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By far the greatest amount of radio energy reaching the Earth comes from the galactic plane and in particular, the center of our galaxy. In addition to these vast extended radio regions, many thousands of discrete or point sources of radio emission have been detected. These may be divided into three major qroups:
Objects within our solar system.
Radio emission has been detected from the quiet and the disturbed Sun (flares, etc.), the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Jupiter appears to radiate as both a thermal and non-thermal source.
Radio emission from within our galaxy
Emission within our galaxy, aside from background emission, consists of several types:
(A) Supernovae Remnants: The most intense discrete radio source Cassiopeia A -- a non-thermal source. Cassiopeia A is believed to be the remains or remnant of a supernova detonation around the year 1700 A.D. (SN II type). The first identification of a radio source with an optical object other than the Sun was the strong non-thermal Taurus A with the Crab Nebula -- another remnant of a supernova explosion in A.D. 1054 (SN I type). Other strong non-thermal sources include Puppis A and the Tycho and Kepler supernovae remnants.
(B) Ionized hydrogen Clouds: The interstellar hydrogen in our galaxy tends to be distributed in vast clouds. When a hot star is in or near one of these clouds, its ultra-violet radiation tends to ionize the cloud and causes it to emit (thermal) continuum radiation. The young and hot O and B stars are often the exciting sources. Well-known examples of thermal -hydrogen (H II) cloud radio sources are the Orion and Rosette nebulae, the Cygnus X source, and the North American, Omega, and Lagoon Nebulae.
(C) Neutral Hydrogen (21cm) Emission: One of the more important results of radio astronomy has been the detection of 21-cm emission in the clouds of neutral hydrogen that occurs in the spiral arms of the galaxy. A tendency in the hydrogen atom toward a lower energy state results in the emission of radiation at a wavelength of 21 cm. Radio astronomers can detect this radiation and the resulting maps have provided us with the first real ppicture of the spiral arm structure of the galaxy.
(D) Flare Stars: Radio emission has been detected from certain red dwarf stars that show occasional sudden increases in optical brightness or flares. Some of these stars are among those listed in the section on flare stars given elsewhere.
Copyright (c) 1997-99 Michael Erlewine
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