Radio Astronomy

I

Introduction

Radio Astronomy, branch of astronomy in which celestial objects and astrophysical phenomena are studied by examining their emission of electromagnetic radiation in the radio portion of the spectrum. See Astronomy; Astrophysics; Electromagnetic Radiation; Spectroscopy; Spectrum.

II

History

Unsuccessful attempts to detect celestial radio emission were made during the latter part of the 19th century. The American radio engineer Karl G. Jansky, while working at Bell Telephone Laboratories, in 1932, was the first to detect radio noise from the region near the center of the Milky Way, during an experiment to locate distant sources of terrestrial radio interference. The distribution of this galactic radio emission was mapped by the American engineer Grote Reber, using a 9.5-m (31-ft) paraboloid that he built in his backyard in Wheaton, Illinois. In 1943 Reber also discovered the long-sought-after radio emission from the Sun. It was later realized, however, that solar radio emission had been detected a few years earlier, when strong solar bursts had interfered with the operation of British, American, and German radar systems designed to detect aircraft.

As a result of the great improvements made during World War II in radio antennas and sensitive receivers, radio astronomy flourished in the 1950s. Radio scientists adapted their wartime radar techniques to the construction of a variety of radio telescopes in Australia, the United Kingdom, the Netherlands, the United States, and the USSR, and the interest of professional astronomers was soon aroused by a series of remarkable discoveries.

Discrete sources of radio emission were cataloged in increasing numbers, and beginning in the 1950s many radio sources were identified with distant visible galaxies. In 1963 the continuing investigation of very small radio sources led to the discovery of quasi-stellar radio sources, called quasars (see Quasar), which, because of redshift of unprecedented magnitude, could be placed at enormous distances from the Earth. Soon afterward, in 1965, the American radio astronomers Arno Penzias and Robert W. Wilson announced the discovery of a 3 K (-454° F) cosmic background radio emission, which has many implications for theories of the origin and evolution of the universe (see Cosmology). An entirely new type of radio source, the pulsar, was discovered in 1968 and was quickly identified as a rapidly rotating neutron star (see Star).

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For many years radio astronomers concentrated on studying relatively long wavelengths near 1 m (about 3.3 ft), for which large antenna structures and sensitive receivers were easy to build. As techniques were developed to build larger and more precise structures, and as sensitive short-wavelength receiving equipment was perfected, the wavelength bands down to 1 mm (about 0.04 in) received increased attention. At the same time, the development of space technology (see Space Exploration) allowed observations to be made at very long wavelengths from above the ionosphere, which is normally opaque to radiation longer than about 20 m (about 66 ft).

III

Principles of Radio Astronomy

Cosmic radio emission, insofar as is known, comes entirely from natural processes, although from time to time radio telescopes are also used to search (so far unsuccessfully) for possible sources of radio emission from extraterrestrial intelligence (see Exobiology). Several physical mechanisms are recognized that produce the observed radio emission.

A

Types of Emission

Because of the random motions of electrons, all bodies emit thermal, or heat, radiation characteristic of their temperature. Careful measurements of the intensity and spectrum of emissions are used to calculate the temperature of distant celestial bodies, such as the planets in the Earth’s solar system, as well as of hot clouds of ionized gas located throughout the Galaxy.

Radio astronomy measurements, however, are often concerned with the much more intense nonthermal emission arising from charged particles such as electrons and positrons moving through weak galactic and intergalactic magnetic fields. When the particle energy is so high that its velocity is close to the speed of light, the radio emission from these “ultra-relativistic” particles is referred to as synchrotron radiation, a term borrowed from the high-energy physics laboratory, where this type of radiation was first discovered.

Both the synchrotron (nonthermal) and thermal radio sources radiate over a wide range of wavelengths. By contrast, a third category of matter—excited atoms, ions, and molecules—radiate at discrete wavelengths characteristic of the atom or molecule and the state of excitation. Wide-range radio emission is referred to as continuum emission, and discrete radio emission as line emission.

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