The study of celestial bodies by examination of the energy they emit at radio frequencies. Celestial radio noise originates by one of several processes. These include both broadband continuum radiation owing to (1) thermal radiation from solid bodies such as the planets, (2) thermal or bremsstrahlung radiation from hot gas in the interstellar medium, (3) synchrotron radiation from ultrarelativistic electrons moving in weak magnetic fields, (4) coherent processes as found in the Sun and on Jupiter, and (5) pulsed radiation from the rapidly rotating neutron stars, as well as (6) narrow “spectral line” radiation from atomic or molecular transitions that occur in the interstellar medium or in the gaseous envelopes around stars.