A subfield of astronomy that studies celestial objects at radio frequencies; a frequency that is useful for radio transmission, usually between 10 kHz and 300,000 MHz.

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.