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.

Extra radiation at certain specific wavelengths in a spectrum, compared with neighboring wavelengths (colors): An emission line is a spectral line occurring from emissions of electromagnetic radiation by atoms, ions, or molecules in the course of changes from agitated states to conditions of lower energy.

An immense cloud in interstellar space which, as a result of ionized gas within it, shines by its own light: An emission nebula is a gas cloud that receives energy from a hot star, allowing it to give off radiation in emission lines, such as those of hydrogen.

The characteristic reddish radiation of many emission nebulae is mostly from the hydrogen-alpha line.

A spectrum of electromagnetic radiation sent off by a self-luminous source: An emission spectrum is the spectrum formed by the emission of electromagnetic radiation by a source, such as a star.

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.