Lecture
This lecture covers the size of nebulae, ionization, recombination, and the nomenclature of hydrogen atoms. It also discusses the absorption, diffusion, and reflection of light in celestial bodies, as well as the intrinsic color index. The instructor explains the impact of absorption and diffusion on the color of celestial bodies, emphasizing the efficiency of short wavelengths. Various concepts related to astrophysics, such as the Stromgren sphere and the Rayleigh scattering, are explored in detail.