Radio Astronomy
Radio Astronomy is the study of the Universe and astrophysical phenomena, by examining their emission of electromagnetic radiation in the radio portion of the spectrum. Radio astronomy has greatly improved our understanding of the evolution of stars, the structure of galaxies, and the origin of the universe.
Radio astronomy covers a frequency range from a few megahertz (100 m) up to frequencies of about 300GHz (1 mm). The low-frequency limit of the radio band is determined by the opacity of the ionosphere, while the high-frequency limit is due to strong absorption from oxygen and water bands in the lower atmosphere. Under favourable conditions, radio astronomers can work into the submillimetre region or through ionospheric holes during sunspot minima.
Brief History
The first observations of cosmic radio emission were made by the American engineer Karl G. Jansky in 1932, while studying thunderstorm radio disturbances at a frequency of 20.5 MHz (14.6 m). He discovered radio emission of unknown origin, which varied within a 24-hour period. Later he identified the source of this radiation to be in the direction of the centre of our Galaxy.
The first radio telescope, built in 1937 by Grote Reber of Wheaton, Ill., U.S., was a steerable 9.5 metre paraboloid--i.e., a device with a parabolically shaped reflector, dubbed the "dish," that focuses the incoming radio waves onto a small pickup antenna, or "feed." Thereafter, radio astronomy developed quite rapidly, and has greatly improved our knowledge of the Universe.
Radio Sources
Small, hot sources such as stars can be detected but are not a major source of emission. Large, cold sources such as the gas and dust clouds of interstellar medium and hot, large sources such as HII regions are important sources of emission.
Solar and Galactic
Radio waves penetrate much of the gas and dust in space as well as the clouds of planetary atmospheres. Radio astronomers can therefore obtain a much clearer picture of the central region of the Galaxy, behind clouds of small particles, than is possible by means of optical observation. A better understanding of the Milky Way has resulted from measurements of radio signals produced at the 21-centimetre wavelength by cold clouds of interstellar neutral hydrogen atoms distributed in its spiral arms.
Among radio sources in our Galaxy are the remains of supernova explosions, such as the Crab nebula and pulsars. Short-wavelength radio waves have been detected from complex molecules in dense clouds of gas where stars are forming.
Much of our knowledge about the structure of our Milky Way comes from radio observations of the 21-cm line of neutral hydrogen and, more recently, from the 2.6-mm line of the carbon monoxide molecule.
Searches have been undertaken for signals from other civilizations in the Galaxy, so far without success.
Extra-Galactic
Strong sources of radio waves beyond our Galaxy include radio galaxies and quasars. Their existence far off in the universe demonstrates how the universe has evolved with time.
Radio astronomers have also detected weak background radiation from the Big Bang explosion that marked the birth of the universe.
Radio astronomy has resulted in many important discoveries; e.g. both pulsars and quasars were first found by radio astronomical observations. Quasars are distant galaxies that release enormous amounts of energy primarily in the form of radio waves. Pulsars are neutron stars that emit highly rhythmic radio pulses.
In recent years these and other related objects have been studied at extremely high resolution by means of very-long-baseline interferometry, a technique involving the synchronized observation of a cosmic radio source with multiple-dish interferometers located many kilometres apart.
Physical Mechanisms
Cosmic radio emission, as far as we know, 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.
Several physical mechanisms are recognized that produce the observed radio emission. Generally speaking, electromagnetic radiation is emitted when electric charges change velocity Radio emission comes from hot gases (thermal radiation); electrons spiraling in magnetic fields (synchrotron radiation); and specific wavelengths (lines) emitted by atoms and molecules in space, such as the 21-cm/8-in line emitted by hydrogen gas. Observations are made both in the continuum (broad band) and in spectral lines (radio spectroscopy).
Thermal Emission
At short wavelengths the emission is dominated by thermal sources. At short radio wavelengths thermal emission sources dominate the sky, and at long radio wavelengths the sky is dominated by non-thermal emission sources.
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.
Synchrotron Radiation
At long wavelengths the emission occurs primarily from synchrotron emitting sources such as pulsars, supernovae remnants, radio galaxies, and quasars. SR arises 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.
Line Emission
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. An example of radio line emission is the 21-cm line of neutral atomic Hydrogen.
