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radio astronomy,study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally.
Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes, whose use corresponds to that of the optical telescopetelescope,
traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of
..... Click the link for more information. in observing visible light. In the most common design, a parabolic "dish" replaces the mirror of the reflecting optical telescope. This dish serves to focus the radio waves into a concentrated signal that is then filtered, amplified, and finally analyzed using a computer. The radio signals received from outer space are extremely weak, and long observing times are required to collect a useful amount of energy. Therefore, most radio telescopes are mounted so that they can automatically track a given object as its position changes because of the rotation of the earth.
Galactic Sources of Radio Waves
Naturally occurring radio emission from the sky was accidentally discovered in 1931 by Karl JanskyJansky, Karl Guthe,
1905–50, American radio engineer; b. Norman, Okla. After graduating (1927) from the Univ. of Wisconson, he joined the Bell Telephone Laboratories.
..... Click the link for more information. . An inexplicable source of radio noise was identified in 1940 by Gröte ReberReber, Gröte,
1911–2002, American radio engineer, b. Chicago, Ill. After graduating from the Armour Institute of Technology (now the Illinois Institute of Technology) in 1933, Reber worked for several radio manufacturers and radio stations.
..... Click the link for more information. , using a radio telescope in the backyard of his home, as originating from our own galaxy, the Milky WayMilky Way,
the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky.
..... Click the link for more information. . This radiation is spread over a wide band of radio frequencies and originates in the ionized interstellar gases surrounding hot, bright stars. In these so-called H II regions, free electrons emit radio waves when they are scattered by collisions with the heavier ions. Other sources of radio waves within our galaxy are the remnants of supernovassupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
..... Click the link for more information. , or exploding stars. The most famous example of a supernova remnant is the Crab NebulaCrab Nebula,
diffuse gaseous nebula in the constellation Taurus; cataloged as NGC 1952 and M1, the first object recorded in Charles Messier's catalog of nonstellar objects (see Messier catalog).
..... Click the link for more information. in Taurus.
Because there are strong magnetic fields (see magnetismmagnetism,
force of attraction or repulsion between various substances, especially those made of iron and certain other metals; ultimately it is due to the motion of electric charges.
..... Click the link for more information. ) in the vicinities of supernovas remnants, an additional mechanism is present for producing radio waves. This is the synchrotron radiationsynchrotron radiation,
in physics, electromagnetic radiation emitted by high-speed electrons spiraling along the lines of force of a magnetic field (see magnetism). Depending on the electron's energy and the strength of the magnetic field, the maximum intensity will occur as
..... Click the link for more information. emitted by energetic electrons as they rapidly spiral around the magnetic lines of force, instead of simply being deflected by collisions with ions.
A third source of radio waves within our own galaxy consists of the atoms and molecules in the interstellar matterinterstellar matter,
matter in a galaxy between the stars, known also as the interstellar medium. Distribution of Interstellar Matter
Compared to the size of an entire galaxy, stars are virtually points, so that the region occupied by the interstellar matter
..... Click the link for more information. . This radiation is at discrete frequencies instead of over a broad band, or continuum, of frequencies. The first of these "radio lines" to be discovered was the line at a wavelength of 21 cm produced by the hydrogen atom (as opposed to the hydrogen molecule, which is composed of two atoms). The intensity of this line in the radiation from a given region is a direct measure of the amount of hydrogen there. Because hydrogen is a major constituent of the interstellar medium, the 21-cm line has provided astronomers with a means of mapping the spiral structure of the Milky Way. The visible light is blocked off by the same interstellar material in which the hydrogen giving rise to a 21-cm line lies, so that the view of the galaxy is obscured in certain directions, particularly in the direction of the center of the galaxy. Thus, before the advent of radio astronomy, the spiral structure of the Milky Way had not actually been observed but was only inferred from comparison with the Andromeda GalaxyAndromeda Galaxy,
cataloged as M31 and NGC 224, the closest large galaxy to the Milky Way and the only one visible to the naked eye in the Northern Hemisphere. It is also known as the Great Nebula in Andromeda. It is 2.
..... Click the link for more information. and from other indirect studies. Besides atomic hydrogen, certain simple organic (carbon-based) molecules, including cyanogen (CN) and formaldehyde (H2CO), have been discovered in the interstellar medium by means of their radio lines.
Extragalactic Sources of Radio Waves
Radio waves also come from outside the Milky Way. These extragalactic radio sources have great implications for cosmologycosmology,
area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe. Modern Cosmological Theories
..... Click the link for more information. , the theory of the overall structure of the universe. Spiral and barred spiral galaxies, such as the Milky Way, are only weak sources of radio waves, but certain giant elliptical and irregular galaxiesgalaxy,
large aggregation of stars, gas, and dust, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926–29) by Edwin P.
..... Click the link for more information. emit more than a million times as much radio energy as ordinary galaxies. Such galaxies are usually marked by dust lanes, which are unusual for galaxies lacking spiral arms. Some of these objects can be detected only by their radio emission, but in other cases the position of the radio source has been determined accurately enough to allow astronomers to identify the radio source with a galaxy visible in an image taken with a large optical telescope.
Other radio sources were optically identified with what at first appeared to be faint blue stars. However, it was discovered that these "stars" had enormous red shifts (shifting of the spectral lines toward the red end of the spectrum) that implied, according to Hubble's lawHubble's law,
in astronomy, statement that the distances between galaxies (see galaxy) or clusters of galaxies are continuously increasing and that therefore the universe is expanding.
..... Click the link for more information. , that they were the most remote objects ever detected and that their intrinsic intensities were about 1000 times greater than an entire galaxy. These extraordinary objects were named quasi-stellar radio sources, which was soon shortened to quasarsquasar
, one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and currently believed to be the most distant and most luminous objects in the universe; the name is shortened from quasi-stellar radio source (QSR).
..... Click the link for more information. . Their nature is still not completely understood.
Many thousands of extragalactic radio sources are known. Of those optically identified radio sources, roughly one third are quasars, and the remainder are radio galaxies. In addition to these localized radio sources, there is uniform low-level radio noise from every direction in the sky. This cosmic background radiation is believed to be an indication that the universe began with an explosive big bang rather than having always existed in an unchanging steady state. More recently radio astronomy has discovered pulsarspulsar,
in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge built a primitive radio telescope to study a
..... Click the link for more information. , thought to be rapidly spinning neutron stars that radiate bursts of energy on and off regularly between 1 and 30 times a second.
See J. D. Kraus, Radio Astronomy (1966); G. Verschuur, The Invisible Universe Revealed (1987).
radio astronomyThe study of celestial bodies and the media between them by means of the radio waves they emit. Extraterrestrial radio waves were first detected in 1932 by the American radio engineer Karl Jansky. The first radio telescope was built and operated by Grote Reber in the late 1930s but radio astronomy only developed after the war. It has benefitted greatly from advances in electronics and solid-state physics and is now a major branch of observational astronomy. It is particularly useful in the study of gaseous nebulae such as H I and H II regions, pulsars, supernova remnants, radio galaxies, quasars, and the microwave background. See also radio source.
the branch of astronomy dealing with the study of celestial objects, such as the sun, stars, and galaxies, by observing the radio waves they emit in the wavelength range from fractions of a millimeter to several kilometers. Radio astronomy sometimes also includes radar astronomy, in which case it is referred to as active radio astronomy, in contradistinction to passive radio astronomy, which deals with observations of the natural radio emissions of celestial objects.
Observations of electromagnetic waves in the radio-frequency range substantially complement observations of celestial bodies in the optical and shorter-wavelength bands, including the X-ray band. As early as the 19th century, it was proposed that the sun emits radio radiation, and attempts were made to detect such radiation; however, the radiation detectors used were not nearly sensitive enough for this purpose. It was not until 1931 that K. Jansky of the USA accidentally discovered appreciable radio emission from the Milky Way at a wavelength of 14.6 m. In 1942 radio emission was discovered from the quiet sun, and in 1945, from the moon. In 1946 the first discrete, or localized, source of radio emission was discovered in the constellation Cygnus. Its physical nature remained unknown until 1954, when a distant galaxy at the location of the radio emission was finally detected in the optical band. In the 1960’s results of radio observations were widely used in the study of physical phenomena occurring in celestial objects.
Theoretical investigations have established that nearly all observed radio-astronomical phenomena involve radio emission mechanisms known in physics. These include the following: thermal radiation of solid bodies, as exhibited by planets and small bodies in the solar system; bremsstrahlung of thermal electrons in the ion fields of the cosmic plasma, as observed in gaseous nebulas in the Milky Way and in the atmosphere of the sun and stars; magnetic braking radiation of subrelativistic electrons in cosmic magnetic fields, as observed in active regions on the sun, in radiation belts around certain planets, and in radio galaxies and quasars; and various combinations of processes in a plasma, as in radio bursts on the sun and Jupiter and other phenomena. In addition to the continuous radiation spectrum from the above phenomena, monochromatic radiation from celestial objects has also been observed. The basic mechanisms by which spectral lines are produced are quantum transitions between different atomic and molecular energy levels. The line of neutral hydrogen, with a wavelength of 21 cm, arises from transitions between hyperfine sublevels of the hydrogen atom; this line and the recombination lines of excited hydrogen are of major importance in radio astronomy. Most of the many tens of molecular radio lines observed involve transitions between energy levels resulting from the rotation of molecules (rotational sublevels).
Cosmic radio emission is studied with radio telescopes. Broadband radiometers are used to observe continuous spectra, and spectral lines are recorded using different types of radio spectrographs. Special radio-telescope instruments, such as radio spectrometers and radio polarimeters, enable us to study the spectral composition, intensity, polarization, and other characteristics of radio emission. Signals from cosmic sources are, as a rule, extremely weak; consequently, radio telescopes used in research are equipped with very large antennas, and the most sensitive detection devices are used. The antenna of the largest radio telescope—the T-shaped telescope near Kharkov, USSR—has an area of approximately 100,000 m2, and the most sensitive radiometers can register temperature changes of 0.001°–0.0001 °K. Radio images of celestial objects are obtained by using single (for example, parabolic) reflectors, as in optical astronomy, and also by more complex radio interfero-metric methods of observing. These methods make it possible to synthesize radio images of celestial bodies by accumulating the emission from a given object over a certain period of time. Advances in the recording of high-frequency electric oscillations and in frequency stabilization have made it possible to carry out interferometric observations by comparing recordings obtained from widely separated stations not linked by radio-frequency channels of communication. Great distances between stations ensure high resolution in determining the directions of radio sources.
Radio telescopes can be used to conduct sky surveys and detailed studies of individual objects. Once radio sources are discovered, they are entered into catalogs; by 1974 approximately 100 catalogs had been published containing data on tens of thousands of objects, most of which are located far beyond the Milky Way Galaxy.
Depending on the objects investigated, radio astronomy is conventionally divided into solar, planetary, galactic, and ex-tragalactic radio astronomy. Solar radio astronomy involves the study of the solar atmosphere, including the chromosphere, the corona, the outer corona, and the solar wind. The main problem is the explanation of solar activity. The nature of radio emission from the sun differs in the different wavelength ranges. Millimeter-wavelength emission, which is associated with bremsstrahlung of electrons of the solar chromosphere plasma in the electric fields of ions, is relatively quiet. Centimeter-wavelength emission strongly depends on bremsstrahlung and magnetic braking radiation from the hot magnetized plasma above sunspots. Finally, meter-wavelength emission from the sun is highly unstable and resembles bursts over the relatively stable level of bremsstrahlung from the solar corona. The intensity of the bursts is sometimes tens of millions of times greater than the radiation from the quiet corona. These bursts are apparently the result of fluxes of high-speed particles traveling through the solar atmosphere. Studies of the solar wind are based on the wind’s scattering of radio waves from distant radio sources.
Planetary radio astronomy involves the study of the thermal and electrical properties of the surface of planets and their satellites; it also studies the atmosphere and radiation belts of the planets. Radio observations substantially add to results obtained in the optical band; this is especially true for planets whose surfaces are hidden from the terrestrial observer by dense clouds. Such observations have made it possible to measure the temperature of the surface of Venus and to estimate the density of the planet’s atmosphere. They have also led to the discovery of Jupiter’s radiation belts and the high-intensity radio bursts in Jupiter’s atmosphere. Radar methods make possible extremely precise measurements of planetary distances and rotation periods, as well as the mapping of the surface of planets.
Galactic radio astronomy studies the structure of the Milky Way Galaxy and the activity in its nucleus, the physical state of interstellar gas, and the nature of the various galactic radio sources. The remnants of supernovas and clouds of gas ionized by ultraviolet radiation from stars are both powerful galactic radio sources. In 1967 pulsating radio sources, called pulsars, were discovered. These objects are apparently associated with rapidly rotating neutron stars whose powerful magnetospheres give rise to radio emission. In the same year, sources of exceptionally bright and narrow emission lines of the hydroxyl radical (OH) were discovered; later, lines of several molecules were also detected. These lines probably originate in a maser mechanism of radiation. A second powerful cosmic maser is water vapor, which is found under certain conditions in compact clouds of interstellar gas. Physical conditions in the interstellar gas have also been studied using radio-frequency spectral lines of excited hydrogen and a large number of molecular lines. Radio emission has been detected from certain other types of new stars. The radio emission from close binary systems in which one of the components may be a “black hole,” has attracted particular attention. Galactic radio astronomy also involves the study of the structure of the magnetic field of the Milky Way Galaxy and has proved useful in investigations into the origin of cosmic rays.
Extragalactic radio astronomy involves the study of all objects outside the Milky Way Galaxy. The overwhelming majority of these objects are known as “normal” galaxies. Such galaxies are characterized by relatively weak radio emission, resulting from the motion of high-speed electrons in the galaxies’ magnetic fields. Galaxies with more active nuclei exhibit radio emission hundreds of times more intense than that of normal galaxies. Radio emission hundreds and even thousands of times more intense is characteristic of radio galaxies. The vast majority of radio galaxies have a two-component structure, in which the optical object—usually an immense elliptical galaxy—is situated between the components and is often itself an extremely weak radio source. Each component usually has a bright feature near the edge. The components of radio galaxies were apparently ejected from the nuclei of optical galaxies and moved off with high velocities.
The energy of relativistic electrons and of the magnetic field in the components of radio galaxies attains enormous values —up to 1061 ergs—and probably increases in the course of intermittent explosions in the galactic nuclei. As of 1975, the cause of such violent activity in these nuclei remains a mystery.
The most powerful extragalactic radio sources, however, are quasars, which, although visible in the optical band, bear no resemblance to normal galaxies. Quasars exhibit radio emission that varies noticeably over periods ranging from several weeks to several years. Such variability is possible only if the radio-emitting regions have relatively small linear dimensions; this has been confirmed by direct observations of the structure of quasars. Wide-base interferometers have detected features measuring less than 10–3 sec of arc; these may be clouds or fluxes of ultrarelativistic particles moving in magnetic fields. The detailed structure of quasars has not yet been studied sufficiently, and the nature of quasars remains unknown.
In addition to discrete extragalactic radio sources, background emission has also been detected outside the Galaxy. This background radiation results from the combined radio emission of a large number of weak sources not individually distinguishable and of isotropic emission corresponding to a temperature of approximately 2.7°K. The latter emission was radiated by matter that filled the metagalaxy at an early stage in the evolution of the universe. This matter (plasma) was then of greater density than now and had a temperature of 3000°–5000°K. This residual emission indicates that the universe today differs from what it was in ages past, when it was dense and hot. Calculations of the number of extragalactic radio sources support the contention that at an earlier time either the concentration of radio sources in the neighborhood of the Milky Way Galaxy was higher or the average intensity of the sources was much greater. It would also seem that the apparent concentration of radio sources decreases rapidly at great distances, that is, at still earlier stages in the evolution of the universe. This may be attributed to the absence of radio sources—and, perhaps, of galaxies in general—at this earlier stage. However, the decrease in concentration may also be the result of intense scattering of radio emission in the extragalactic gas.
Radio-astronomical research is conducted at many astronomical observatories and institutes, and there are specialized radio observatories as well. In the USSR such activities are coordinated by the Scientific Council on Radio Astronomy of the Academy of Sciences of the USSR and by the Astronomical Council of the Academy of Sciences of the USSR. Radio-astronomical research on an international scale is coordinated by the International Astronomical Union.
REFERENCESShklovskii, I. S. Kosmicheskoe radioizluchenie. Moscow, 1956.
Kaplan, S. A., and S. B. Pikel’ner. Mezhzvezdnaia sreda. Moscow, 1963.
Kaplan, S. A. Elementarnaia radioastronomiia. Moscow, 1966.
Kraus, J. D. Radioastronomiia. Moscow, 1973. (Translated from English.)
Pacholcyzk, A. Radioastrofizika. Moscow, 1973. (Translated from English.)
IU. N. PARIISKII