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.

A subfield of astronomy that deals with celestial objects at radio frequencies; a frequency that is useful for radio transmission, usually between 10 kHz and 300,000 MHz: Radio astronomy uses radio telescopes to study the celestial bodies in the sky.

In addition to celestial bodies that radiate visible light, the universe contains many objects that emit radiation of various non-visible wavelengths, such as radio waves.

Some astronomical objects emit very little light or even none at all, however they may radiate relatively large amounts of energy at non-visible wavelengths.

Radio observations of celestial objects can be made from the Earth's surface during the day and at night, both in cloudy and clear weather.

An instrument used to make observations of celestial bodies at radio wavelengths: The radio telescope usually consists of an aerial which collects the radiation and feeds it to a processing computer.

A form of electromagnetic radiation traveling at the speed of light whose frequency lies between about 10 kilohertz and about 100,000 megahertz: Radio waves are low frequency and, therefore, long wavelength electromagnetic radiations.

The radio part of the electromagnetic spectrum lies beyond the infrared region and comprises all radiation with a frequency lower than about 300,000 million hertz (300,000 MHz) or with a wavelength longer than about one millimeter.