ABSTRACT
This chapter considers a warmer — and ionized — component of the interstellar medium. It discusses two examples of photo-ionized regions: HII regions and planetary nebulae. HII regions are one of the more common manifestations of thermal bremsstrahlung emission at radio wavelengths. The principal heating mechanism for HII regions is the photo-ionization of hydrogen. HII regions Strictly speaking, an “HII region” simply means a region of ionized hydrogen. The size of the HII region as a function of time is found from the velocity of the advancing ionization front. The preceding calculations were for pure hydrogen nebulae; however, real nebulae contain helium, heavy elements and dust. The heavy elements are trace constituents and have little effect on the ionization structure of the nebulae, although they do produce readily observable emission lines. The nebulae were first identified from their optical spectral line emission, which resulted in a selection effect.
