Radio astronomy

In 1932, Karl Guthe Jansky made observations that led him to conclude that Earth was receiving radio waves not produced by humans. This event opened a new observational area of study for astronomers and gave birth to the science of radio astronomy, which produced a new picture of the universe as a very active place with pulsars, quasars, and radio galaxies.

Overview

Until radioexperiments by Karl Guthe Jansky (1905-1950) in the 1930s, virtually all astronomical knowledge was a product of analyzing visible starlight. Historically, optical telescopes and photographic techniques enhanced scientists’ vision of the small range of frequencies in the electromagnetic spectrum perceived as visible light (400 to 700 nanometers in wavelength). Radio signals are found in the long-wavelength part of the electromagnetic spectrum, ranging from millimeters to several thousand meters. Jansky’s 1932 discovery of radio sources beyond Earth ultimately led astronomers to imagine millions of discrete radio transmitters broadcasting throughout the universe. The small slit of visible light through which astronomers had previously viewed the cosmos suddenly expanded to a wide-open window with the addition of radio signals. Unfortunately, the radio universe is invisible to humans’ biological eyes, and consequently, it is necessary to convert radio signals into a visual format for the interpretation of data.

A radio telescope collects and converts radio signals through its basic components: the antenna, which collects the signals; an amplifier, which magnifies these incredibly weak waves; and a recorder, which translates this information into a medium that can be viewed. A radio telescope is a directional antenna that may take several forms. Jansky’s design was a square wooden frame around which he wrapped wire. Another type of antenna is the dipole, a pair of metal poles (the length of the wavelength to be received) 180° to each other joined by insulating material. A television antenna is a common example. Television antennas often have several dipoles in front of the collector element (where the wires attach from the television) that focus its directivity and a reflector element behind it to increase the strength of the incoming signal. Radio telescopes can be very similar, with only the length of their dipoles being different. The size, shape, and design of a radio telescope are largely dictated by the radio-wave frequency being investigated by the telescope.

Nevertheless, as signals from extraterrestrial sources are often weaker than television broadcasts, radio astronomers must use the greatest directivity and sensitivity antennae. The parabolic reflector efficiently satisfies these requirements. These reflectors are generally larger versions of the home television satellite dish for radio astronomy.

While the advantage of a larger observational window is attractive, radio astronomy has limitations. The ability to see or resolve detail with a radio telescope depends on the size of its reflecting elements, proportional to its diameter, much like the resolution parameters of optical telescopes. Radio waves, longer than optical wavelengths by ten thousand, require reflectors to be equally large. Since this requirement is impractical to achieve, most radio telescopes have, by optical standards, poor resolution. A parabolic reflector thirty meters in diameter has a resolving power of about 0.5°, or about the size of the full Moon. Fortunately, computer advances allow radio telescopes kilometers apart to be linked together, forming the equivalent of a collector equal to that distance.

Another difficulty facing radio astronomers is the extremely weak nature of the signals. Even large telescopes, such as the 300-meter dish at Arecibo, Puerto Rico, collect only minute quantities of radio signals. To illustrate the incredibly faint nature of these signals, one must realize that the energy received by all radio telescopes on Earth is equivalent to less than the energy transferred by a snowflake hitting the ground. Additionally, radio astronomers have to filter unwanted radio signals created by their own technology. Human-produced radio noise can range from automobile ignitions to satellite transmissions.

As severe as these limitations may appear, radio astronomy provides distinct advantages over optical telescopes. Radio telescopes are not affected by weather, light pollution, interstellar dust, or the Sun, which obliterates observations for optical telescopes. The ability of radio telescopes to identify optically invisible sources of matter between the stars, such as nonluminous clouds of hydrogen, makes them an invaluable tool. Hydrogen emits a distinctive radio signature originating from a spin-flip transition in the ground state that emits photons of twenty-centimeter wavelength (or a frequency of 1,420 megahertz). The universe is dominated by hydrogen (about 75 percent), and studying its distribution contributes to understanding the cosmos. Additional advantages of the radio window are the property of these long wavelengths to penetrate optically opaque dust clouds in the Milky Way and allow astronomers to “see” into the Galaxy. The radio window will enable astronomers to detect " brighter " objects in the radio frequencies than in the visible wavelengths. Indeed, some of the most distant objects in the universe are detectable only by their radio emissions.

Radio waves are generated in space by several methods. Thermal emissions—frequencies radiating from hot ionized gas clouds—indicate the places of star birth. However, most radio emissions are of nonthermal origin. Cosmic-ray electrons (charged particles) ejected from supernova explosions are typical of this type of radio emission. Upon encountering magnetic fields, such as a galaxy, these electrons move around and within the magnetic field in helical paths, losing kinetic energy that radiates into space as radio waves. This radiation source yields information about cosmic rays and the nature of interstellar magnetic fields. Radio waves also originate from the ground state spin-flip transition of hydrogen atoms scattered throughout interstellar space. The frequency is so specific that it has become a subdiscipline in radio astronomy: studying the twenty-one-centimeter emission line, hydrogen’s radio signature. The importance of this radio signature is that it is possible to determine if a hydrogen nebula is moving toward or away from Earth by utilizing the Doppler shift. Hydrogen atoms also absorb radiation from more distant sources. Studying the combined effects of absorption and transmission can determine if the radio source lies beyond the hydrogen cloud. In this way, studies in the twenty-one-centimeter wavelength have helped define much of the galaxy's structure.

Earth’s atmosphere is opaque to most radiation, except the visible and radio wavelengths. The atmosphere absorbs most of the other wavelengths before they reach the ground. Radio telescopes enable astronomers to detect radio emissions from many sources: the Moon, Venus, Mars, Jupiter, Saturn, the Sun’s atmosphere (corona), clouds of ionized hydrogen within the Milky Way, and cosmic-ray electrons spiraling in the magnetic field of the Milky Way as well as other galaxies. Astronomers have learned much about the nature and structure of the Milky Way and the universe through radio observations.

Applications

Before the introduction of radio astronomy, the universe appeared relatively stable, if not static, apart from the occasional supernova. Then, a new picture of an active universe emerged with the development of radio and the associated radio telescope in the early twentieth century. The discovery of radio sources identified as pulsars underlies this dynamic element.

In 1934, two astronomers, Walter Baade and Fritz Zwicky, proposed that the remaining stellar core from a supernova explosion represents the transition of a star to a neutron star. This theory received little attention until 1967.

In the mid-1960s in Cambridge, England, Antony Hewish and his colleagues built a radio telescope to observe the scintillation, or twinkling, of radio sources known as quasars. By coincidence, the parameters for the telescope matched the characteristics of pulsars (later to be described as rapidly rotating neutron stars). Jocelyn Bell, a graduate student, detected a regular pulsating source of radio emissions that kept appearing from the same area in the sky, with a regularity of every 1.33 seconds. Bell’s discovery of pulsars began a small revolution in astronomy. Their signals were unlike any previously detected from stars or galaxies. The intriguing idea of the Little Green Men (LGM) phenomenon was entertained briefly. Then, another pulsar was discovered, which was pulsating at 1.2 seconds. The regularity of the pulses distinguished them from the other celestial radio sources. As astronomers rushed to map the radio sky, they found several more pulsars within the following year. There are more than 1,500 known sources. The fastest pulsars were associated with supernova remnants (nebulae); the slower ones had no visible nebula.

The evolution of a pulsar began in the fires of a supernova explosion. The rapidly spinning core pulsed radio signals and slowed as the star’s outer shell expanded into space (the nebula). There was a correlation between the spin rate and the presence or absence of a nebula. The twenty-thousand-year-old Vela Pulsar has a rate of about one second, while the one-thousand-year-old Crab Nebula has a rate of about thirty-three pulses per second. The Baade-Zwicky neutron star theory was revived and developed momentum as a model to explain the phenomena. Indeed, pulsars seem to be rotating neutron stars, the final products of supernova explosions.

Interestingly, no one has seen a neutron star; they are too small to be seen at the distances they present themselves. Were it not for their radio pulses and the ability to detect them, astronomers would remain ignorant of the relationship between supernovae and neutron stars. However, using very long baseline interferometry (VLBI), astronomers have identified differences across the surface of neutron stars.

Although the LGM theory did not last long after Bell’s first pulsar discovery, it did begin a series of observations and experimental transmissions through which it is hoped to detect the existence of extraterrestrial intelligence, known as the Search for Extraterrestrial Intelligence (SETI). The search for signals from other civilizations began in 1959 at the Green Bank National Radio Observatory in West Virginia. In 1974, at the dedication of the radio telescope at Arecibo, Puerto Rico, astronomers transmitted the first message from Earth intended for other civilizations.

The discovery of pulsars and neutron stars had many consequences for astronomy and radio astronomy. Fundamentally, it caused physicists to reassess the states of matter to accommodate a new phenomenon of “degenerate matter.” Radio astronomy has enabled astronomers to “listen” to the background radiation of the Big Bang, determine the nature of quasars and their probable relation to black holes, discover molecular hydrocarbons in space, some of the fundamental building blocks of life, and begin an active search for intelligent extraterrestrial life. Radio astronomy has increased astronomers’ knowledge of the Milky Way and the universe.

Context

The ability to see the universe through the radio window is a technological development whose evolution parallels the maturation of radio and its associated technology. Transmission of radio waves through space was demonstrated in 1887 by German physicist Heinrich Rudolph Hertz (1857-1894) in experiments based on predictions of James Clerk Maxwell (1831-1879). This successful demonstration began the age of radio.

Some astronomers suspected that stars might be a source of radio waves; the most easily examined star is the Sun. The search for solar radio signals began in 1890 with American inventor Thomas Alva Edison (1847-1931), followed in England by Sir Oliver Joseph Lodge (1851-1940) in 1894 and Charles Nordman of France in 1900. Negative results from their experiments diminished interest in this line of research. The idea of extraterrestrial radio waves generally receded from the minds of the astronomical community at this point. Scientists of the period were unaware of the ionized reflecting layer in the upper atmosphere (ionosphere), which effectively filters radio frequencies longer than about twenty meters.

This apparent failure of radio science did not deter its technological development. The radio caught the public's and scientists' imagination with Guglielmo Marconi’s (1875-1937) transatlantic transmission in 1901. This achievement led Arthur Edwin Kennelly (1861-1939) and Oliver Heaviside (1850-1925) in 1902 to propose an electrified layer in the upper atmosphere (ionosphere), which reflected radio waves around the curve of Earth. Research into this new technology and “atmospherics” intensified during World War I. By the 1920s, radio research flourished in major universities, especially in commercial laboratories such as Bell Telephone Laboratories in the United States and Marconi Telegraph Company in England.

During the exploration of radio technology in the 1920s, meteorological conditions, such as lightning, were known to affect the quality of radio transmissions. Still, details of atmospheric disturbances and the Kennelly-Heaviside layer (ionosphere) were only understood partially. Nevertheless, radio technology advanced, and in 1929, shortwave transatlantic radio telephone service became available to the public. Unfortunately, the technology of the 1920s also introduced its source of radio noise—specifically, the automobile and the intrinsic noise of the vacuum-tube receivers. Research into atmospheric radio noise led by Sir Robert A. Watson-Watt (1892-1973, the inventor of radar) in the 1920s began a sequence of studies resulting in the foundation of radio astronomy.

Karl Guthe Jansky (1905-1950) joined Bell Telephone Laboratories in 1928. His assignment was to build a radio receiver to record and study the intensity of shortwave atmospheric interference (“atmospherics”) of about fifteen meters, a frequency used for ship-to-shore and transatlantic communication. Jansky observed three types of static in his receiver. He identified two static types as atmospheric disturbances, but the third was a steady type of hiss-static. This steady hiss was the most intriguing because the most substantial part of the signal advanced in time throughout the year. At first, Jansky thought it might be connected with the Sun, but radio recordings of a partial solar eclipse of the Sun on August 31, 1932, indicated that the Sun was not the source. (This period was a time of sunspot minimum. If the Sun had been in a sunspot maximum phase, Jansky might have recorded solar radio emissions.)

Jansky spent 1932 analyzing the radio signals and trying to distinguish a single source and the apparent angle of arrival. He observed that the maximum signal strength coincided with the passage of the Milky Way in front of his antenna. On June 27, 1933, at the Institute of Radio Engineers’ Eighth Annual Convention in Chicago, he presented a paper entitled “Electrical Disturbances Apparently of Extraterrestrial Origin.” He described the source as having a periodicity of twenty-three hours, fifty-six minutes, characteristic of sidereal objects, and the direction of the source at approximately eighteen hours right ascension. The front page of The New York Times of May 5, 1933, read “New Radio Waves Traced to Center of Milky Way.” Interestingly, with both popular and scientific publicity, no university pursued the research. In 1936, G. W. Potapenko and D. F. Folland of the California Institute of Technology confirmed Jansky’s results but were not funded for further research after their initial trial.

Grote Reber, a radio engineer in Chicago, built the next radio telescope. During 1937 and 1938, Reber built a parabolic radio telescope in his backyard. With this new type of telescope, he identified radio sources in the constellations Cygnus, Cassiopeia, and Sagittarius, as well as signals from the Sun that had eluded earlier researchers. He also constructed a detailed radio map of the Milky Way.

World War II interrupted many scientists’ research, but advanced radar astronomy and the new radar field. During the war, some scientists made near-simultaneous discoveries in radio and radar astronomy but, for military or political reasons, were unaware of one another’s work. In 1942, J. S. Hey in England correlated radio noise with a large solar flare and identified the Sun as a radio source, but it was not announced until after the war. After 1945, benefits were gained from technological developments such as radar, high-frequency receivers, and antennas of large aperture and higher gain. Radio observations of the Sun and the galaxy were vastly improved, and the discovery of other radio sources began a race to build radio telescopes in the early 1950s. Like its optical counterpart, radio astronomy became a big science requiring funding from the federal government and underwriting by wealthy philanthropists.

Historically, radio astronomy was concerned with continuous emissions from various extraterrestrial sources. In 1951, however, a subdiscipline developed: the study of twenty-one-centimeter line emissions. These emissions are the radio signatures of largely neutral hydrogen atoms first detected by Harold Ewen and Edward Mills Purcell of Harvard University. Observing the twenty-one-centimeter spectral line of neutral hydrogen provided a method of mapping the galaxy previously unclear to astronomers. It was discovered that hydrogen was a significant component of interstellar space and the universe, accounting for about 75 percent of its mass.

During the 1950s, there was a rush to build radio telescopes and identify as many discrete sources as possible. Most radio sources coincided with optical sources, such as galaxies and supernova remnants. The culmination of this data gathering produced new catalogs of the heavens and a reexamination of the more familiar optical sources. The discovery of interstellar molecules, radio galaxies, quasars, pulsars, and the solar wind have all influenced the direction of astronomy since that time. Radio astronomy has contributed significantly to theories of radio emissions, the origin of the universe, and the physics of energy mechanisms.

Bibliography

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Jansky, Karl G. “Electrical Disturbances Apparently of Extraterrestrial Origin.” Proceedings of the Institute of Radio Engineers, vol. 21, 1933, pp. 1387-1398.

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