Radio astronomy

Matter in the universe emits radiation (energy) from all parts of the electromagnetic spectrum, the range of wavelengths produced by the interaction of electricity and magnetism. The electromagnetic spectrum includes light waves, radio waves, infrared radiation, ultraviolet radiation, X rays, and gamma rays.

Radio astronomy is the study of celestial objects by means of the radio waves they emit. Radio waves are the longest form of electromagnetic radiation. Some of these waves measure up to 6 miles (more than 9 kilometers) from peak to peak. Objects that appear very dim or are invisible to our eye may have very strong radio waves.

Words to Know

Big bang theory: Theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago.

Electromagnetic radiation: Radiation that transmits energy through the interaction of electricity and magnetism.

Gamma rays: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.

Infrared radiation: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

Pulsars: Rapidly spinning, blinking neutron stars.

Quasars: Extremely bright, starlike sources of radio waves that are the oldest known objects in the universe.

Radio waves: Longest form of electromagnetic radiation, measuring up to 6 miles from peak to peak.

Ultraviolet radiation: Electromagnetic radiation (energy) of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.

Wavelength: The distance between two peaks in any wave.

X rays: Electromagnetic radiation of a wavelength just shorter than ultraviolet radiation but longer than gamma rays that can penetrate solids and produce an electrical charge in gases.

In some respects, radio waves are an even better tool for astronomical observation than light waves. Light waves are blocked out by clouds, dust, and other materials in Earth's atmosphere. Light waves from distant objects are also invisible during daylight because light from the Sun is so bright that the less intense light waves from more distant objects cannot be seen. Radio waves, however, can be detected as easily during the day as they can at night.

Origins of radio astronomy

No one individual can be given complete credit for the development of radio astronomy. However, an important pioneer in the field was Karl Jansky, a scientist employed at the Bell Telephone Laboratories in Murray Hill, New Jersey. In the early 1930s, Jansky was working on the problem of noise sources that might interfere with the transmission of short-wave radio signals. During his research, Jansky discovered that his instruments picked up static every day at about the same time and in about the same part of the sky. It was later discovered that the source of this static was the center of the Milky Way galaxy.

Grote Reber, an amateur radio enthusiast in Wheaton, Illinois, took it upon himself to begin examining the radio signals from space. In 1937, he built the world's first radio dish—out of rafters, galvanized sheet metal, and auto parts—to collect radio signals in his back yard. He mounted a receiver above the dish. Reber produced the first radio maps of the sky,

discovering points where strong radio signals were being emitted. He worked virtually alone until the end of World War II (1939–45), when scientists began adapting radar tracking devices for use as radio telescopes.

What radio astronomy has revealed

Scientists have found that radio signals come from everywhere. 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.

In 1955, astrophysicists detected radio bursts coming from Jupiter. Next to the Sun, this planet is the strongest source of radio waves in the solar system. Around this time, Dutch astronomer Jan Oort used a radio telescope to map the spiral structure of the Milky Way galaxy. In 1960, several small but intense radio sources were discovered that did not fit into any previously known classification. They were called quasi-stellar radio sources. Further investigation revealed them to be quasars, the most distant and therefore the oldest celestial objects known. And in the late 1960s, English astronomers Antony Hewish and Jocelyn Bell Burnell detected the first pulsar (neutron star), a strong radio source in the core of the Crab Nebula.

Evidence of the big bang. In 1964, radio astronomers found very compelling evidence in support of the big bang theory of how the universe began. Americans Arno Penzias and Robert Wilson discovered a constant background noise that seemed to come from every direction in the sky. Further investigation revealed this noise to be radiation (now called cosmic microwave background) that had a temperature of −465°F (−276°C). This corresponded to the predicted temperature to which radiation left over from the formation of the universe 12 to 15 billion years ago would have cooled by the present.

Today astronomers use radio astronomy and other sophisticated methods including gamma ray, infrared, and X-ray astronomy to examine the cosmos. The largest single radio telescope dish presently in operation, with a diameter of 1,000 feet (305 meters), is in Arecibo, Puerto Rico.

[See also Galaxy; Pulsar; Quasar; Telescope ]

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 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 . An inexplicable source of radio noise was identified in 1940 by Gröte Reber , using a radio telescope in the backyard of his home, as originating from our own galaxy, the Milky Way . 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 , or exploding stars. The most famous example of a supernova remnant is the Crab Nebula in Taurus.

Because there are strong magnetic fields (see magnetism ) in the vicinities of supernovas remnants, an additional mechanism is present for producing radio waves. This is the synchrotron radiation 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 . 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 and from other indirect studies. Besides atomic hydrogen, certain simple organic (carbon-based) molecules, including cyanogen (CN) and formaldehyde (H ₂ CO), 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 , 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 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 , 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 . 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 , 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

Light Astronomy

In the 1950s light astronomy, that is, scientific advancement based on what one could see through various telescopes that magnified images, was nearing its technological limit. The giant telescope at Mount Palomar was producing sharp, clear views of celestial bodies, including the planets. These images would not be greatly improved until satellites began transmitting pictures of the planets from close range decades later. The next big technological breakthrough in astronomy during the 1950s was the radio telescope.

Celestial Static

Radio astronomy was born in 1932, when K. G. Jansky noted that radios picked up static from some source. He traced this to radio waves emitted by celestial bodies, especially certain stars. This curious finding became the field of radio astronomy. It was subsequently discovered that all stars emit various waves: light is one form of electromagnetic wave; radio is a nother, with a longer wavelength. Using a telescope with a special antenna, an observer can detect these radio waves on ground stations. These receivers are usually made of a large wire-mesh concave dish with a central antenna. They can be grouped to scan wider areas of the sky.

Versatility

Radio telescopes are versatile. They can be mounted on pivots so that they can be pointed accurately in a particular direction. They can also have their pivots mounted. If the pivots are mounted on a circular track, the telescope can detect waves from three dimensions at any point above the horizon.

The Sounds of Jupiter

Just as each star has a characteristic light signature on a spectrograph, it also has a characteristic radio signature. Some stars are very strong emitters (radio stars). Their emissions may change over time, as our own sun does when it has a solar storm. Even planets may have radio emissions. Jupiter was first heard in 1955. By the end of the decade, radio telescopes out-numbered light telescopes in use in America for academic astronomy.

Sources:

Trevor Illtyd Williams, Science: A History of Discovery in the Twentieth Century (New York and Oxford: Oxford University Press, 1990);

Life (17 November 1952): 130;

Popular Mechanics (November 1952): 148.

radio astronomy The study of the Universe in the radio part of the electromagnetic spectrum. There is a wide radio window covering wavelengths from about 1 mm to 30 m, almost all of it accessible from ground-based observatories, both day and night. Emission mechanisms are either thermal (black-body radiation, free–free transition) or non-thermal (mainly synchrotron radiation). Maser sources are also common. Study of emission and absorption lines from molecules provides information about conditions in interstellar space. The 21-cm hydrogen line has proved to be a particularly valuable probe of the structure of our Galaxy and others.

Radio astronomy began in the 1930s with the pioneering work of K. G.Jansky and G.Reber, but it was not until after World War II that major research groups were set up. The first sources to be identified were the Sun, the Milky Way, and supernova remnants such as the Crab Nebula. As radio telescopes improved in sensitivity and resolution, radio galaxies and quasars were identified. New understanding of the late stages of stellar evolution came with the discovery of pulsars. The counting of radio sources of different brightnesses (and hence presumably different distances) has helped astronomers understand how the Universe has evolved, and the discovery of the cosmic microwave background has provided direct support for the Big Bang theory of cosmology.

Radio telescopes employ several different techniques to collect the very weak radio waves coming from the sky. The most familiar is the circular parabolic dish that gathers radio waves and brings them to a focus, but the simplest radio telescopes may consist of little more than a metal rod (a dipole antenna). Several antennas or telescopes may be used together as an interferometer to achieve high resolution, notably in the sophisticated techniques of aperture synthesis and very long baseline interferometry.

radio galaxy A galaxy that is an unusually powerful emitter of radio waves. The output of a radio galaxy can be up to 10³⁸ watts, a million times greater than a normal galaxy such as our own. Radio galaxies have a compact radio nucleus coincident with the core of the visible parent galaxy, a pair of opposed jets emerging from the nucleus, and a pair of lobes (1) far outside the visible confines of the galaxy. The galaxy is almost always a giant elliptical, which may be the result of the collision and merger of two or more smaller galaxies. The source of the radio galaxy's energy is believed to be a massive black hole in the galactic nucleus from which the jets emerge, delivering energy to the lobes. Notable radio galaxies include Centaurus A, Cygnus A, and Virgo A.

radio astronomy Study of radio waves (electromagnetic radiation with wavelengths from about 1mm to many metres) that reach the Earth from objects in space. Observations can be made using a radio telescope. Karl Jansky discovered radio noise from the Milky Way in 1931, and the subject grew rapidly after World War II. The number of radio sources increases with distance, demonstrating that the universe evolves with time. This fact, combined with the discovery at radio wavelengths of the cosmic microwave background, is strong evidence in support of the Big Bang theory of the origin of the universe.

radio galaxy Galaxy that emits strong electromagnetic radiation of radio frequency. These emissions seem to be produced by the high-speed motion of elementary particles in strong magnetic fields.

ra·di·o as·tron·o·my • n. the branch of astronomy concerned with radio emissions from celestial objects.