and discrete spectrum, and spectral lines. It describes instruments used for astronomical observations as well as how the radiation received is characterized and interpreted. The book also examines the structure of stars, the basic equations which explain stars in equilibrium, and the fusion reactions occurring in stellar cores. Plus, it discusses the evolution of stars, the solar system, the dynamics of galaxies, and the fundamentals of modern cosmology.