The distribution with frequency of radiation from a source is called a spectrum. It can be plotted as an energy spectrum or as a number spectrum and as a function of either frequency or wavelength. Conversions from one to another are possible and useful. Continuum spectra are without spectral lines though spectral lines may be superposed upon them. They can arise from interactions of atoms and free electrons, for example in the solar atmosphere. Three kinds of such spectra encountered in astronomy are thermal bremsstrahlung from an optically thin gas, blackbody radiation from an optically thick gas in thermal equilibrium, and synchrotron radiation from a gas of extremely energetic electrons in the presence of magnetic fields. Antenna temperatures used by radio astronomers are a measure of specific intensity. The total power radiated by unit area of a blackbody, σT4, allows one to relate approximately the radius of a star to its luminosity and temperature.

Spectral lines arise from atomic transitions in emitting or absorbing gases. They provide powerful diagnostics of the regions that form the lines. Stars exhibit mostly absorption lines while gaseous nebulae exhibit emission lines. Some of the latter are forbidden lines which occur only at the extremely low gas densities found in space. The shapes of spectral lines reveal the presence of turbulent motions and the effects of collisions, the latter providing the local density. […]