Radio Astronomy
Before 1931, astronomers could gain information about the cosmos only by visual means, using the naked eye aided by magnifying lenses and reflective mirrors. In the early part of the twentieth century, however, a new way of assessing the nature of the stars and the planets was found.
Visible light is a small segment of a vast electromagnetic spectrum that includes X rays, gamma rays, infrared light, and radio waves. Radio waves are light waves that have a longer frequency than the visible red and the invisible infrared portions of the electromagnetic spectrum. Many of the most important objects in the cosmos are invisible, or very dim, at visible wavelengths, yet extremely active at lower electromagnetic frequencies. Such objects can be detected and studied easily with a receiver tuned to the appropriate frequency.
In 1928 Karl G. Jansky, an engineer employed by Bell Telephone Laboratories, was given the task of finding the source of static that was interfering with trans-Atlantic radio-telephone communications. Jansky built a crude dish antenna from two parallel frameworks of wood and brass; one frame reflected radio waves into the device, and the other received signals. So it could rotate, Jansky mounted his antenna on wheels from a Model-T Ford. By 1932 he had detected three sources of radio waves. Two were related to lightning storms, but the third seemed to be coming from the constellation Sagittarius in the southern Milky Way. Jansky wanted to build a dish antenna nearly 100 ft (about 30 m) in diameter to investigate this phenomenon, but his employer refused to fund the undertaking. Universities did not pursue the matter, either, lacking resources for such a project.
The radio signals from space remained unexamined for years until an amateur radio enthusiast in Wheaton, Illinois took it upon himself to begin exploring the signals. Grote Reber (1911-) had become interested in radio at the age of fifteen. When he learned of Jansky's discovery, he built the world's first radio dish in his back yard out of rafters, galvanized sheet metal, and auto parts. The dish, the portion of the device that collects radio signals, had a diameter of 31 feet (9.5 m); the receiver was mounted above the dish. Reber completed his radio dish in 1937, at a cost of $1,300.
With his radio telescope, Reber was able to produce the first radio maps of the sky. He discovered points in the heavens where strong radio signals were being emitted as if from small star-like objects, in the absence of visible stars. (Astronomer Walter Baade would later identify a pair of galaxies in collision at one of these locations.)
Reber worked virtually alone until the end of World War II allowed professional scientists to direct their attention to the radio signals from space. In 1947 Sir Bernard Lovell (1913-), an astronomer who had been using radar to detect daytime meteor activity, oversaw the construction of a parabolic radio reflector 216 ft (66 m) in diameter at Jodrell Bank, England. A 250-ft (76 m) dish, the world's largest at the time, went into operation in August 1957.
Sir Martin Ryle improved the reflector in 1949 to study intense radio waves in the constellations Cassiopeia and Cygnus. It showed that the sources were too large to be stars. The one in Cassiopeia turned out to be the remnant of a supernova explosion; in Cygnus, the object was a radio galaxy. There was skepticism that an extragalactic object could be responsible, but as more radio galaxies were found, the evidence became overwhelming.
Ryle then devised an ingenious method by which multiple radio telescopes, separated by a distance, could act as a single telescope with a diameter equal to the distance separating them. In 1955 he used twelve individual telescopes to observe the same object simultaneously. The data were recorded and sent to a single receiver, where a computer was used to synchronize and analyze all the data. This was the birth of the radio interferometer. It was a crucial advance, because radio astronomy was limited by poor resultion, stemming from the very long wavelength of the light being studied. But large radio interferometers could approach the resolution of smaller optical telescopes.
Ryle used three telescopes in 1964 that were far enough apart to synthesize an aperture of 10 mi (16 km). In 1977 the Very Large Array ( VLA) interferometer was constructed near Socorro, New Mexico. It consists of 27 dishes in a Y pattern, each arm with a maximum length of about 12 mi (19 km), giving extremely high resolution.
Even more powerful is the new VLBA (Very Long baseline Array) interferometer, which was completed in 1993. It comprises ten antennas from the Hawaiian Islands to the U.S. Virgin Islands. It provides resolution in the radio regime down to one milliarcsecond (one one-thousandth of one second of arc, which is comparable to being able to see a dime in New York with a telescope located in Los Angeles).
Other single-dish radio telescopes have been in existence longer. An 85-ft (26 m) diameter dish was put into operation in 1959 in Green Bank, West Virginia, and a 300-ft (91 m) behemoth was put into operation in 1962. It was the largest steerable radio telescope in the world, until it collapsed from metal fatigue in 1989.
The world's largest individual dish is in Arecibo, Puerto Rico. Here American engineer Thomas C. Kavanagh (1912-) designed and constructed a radio telescope out of a natural depression within a ring of mountains in 1963. The reflecting surface, rebuilt in 1974, has a diameter of 1,000 ft (305 m). Radio objects were everywhere; there were sources, both "bright" and "dim," at a wide variety of wavelengths including the Sun, interstellar gas and dust, the Milky Way and beyond.
Today, radio astronomy is administered in the United States by the National Radio Astronomy Observatory, which operates with offices in Charlottesville, Virginia, Green Bank, West Virginia, Socorro, New Mexico, and Tucson, Arizona.
Our knowledge of nearly every object in the cosmos has been improved by the use of radio telescopes. Radio astronomy has amassed an incredible amount of information, much of it surprising and unexpected. The Crab nebula, in the constellation of Taurus, was found to be a strong radio source by Australian astronomer John Boulton in 1947. Twenty years later, Anthony Hewish identified a pulsar ( neutron star) within the core of the Crab. In January, 1955, American astrophysicists Bernard Burke (1928-) and Kenneth Franklin (1923-) detected radio bursts coming from the planet Jupiter. Next to the sun, this planet is the strongest source of radio waves in the solar system.
Arno A. Penzias and Robert W. Wilson (1936-), in 1963, found themselves in much the same boat as Karl Jansky 31 years earlier. Background noise was interfering with a horn antenna that was to be used to collect radio signals bounced off the Echo satellite. Their discovery turned out to be the residual "noise" from the " big bang " that initiated the creation of the universe billions of years ago.
When several very small, but intense radio sources were discovered which did not fit into any previously known classification, they were called "quasi-stellar radio sources." Investigation of their red-shifts by Maartin Schmidt (1929-) revealed them to be the most distant, and therefore the oldest, objects known; quasars. When astronomers look at a quasar, they are getting a glimpse into the early universe.
What is still to come? Glimpses of the inner regions of quasars and galactic nuclei; accurate measurements of cosmic distances; relativistic bending of radio waves to match that of visible light, predicted by Albert Einstein. There are even possibilities for increasing our knowledge about our own planet through accurate measurements of the positions of quasars from various points on the Earth, gleaning information about continental drift, shifts in fault lines, and changes in the Earth's rotation.
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