RADIOASTRO

RADIO ASTRONOMY

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.

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 (1911–    ), using a 9.5-m (31-ft) paraboloid that he built in his backyard in Wheaton, Ill. 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, Great Britain, the Netherlands, the U.S., 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 , which, because of red shift 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. An entirely new type of radio source, the pulsar, was discovered in 1968 and was quickly identified as a rapidly rotating neutron star.

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 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).

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. Several physical mechanisms are recognized that produce the observed radio emission.

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.

Radio telescopes

Radio wavelengths are relatively long, extending from about 1 mm (about 0.04 in) to more than 1 km (about 0.6 mi), and radio telescopes must be extremely large in order to focus the incoming signals to produce a sharp radio image. The world’s largest stationary radio telescope, Arecibo Observatory (q.v.) in Puerto Rico, is a bowl-shaped dish 305 m (1000 ft) in diameter. The largest fully steerable parabolic dish-type antennas are 50 to 100 m (about 165 to 330 ft) in diameter, and they have a resolution of about 1 arc minute, equivalent to that of the unaided human eye at optical wavelengths. Incoming radio waves are focused by the parabolic surface onto a small horn antenna that leads to an extremely sensitive radio receiver. These receivers, although similar in principle to the home radio, are able to detect signals as weak as 10^–17 W. The critical parts of the receiver are often cooled to temperatures close to absolute zero in order to obtain the best possible performance. For spectral line observations, specialized receivers are used that can be tuned to as many as 1000 frequencies simultaneously.

In order to obtain higher resolution, arrays of antennas are used as interferometers giving resolutions of approximately 1 arc second, equivalent to that of large optical telescopes under ideal viewing conditions. The largest radio telescope of this type is the Very Large Array, or VLA, located on an isolated plain near Socorro, N.Mex. The VLA contains a total of 27 parabolic dishes, each 25 m (82 ft) in diameter, located along three 21-km (13-mi) arms in a Y configuration. Each antenna element contains its own receiver, and the signals from each receiver are sent to a central building where they are combined to form the high-resolution image by a technique that is known as aperture synthesis. Other interferometers may use antennas like huge television antennas. One at Cambridge, England, uses 60 antennas to detect radiation at wavelengths of 2 m (6.6 ft).

Even higher resolutions may be achieved if individual antenna elements are spaced thousands of kilometers apart. With these spacings it becomes impractical to send the signals from each antenna directly to a common point. Instead, separate broadband tape recordings are made at each antenna, and the individual tapes are then shipped to a central processing facility. This technique of very long baseline interferometry (VLBI) involves using atomic clocks at each telescope to synchronize the individual recordings to an accuracy of better than one-millionth of a second. In this way, angular resolutions of one-thousandth of an arc second are achieved, equivalent to the apparent angular dimensions of a basketball at the distance of the Moon. In 1984, the U.S. government appropriated funds for the construction of an installation called the Very Long Baseline Array (VLBA), a network of 10 radio antennas spread across the U.S. from Hawaii to the Virgin Islands. The VLBA, which became operational in 1993, is expected to provide angular resolutions in the range of 200-millionths of an arc second. Canada and Australia are both planning similar programs.

Classes of Radio Sources

Many discrete radio sources have been discovered and studied in our solar system, in our galaxy, and in the wide extent of the universe beyond our galaxy.

Solar system radio astronomy

The Sun is the brightest radio source in the sky. Its radio emission is much more intense than would be expected from the thermal emission of its visible surface, which has a temperature near 6000 K (about 10,300° F). This is because most of the radio emission observed at longer radio wavelengths comes from the much hotter, but optically invisible, outer atmosphere, which has temperatures near 1,000,000 K (near 1,800,000° F). In addition to the thermal emission, numerous nonthermal storms and bursts occur, particularly during periods of high sunspot activity when the intensity of radio emission may dramatically increase by a factor of 1 million or more for brief periods of about an hour.

The only other source of nonthermal radio emission in the solar system is the planet Jupiter . At wavelengths near 15 m (about 49 ft), Jupiter emits strong bursts of radiation that come from relatively small regions, near the cloud surface, that rotate with the planet. The intensity of these bursts appears to be greatly influenced by the location of the satellite Io. In addition, Jupiter is surrounded by extensive radiation belts that radiate in the microwave band at wavelengths that are shorter than about 1 m (about 3.3 ft).

Thermal radiation has been observed to emanate from the surface or atmosphere of all of the planets except Pluto. These emissions have been used by instruments aboard spacecraft to derive information on planetary meteorological conditions and other phenomena.

Galactic radio sources

The galaxy, or the Milky Way, emits radio waves as a result of synchrotron radiation from cosmic ray electrons moving through the weak galactic magnetic field. The 21-cm line emission from neutral hydrogen is also observed throughout the Galaxy. Small changes in the observed wavelength of the 21-cm line are caused by the motion of the hydrogen clouds toward or away from an observer. These changes are an example of the phenomenon known as the doppler effect, or red shift. Clouds that are most distant from the center of the Galaxy revolve around the center with the greatest velocity, and observations of the Doppler effect are used to measure the velocity and locate the position of hydrogen clouds. In this way it has been possible to trace the shapes of the Milky Way’s spiral arms, which are not readily observed at optical wavelengths.

In addition to the diffuse background radiation, numerous discrete sources of radio emission exist in the Galaxy. These discrete sources include the following: supernova remnants, radio stars, emission nebulas, molecular clouds, and pulsars.

Supernova remnants are the clouds of debris remaining from stars that have exploded. Relativistic electrons produced in a supernova explosion are captured by the magnetic field surrounding the location of the explosion. As these electrons spiral around the magnetic field lines, they continue to radiate for thousands of years. In some cases the star itself continues to be a source of radio emission and is referred to as a radio star. Another important class of radio star comprises the binary (double) star systems that emit radio waves when mass is transferred from one component to the other. Radio stars are often X-ray sources as well.

Thermal radio emission is observed from clouds of ionized hydrogen (termed H II regions) located along the spiral arms of the Galaxy. When free electrons recombine with ions of hydrogen or other light elements, radio energy is released that can be observed as recombination lines in the radio portion of the spectrum.

Spectral lines also result from vibrational and rotational transitions of such interstellar molecules as water vapor (H₂O), ammonia (NH₃), formaldehyde (H₂CO), and carbon monoxide (CO). More than 50 interstellar molecules are now known, including many complex organic molecules. In some interstellar clouds, the radio molecular lines are unusually intense due to the maser (microwave amplification by the stimulated emission of radiation) effect.

The intensity of most cosmic radio sources is steady, or only varies slowly with time. The pulsars, however, emit short periodic bursts or pulses of radiation about once per second. Although first discovered because of their intense pulsed radio emission, some were later found to emit optical and X-ray pulses as well. Pulsars are thought to form when stars like the Sun collapse under their own gravity to dimensions of about 10 km (about 6 mi). The density then becomes extremely great, and electrons are stripped from their atoms, leaving a so-called neutron star.

Radio galaxies

Most galaxies probably emit radio waves and do so at energies comparable to that of our own galaxy—about 10³² W. In the cases of the so-called radio galaxies, however, the radio emission is up to 100 million times stronger. Most of this energy originates not in the galaxies themselves but in clouds of superheated, ionized gases, or plasma, located hundreds of thousands or even millions of light-years away from the parent galaxy. These giant radio clouds may be 100 times the size of the galaxy itself and are among the largest known objects in the universe.

A great deal of energy is required to generate the powerful radio emissions from radio galaxies, and it may amount to a significant fraction of the total energy that would result from the nuclear burning of a whole galaxy. The origin of this energy and the manner in which it is converted to radio emissions have been major problems of astrophysics since the discovery of radio galaxies more than two decades ago.

Recent detailed pictures of radio galaxies, obtained with high resolution radio telescopes such as the VLA, often show a prominent jet of material connecting a bright, compact radio source at the galactic nucleus to the more extended radio lobes (clouds). It is widely speculated that these jets or beams transport energy away from the galactic nucleus to the radio-emitting plasma and that the source of energy lies in a massive object, possibly a black hole located at the galactic center. Frequently, a compact radio source is found at the center of radio galaxies. In one unusual radio galaxy observed in the mid-1980s, two bright clusters of stars near its center are emitting jets apparently braided together.

Quasars

Quasars appear to radiate with the luminosity of hundreds of galaxies, but each quasar is smaller than a typical galaxy by a factor of nearly a million. Quasars have very large red shifts, and they are therefore believed to lie at great distances from the Milky Way. Because quasars appear to be so powerful, and because their radiation often varies rapidly, it was once thought they might be relatively nearby weak objects rather than distant powerful ones. However, evidence has accumulated supporting the cosmological interpretation of the red shifts. Radio galaxies, quasars, and bright objects called BL Lacertae Objects are probably closely related phenomena.

Like the radio galaxies, some quasars are also surrounded by extended lobes of powerful radio emissions, but most of the radio emission from quasars usually comes from a bright core only a few light-years or less in diameter and coincident with the optically visible quasar.

When observed with very high resolution radio interferometers, this radio core is often found to consist of two or more smaller regions, which may appear to be moving away from each other with velocities considerably greater than the speed of light. Although these remarkably high velocities may seem at first to violate Albert Einstein’s special theory of relativity, they in fact can be explained as a result of motion just under the speed of light, which is directed almost toward the observer. Because the moving radio source is nearly catching up with the emitted radiation, the observed time interval between successive positions of relativistic jets of material appears shortened, and the velocity appears to be increased by a large factor over the true velocity. This phenomenon is termed apparent superluminosity.

Cosmology

Because radio galaxies and quasars are such powerful radio sources, they can be detected from a great distance. Because of the long time it takes for signals to reach the Earth from distant radio sources, radio astronomers are able to see the universe as it appeared more than 10 billion years ago, or far back in time toward the origin of the universe—the so-called big bang. Unfortunately, determining the distance to a radio source is not possible from radio measurements alone, so that distinguishing between a powerful distant source and a relatively weak nearby one is impossible. The distance may be determined only if that source is optically identified with a galaxy or quasar that has a measurable red shift. Nevertheless, from studies of the statistical distribution of large numbers of radio sources, it appears that when the universe was only a few billion years old, the number of intense radio sources was much greater and their dimensions smaller.