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Radio Astronomy

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 dishout of rafters, galvanized sheet metal, and auto partsto 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 (193945), 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 ]

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radio astronomy

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 (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, 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 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).

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radio astronomy

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.

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radio galaxy

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.

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radio astronomy

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

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Radio Astronomy

Radio Astronomy

Origins of radio astronomy

Radio vs. optical astronomy

Radio telescopes

Increasing resolution in a radio telescope

Discoveries made in radio astronomy

Radio studies of the Milky Way

Radio galaxies

Quasars and pulsars

Resources

Radio astronomy is the field of science in which information about the solar system and outer space is collected by using radio waves rather than visible light waves. In their broadest principles, radio astronomy and traditional optical astronomy are quite similar. Both visible radiation and radio waves are forms of electromagnetic radiation, the primary difference between them being the wavelength and frequency of the waves in each case. Visible light has wavelengths in the range between about 4,000 and 7,000 angstroms and frequencies in the range from about 1014 to 1015 cycles per second. (An angstrom is a unit of measurement equal to 10-8 centimeter.) In contrast, radio waves have wavelengths greater than 1 meter and frequencies of less than 109 cycles per second.

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 American engineer Karl Janskey (19051945), 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 made the surprising discovery 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.

Radio vs. optical astronomy

The presence of radio sources in outer space was an important breakthrough for astronomers. Prior to the 1930s, astronomers had to rely almost entirely on visible light for the information they obtained about the solar system and outer space. Sometimes that light was collected directly by the human eye, and others time by means of telescopes. But in either case, astronomers had at their disposal only a small fraction of all the electromagnetic radiation produced by stars, planets, and interstellar matter.

If an observer is restricted only to the visible region of the electromagnetic spectrum, she or he obtains only a small fraction of the information that is actually emitted by an astronomical object. Janskys discovery meant that astronomers were now able to make use of another large portion of the electromagnetic spectrumradio wavesto use in studying astronomical objects.

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

Radio telescopes

Radio telescopes and optical telescopes have some features in common. Both instruments, for example, are designed to collect, focus, and record the presence of a certain type of electromagnetic radiationradio waves in one case and light waves in the other. However, the details of each kind of telescope are quite different from one other.

One reason for these differences is that the human eye cannot detect radio waves as it can light waves. Therefore, an astronomer cannot look into a radio telescope the way he or she can look into an optical telescope. Also, radio waves have insufficient energy to expose a photographic plate, so an astronomer cannot make a picture of a radio source in outer space as she or he can of an optical source.

The first difference between an optical telescope and a radio telescope is in the shape and construction of the collecting apparatusthe mirror in the case of the optical telescope and the dish in the case of the radio telescope. Because the wavelength of visible light is so small, the mirror in an optical telescope has to be shaped very precisely and smoothly. Even slight distortions in the mirrors surface can cause serious distortions of the images it produces.

In a radio telescope, however, the mirror does not have to be so finely honed. The wavelength of radio waves is so long that they do not recognize small irregularities in the mirror. (The word mirror is used as a synonym for the collecting surface of the radio telescope because in effect it acts like one, although it looks nothing like a mirror.) In fact, it can be made of wire mesh, wire rods, or any other kind of material off which radio waves can be reflected.

For many years, the largest radio telescope in the world was located in a natural bowl in a mountain outside Arecibo, Puerto Rico. The bowl, which is 1,000 ft (305 m) wide and occupies 20 acres (8 ha), was lined with wire mesh, off which radio waves were reflected to a wire antenna at the focus of the telescope. The radio waves collected along the antenna were then converted to an electrical signal, which was used to operate an automatic recording device that traced the pattern of radio waves received on the wire mesh.

Increasing resolution in a radio telescope

A major drawback of the radio telescope is that it resolves images less accurately than does an optical telescope. The resolving power of a telescope is its ability to separate two objects close to each other in the sky. The resolving power of early radio telescopes was often no better than about a degree of arc compared to a second of arc that is typical for optical telescopes.

Since the resolving power of a telescope is inversely proportional to the wavelengths of radiation it receives, the only way to increase the resolving power of a radio telescope is to increase the diameter of its dish. Fortunately, it is much easier to make a very large dish constructed of metal wire than to make a similar mirror made of glass or plastic. The Arecibo radio telescope was an example of a telescope that was made very large in order to improve its resolving power.

One could, in theory, continue to make radio telescopes larger and larger in order to improve their resolving power. However, another possibility exists.

Instead of making just one telescope with a dish that is many miles in diameter, it should be possible to construct a series of telescopes whose diameters can be combined to give the same dimensions.

The radio telescope at the National Radio Astronomy Observatory near Socorro, New Mexico, is an example of such an instrument. The telescope consists of 27 separate dishes, each 85 ft (26 m) in diameter. The dishes are arranged in a Y-shaped pattern that covers an area 17 mi (27 km) in diameter at its greatest width. Each dish is mounted on a railroad car that travels along the Y-shaped track, allowing a large variety of configurations of the total observing system. The system is widely known by its more common name of the Very Large Array, or VLA.

Discoveries made in radio astronomy

The availability of radio telescopes has made possible a number of exciting discoveries about the solar system, about galaxies, about star-like objects, and about the interstellar medium. The solar system discoveries are based on the fact that the planets and their satellites do not emit visible light themselves (they only reflect visible light), although they do emit radio waves. Thus, astronomers can collect information about the planets using radio telescopes that was unavailable to them with optical telescopes.

As an example, astronomers at the Naval Research Laboratory decided in 1955 to look for radio waves in the direction of the planet Venus. They discovered the presence of such waves and found them considerably more intense than had been predicted earlier. The intensity of the radio waves emitted by the planet allowed astronomers to make an estimate of its surface temperature, in excess of 600°F (316°C).

At about the same time as the Venus studies were being carried out, radio waves from the planet Jupiter were also discovered. Astronomers found that the planet emits different types of radio radiation, some consisting of short wavelengths produced continuously from the planets surface and some consisting of longer wavelengths emitted in short bursts from the surface.

Radio studies of the Milky Way

Some of the earliest research in radio astronomy focused on the structure of the Milky Way galaxy. Studying Earths own galaxy with light waves is extraordinarily difficult because the solar system is buried within the galaxy, and much of the light emitted by stars that make up the galaxy is blocked out by interstellar dust and gas.

Radio astronomy is better able to solve this problem because radio waves can travel through intervening dust and gas and provide images of the structures of which the galaxy is made. Of special importance in such studies is a particular line in the radio spectrum, the 8-inch (21-cm) line emitted by hydrogenatoms. When hydrogen atoms are excited, they emit energy with characteristic wavelengths in both the visual and the radio regions of the electromagnetic spectrum. The most intense of these lines in the radio region is the 8-inch line. Since hydrogen is by far the most abundant element in the universe, that line is widely used in the study of interstellar matter.

The 8-inch line can be used to measure the distribution of interstellar gas and dust within the galaxy. Since the galaxy is rotating around a common center, the motion of interstellar matter with respect to the solar system (and consequently with respect to the galactic center) can often be determined. Because of studies such as these, astronomers have concluded that the Milky Way probably has spiral arms, similar to those observed for other galaxies. One major difference, however, is that the spiral arms in the Milky Way galaxy appear to be narrower and more numerous than those observed in other galaxies.

Radio emission from molecules in the interstellar gas provides radio astronomers with another important tool for probing the structure of Earths galaxy. Gases such as carbon monoxide (CO) emit at specific radio wavelengths, and are found in dark clouds of interstellar gas and dust. Because stars form in these regions, radio astronomy yields unique information on star births and on young stars.

Radio galaxies

One of the earliest discoveries made in radio astronomy was the existence of unusual objects now known as radio galaxies. The first of these, a strong radio source named Cygnus A, was detected by American radio operator Grote Reber (19112002) in 1940 using a homemade antenna in his backyard. (Reber built the first radio telescope in 1937 in Wheaton, Illinois.) Cygnus A emits about one million times as much energy in the radio region of the electromagnetic spectrum as does the Milky Way galaxy in all regions of the spectrum. Powerful radio-emitting sources like Cygnus A are now known as radio galaxies.

Radio galaxies also emit optical (visible) light, but they tend to look quite different from the more familiar optical galaxies with which astronomers had long been familiar. For example, Cygnus A looks as if two galaxies are colliding with each other, an explanation that had been adopted by some astronomers before Rebers discovery. Another radio galaxy, Centaurus A, looks as if it has a dark band running almost completely through its center. Still another radio galaxy, known as M87, seems to have a large jet exploding from one side of its central body.

In most cases, the radio image of a radio galaxy is very different from the optical image. In the case of Cygnus A, for example, the radio image consists of two large lobe-shaped structures extending to very large distances on either side of the central optical image. Studies have shown that these radio-emitting segments are very much younger (about 3 million years old) compared with the central optical structures (about 10 billion years old).

Quasars and pulsars

Some of the most interesting objects in the sky have been discovered by using the techniques of radio astronomy. Included among these are the quasars and pulsars. When quasars were first discovered in 1960, they startled astronomers because they appeared to be stars that emitted both visible and radio radiation in very large amounts. Yet there was no way to explain how stars could produce radio waves in such significant amounts.

Eventually, astronomers came to the conclusion that these objects were actually star-like objects they named quasi-stellar objects (QSOs), or quasars (quasi-stellar radio sources), rather than actual stars. An important breakthrough in the study of quasars occurred when astronomers measured the redshift of the light they produced. That redshift was very great indeed, placing some at distances of about 12 billion light-years from the Earth. At that distance, quasars may well be among the oldest objects in the sky. It is possible, therefore, that they may be able to provide information about the earliest stages of the universes history. It is now thought that quasars are the very bright centers of some distant galaxies, and so energetic probably because there is a supermassive black hole at many of the galaxies centers.

Another valuable discovery made with radio telescopes was that of pulsars. In 1967, British astrophysicist Jocelyn Bell Burnell (1943) noticed a twinkling-like set of radio signals that reappeared every evening in exactly the same location of the sky. Bell finally concluded that the twinkling effect was actually caused by an object in the sky that was giving off pulses of energy in the radio portion of the electromagnetic spectrum at very precise intervals, with a period of

KEY TERMS

Frequency The number of times per second that a wave passes a given point.

Optical astronomy A field of astronomy that uses visible light as its source of data.

Radio galaxy A galaxy that emits strongly in the radio region of the electromagnetic spectrum.

Radio waves A portion of the electromagnetic spectrum with wavelengths greater than 1 meter and frequencies of less than 109 cycles per second.

Resolving power The ability of a telescope to recognize two objects that are very close to each other in the sky.

Wavelength The distance between two consecutive crests or troughs in a wave.

1.3373011 seconds. She later found three more such objects with periods of 0.253065, 1.187911, and 1.2737635 seconds. Those objects were soon given the name of pulsars (for pul sating s tars). Evidence appears to suggest that pulsars are rotating with very precise periods, and they are now believed to be rotating neutron stars that emit a narrow beam of radio waves. Because the star is rotating, the beam sweeps across Earths sky with a precise period.

Radio astronomy is a relatively new field when compared to other fields involved with astronomical study and research. Besides Earth-based telescopes, space-based telescope satellites are now being used to study radio sources. For example, HALCA (Highly Advanced Laboratory for Communications and Astronomy) is an 9-yd (8 m) diameter radio telescope that was launched in February 1997 to an apogee/perigee altitude of 13,297 miles by 348 miles (21,400 km by 560 km), respectively. Its completed mission ended in November 2005. In addition, ASTRO-G is a radio telescope satellite being built by the Japanese Aerospace Exploration Agency (JAXA). It is scheduled to be launched in 2011 into an elliptic orbit about Earth.

See also Galaxy; Pulsar; Quasar.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Aveni, Anthony F. Uncommon Sense: Understanding Natures Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.

Burke, Bernard F. An Introduction to Radio Astronomy. Cambridge, UK: Cambridge University Press, 2001.

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Rohlfs, Kristen. Tools of Radio Astronomy. Berlin, Germany, and New York: Springer, 2004.

Taschek, Karen. Death Stars, Weird Galaxies, and a Quasar-spangled Universe: The Discoveries of the Very Large Array Telescope. Albuquerque, NM: University of New Mexico Press, 2006.

OTHER

The Search for Extraterrestrial Intelligence (SETI) Project (2006).ψ<http://setiathome.ssl.berkeley.edu> (accessed October 24, 2006)

David E. Newton

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Radio Astronomy

Radio astronomy

Radio astronomy is the field of science in which information about the solar system and outer space is collected by using radio waves rather than visible light waves. In their broadest principles, radio astronomy and traditional optical astronomy are quite similar. Both visible radiation and radio waves are forms of electro-magnetic radiation, the primary difference between them being the wavelength and frequency of the waves in each case. Visible light has wavelengths in the range between about 4,000 and 7,000 angstroms and frequencies in the range from about 1014 to 1015 cycles per second. (An angstrom is a unit of measurement equal to 10-8 centimeter.) In contrast, radio waves have wavelengths greater than 1 meter and frequencies of less than 109 cycles per second.


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 made the surprising discovery 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 .

Radio vs. optical astronomy

The presence of radio sources in outer space was an important breakthrough for astronomers. Prior to the 1930s, astronomers had to rely almost entirely on visible light for the information they obtained about the solar system and outer space. Sometimes that light was collected directly by the human eye , and others time by means of telescopes. But in either case, astronomers had at their disposal only a small fraction of all the electro-magnetic radiation produced by stars, planets, and interstellar matter .

If an observer is restricted only to the visible region of the electromagnetic spectrum , she or he obtains only a small fraction of the information that is actually emitted by an astronomical object. Jansky's discovery meant that astronomers were now able to make use of another large portion of the electromagnetic spectrum—radio waves—to use in studying astronomical objects.

In some respects, radio waves are an even better tool for astronomical observation than are 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 intense that the less intense light waves from more distant objects cannot be seen. Such is not the case with radio waves, however, which can be detected as easily during the day as they can at night.


Radio telescopes

Radio telescopes and optical telescopes have some features in common. Both instruments, for example, are designed to collect, focus, and record the presence of a certain type of electromagnetic radiation—radio waves in one case and light waves in the other. However, the details of each kind of telescope are quite different from one other.

One reason for these differences is that the human eye cannot detect radio waves as it can light waves. So an astronomer cannot look into a radio telescope the way he or she can look into an optical telescope. Also, radio waves have insufficient energy to expose a photographic plate, so an astronomer cannot make a picture of a radio source in outer space as she or he can of an optical source.

The first difference between an optical telescope and a radio telescope is in the shape and construction of the collecting apparatus—the mirror in the case of the optical telescope and the "dish" in the case of the radio telescope. Because the wavelength of visible light is so small, the mirror in an optical telescope has to be shaped very precisely and smoothly. Even slight distortions in the mirror's surface can cause serious distortions of the images it produces.

In a radio telescope, however, the "mirror" does not have to be so finely honed. The wavelength of radio waves is so long that they do not "recognize" small irregularities in the "mirror." (The word mirror is placed in quotation marks here because the collecting surface of the radio telescope looks nothing like a mirror, though it does in effect act like one.) In fact, it can be made of wire mesh, wire rods, or any other kind of material off which radio waves can be reflected.

For many years, the largest radio telescope in the world was located in a natural bowl in a mountain outside Arecibo, Puerto Rico. The bowl, which is 1,000 ft (305 m) wide and occupies 20 acres (8 ha), was lined with wire mesh, off which radio waves were reflected to a wire antenna at the focus of the telescope. The radio waves collected along the antenna were then converted to an electrical signal which was used to operate an automatic recording device that traced the pattern of radio waves received on the wire mesh.


Increasing resolution in a radio telescope

A major drawback of the radio telescope is that it resolves images much less well than does an optical telescope. The resolving power of a telescope is its ability to separate two objects close to each other in the sky. The resolving power of early radio telescopes was often no better than about a degree of arc compared to a second of arc that is typical for optical telescopes.

Since the resolving power of a telescope is inversely proportional to the wavelengths of radiation it receives, the only way to increase the resolving power of a radio telescope is to increase the diameter of its dish. Fortunately, it is much easier to make a very large dish constructed of metal wire than to make a similar mirror made of glass or plastic. The Arecibo radio telescope was an example of a telescope that was made very large in order to improve its resolving power.

One could, in theory, continue to make radio telescopes larger and larger in order to improve their resolving power. However, another possibility exists. Instead of making just one telescope with a dish that is many miles in diameter, it should be possible to construct a series of telescopes whose diameters can be combined to give the same dimensions.

The radio telescope at the National Radio Astronomy Observatory near Socorro, New Mexico, is an example of such an instrument. The telescope consists of 27 separate dishes, each 85 ft (26 m) in diameter. The dishes are arranged in a Y-shaped pattern that covers an area 17 mi (27 km) in diameter at its greatest width. Each dish is mounted on a railroad car that travels along the Y-shaped track, allowing a large variety of configurations of the total observing system. The system is widely known by its more common name of the Very Large Array, or VLA.

Discoveries made in radio astronomy

The availability of radio telescopes has made possible a number of exciting discoveries about our own solar system, about galaxies, about star-like objects, and about the interstellar medium. The solar system discoveries are based on the fact that the planets and their satellites do not emit visible light themselves (they only reflect visible light), although they do emit radio waves. Thus, astronomers can collect information about the planets using radio telescopes that was unavailable to them with optical telescopes.

As an example, astronomers at the Naval Research Laboratory decided in 1955 to look for radio waves in the direction of the planet Venus. They discovered the presence of such waves and found them considerably more intense than had been predicted earlier. The intensity of the radio waves emitted by the planet allowed astronomers to make an estimate of its surface temperature , in excess of 600°F (316°C).

At about the same time as the Venus studies were being carried out, radio waves from the planet Jupiter were also discovered. Astronomers found that the planet emits different types of radio radiation, some consisting of short wavelengths produced continuously from the planet's surface and some consisting of longer wavelengths emitted in short bursts from the surface.


Radio studies of the Milky Way

Some of the earliest research in radio astronomy focused on the structure of our galaxy, the Milky Way Galaxy. Studying our own galaxy with light waves is extraordinarily difficult because our solar system is buried within the galaxy, and much of the light emitted by stars that make up the galaxy is blocked out by interstellar dust and gas.

Radio astronomy is better able to solve this problem because radio waves can travel through intervening dust and gas and provide images of the structures of which the galaxy is made. Of special importance in such studies is a particular line in the radio spectrum , the 8-inch (21-cm) line emitted by hydrogen atoms . When hydrogen atoms are excited, they emit energy with characteristic wavelengths in both the visual and the radio regions of the electromagnetic spectrum. The most intense of these lines in the radio region is the 8-inch (21-cm) line. Since hydrogen is by far the most abundant element in the universe, that line is widely used in the study of interstellar matter .

The 8-inch (21-cm) line can be used to measure the distribution of interstellar gas and dust within the galaxy. Since the galaxy is rotating around a common center, the motion of interstellar matter with respect to our own solar system (and consequently with respect to the galactic center) can often be determined. As a result of studies such as these, astronomers have concluded that the Milky Way probably has spiral arms, similar to those observed for other galaxies. One major difference, however, is that the spiral arms in our galaxy appear to be narrower and more numerous than those observed in other galaxies.

Radio emission from molecules in the interstellar gas provides radio astronomers with another important tool for probing the structure of our galaxy. Gases such as carbon monoxide (CO) emit at specific radio wavelengths, and are found in dark clouds of interstellar gas and dust. Because stars form in these regions, radio astronomy yields unique information on star births and on young stars.


Radio galaxies

One of the earliest discoveries made in radio astronomy was the existence of unusual objects now known as radio galaxies. The first of these, a strong radio source named Cygnus A, was detected by Grote Reber in 1940 using a homemade antenna in his backyard. Cygnus A emits about a million times as much energy in the radio region of the electromagnetic spectrum as does our own galaxy in all regions of the spectrum. Powerful radio-emitting sources like Cygnus A are now known as radio galaxies.

Radio galaxies also emit optical (visible) light, but they tend to look quite different from the more familiar optical galaxies with which astronomers had long been familiar. For example, Cygnus A looks as if two galaxies are colliding with each other, an explanation that had been adopted by some astronomers before Reber's discovery. Another radio galaxy, Centaurus A, looks as if it has a dark band running almost completely through its center. Still another radio galaxy, known as M87, seems to have a large jet exploding from one side of its central body.

In most cases, the radio image of a radio galaxy is very different from the optical image. In the case of Cygnus A, for example, the radio image consists of two large lobe-shaped structures extending to very large distances on either side of the central optical image. Studies have shown that these radio-emitting segments are very much younger (about 3 million years old) compared with the central optical structures (about 10 billion years old).

Quasars and pulsars

Some of the most interesting objects in the sky have been discovered by using the techniques of radio astronomy. Included among these are the quasars and pulsars. When quasars were first discovered in 1960, they startled astronomers because they appeared to be stars that emitted both visible and radio radiation in very large amounts. Yet there was no way to explain how stars could produce radio waves in such significant amounts.

Eventually, astronomers came to the conclusion that these objects were actually star-like objects they named Quasi-Stellar Objects (QSOs), or quasars, rather than actual stars. An important breakthrough in the study of quasars occurred when astronomers measured the red-shift of the light they produced. That red-shift was very great indeed, placing some at distances of about 12 billion light-years from Earth . At that distance, quasars may well be among the oldest objects in the sky. It is possible, therefore, that they may be able to provide information about the earliest stages of the universe's history. It is now thought that quasars are the very bright centers of some distant galaxies, and so energetic probably because there is a supermassive black hole at the galaxy's center.

Another valuable discovery made with radio telescopes was that of pulsars. In 1967, British astronomer Jocelyn Bell noticed a twinkling-like set of radio signals that reappeared every evening in exactly the same location of the sky. Bell finally concluded that the twinkling effect was actually caused by an object in the sky that was giving off pulses of energy in the radio portion of the electromagnetic spectrum at very precise intervals, with a period of 1.3373011 seconds. She later found three more such objects with periods of 0.253065, 1.187911, and 1.2737635 seconds. Those objects were soon given the name of pulsars (for pulsating stars). Evidence appears to suggest that pulsars are rotating with very precise periods, and they are now believed to be rotating neutron stars that emit a narrow beam of radio waves. Because the star is rotating, the beam sweeps across our sky with a precise period.

See also Galaxy; Pulsar; Quasar.


Resources

books

Editors of Time-Life Books. Voyage through the Universe: The Far Planets. Alexandria, VA: Time-Life Books, 1991.

Editors of Time-Life Books. Voyage through the Universe: TheNew Astronomy. Alexandria, VA: Time-Life Books, 1991.

Mark, Hans, Maureen Salkin, and Ahmed Yousef, eds. Encyclopedia of Space Science & Technology. New York: John Wiley & Sons, 2001.

Pasachoff, Jay M. Contemporary Astronomy. 4th ed. Philadelphia: Saunders College Publishing, 1989.

Verschuur, Gerrit L. The Invisible Universe Revealed. New York: Springer-Verlag, 1987.

other

The Search for Extraterrestrial Intelligence (SETI) Project [cited 2003]. <http://setiathome.ssl.berkeley.edu>.


David E. Newton

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency

—The number of times per second that a wave passes a given point.

Optical astronomy

—A field of astronomy that uses visible light as its source of data.

Radio galaxy

—A galaxy that emits strongly in the radio region of the electromagnetic spectrum.

Radio waves

—A portion of the electromagnetic spectrum with wavelengths greater than 1 meter and frequencies of less than 109 cycles per second.

Resolving power

—The ability of a telescope to recognize two objects that are very close to each other in the sky.

Wavelength

—The distance between two consecutive crests or troughs in a wave.

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