There are three main types of celestial radio emissions:

1. Thermal emissions which occur as a result of the acceleration of electrically-charged particles in a hot gas.
2. Synchrotron or non-thermal, emissions are produced by the acceleration of charged particles, but the acceleration is caused by a magnetic field.
3. Radio spectral-line emissions are concentrated in a narrow band around one specific frequency; just as an optical spectral line corresponds to a single frequency in the visible electromagnetic spectrum.

Radio line emissions usually originate in clouds of hydrogen gas, a relatively common constituent of the universe, which is found in our galaxy's spiral arms, among other places.

Electromagnetic radiation emitted or absorbed at discrete frequencies or wavelengths: A spectrum of line emission occurs when a hot gas gives off certain wavelengths of illumination which then produces lines on a dark background.

The discharge of electrons from very hot substances: Thermal emission is the type of electromagnetic radiation emitted when electrons and atoms, forming part of a hot gas, interact by collisions. The resulting radiation is continuous, as opposed to discrete, line emission.