radio astronomy, study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally.
Radio Telescopes
Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes, whose use corresponds to that of the optical telescope telescope, 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;
..... 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 Jansky Jansky, 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 Reber Reber, 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 Way Milky 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 supernovas supernova, 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.
..... Click the link for more information. , or exploding stars. The most famous example of a supernova remnant is the Crab Nebula Crab 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.
..... Click the link for more information. in Taurus.
Because there are strong magnetic fields (see magnetism magnetism, 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 radiation synchrotron radiation, in physics, electromagnetic radiation emitted by high-speed electrons spiraling along the lines of force of a magnetic field (see magnetism ).
..... 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 matter interstellar 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
..... 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 Galaxy Andromeda 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 cosmology cosmology, 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 galaxies like the Milky Way are only weak sources of radio waves, but certain giant elliptical and irregular galaxies galaxy, 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 law Hubble'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 quasars quasar (kwā`sär), one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and
..... 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 pulsars pulsar, 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 Univ.
..... 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.
Bibliography
See J. D. Kraus, Radio Astronomy (1966); G. Verschuur, The Invisible Universe Revealed (1987).
radio astronomy a branch of astronomy in which a radio telescope is used to detect and analyse radio signals received on earth from radio sources in space
radio astronomy [
′rād·ē·ō ə′strän·ə·mē]
(astronomy)
The study of celestial objects by measurement and analysis of their emitted electromagnetic radiation in the wavelength range from roughly 1 millimeter to 30 millimeters.