Abstract :
Celestial radio sources have generally been divided into two categories: 1) thermal sources, and 2) nonthermal sources. The thermal sources are usually masses of ionized hydrogen and their radiation is due to the random interaction of the electrons and protons of the ionized gas. The major cause of ionization is the ultraviolet radiation of the hot stars. This paper describes the basic physical processes involved in the radiation, and the methods of obtaining information from radio observations about the interstellar medium and the hot stars.
