Radio characteristics of HII regions and the diffuse thermal background

The properties and the emission mechanisms of the HII regions are discussed. Topics covered are ionized interstellar hydrogen, emission nebulae, the diffuse thermal background, radiation transfer, bremsstrahlung, absorption and emission coefficients, radio recombination lines, line frequencies and shape functions, transfer equations, physical characteristics of HII regions, LTE and departures from LTE, Stark broadening, diffuse regions and pulsar dispersion, turnover frequencies, anomalous recombination lines, relative abundances of helium and hydrogen, and observation studies. (U.S.)