Reber Publishes the First Radio Maps of the Galaxy

Grote Reber built the first intentional radio telescope and used it to record the first radio contour maps of the Milky Way in two complete sky surveys, establishing the foundations of radio astronomy.

Summary of Event

Grote Reber’s work to record the first radio contour maps of the galaxy represented a new and unexpected application of the technology of radio to the science of astronomy. The discovery of radio waves by Heinrich Hertz only fifty years before Reber built his radio telescope in 1937 had confirmed James Clerk Maxwell’s electromagnetic theory of light and demonstrated that electromagnetic waves consist of many different wavelengths. These wavelengths extend from the longer waves, such as radio and infrared, down to shorter waves that include visible light, ultraviolet, and X rays. As wavelength decreases, frequency generally increases, but all electromagnetic waves travel at the speed of light. Reber began the process of converting these varying wavelengths of energy reaching Earth from outer space into a window that could be used to observe astronomical phenomena. Radio astronomy;Milky Way galaxy
Milky Way galaxy
[kw]Reber Publishes the First Radio Maps of the Galaxy (Nov., 1944-Oct., 1948)
[kw]Radio Maps of the Galaxy, Reber Publishes the First (Nov., 1944-Oct., 1948)
[kw]Maps of the Galaxy, Reber Publishes the First Radio (Nov., 1944-Oct., 1948)
[kw]Galaxy, Reber Publishes the First Radio Maps of the (Nov., 1944-Oct., 1948)
Radio astronomy;Milky Way galaxy
Milky Way galaxy
[g]North America;Nov., 1944-Oct., 1948: Reber Publishes the First Radio Maps of the Galaxy[01290]
[g]United States;Nov., 1944-Oct., 1948: Reber Publishes the First Radio Maps of the Galaxy[01290]
[c]Astronomy;Nov., 1944-Oct., 1948: Reber Publishes the First Radio Maps of the Galaxy[01290]
[c]Science and technology;Nov., 1944-Oct., 1948: Reber Publishes the First Radio Maps of the Galaxy[01290]
Reber, Grote
Jansky, Karl G.
Herschel, William
Shapley, Harlow
Hubble, Edwin Powell

William Herschel was one of the first astronomers to recognize the true nature of the dense band of stars across the sky called the Milky Way. By counting stars in various directions in the Milky Way, he concluded in 1785 that the vast majority of stars were contained within a flattened disk, forming a sort of island in space. Herschel’s work reduced the solar system to a tiny speck in the vast universe of stars. Early in the twentieth century, Harlow Shapley was able to use the 2.5-meter Mount Wilson telescope to study star clusters and estimate their distances from Earth. He demonstrated that the Milky Way galaxy was far larger than any previous estimate and that the Sun was far away from the galactic center, which he located in the direction of the constellation Sagittarius.

Grote Reber.

(Courtesy, National Radio Astronomy Obeservatory)

Much of the modern understanding of the Milky Way and other galaxies comes from the work of Edwin Powell Hubble. (Coincidentally, Reber graduated from the same high school as did Hubble in Wheaton, Illinois, and Hubble’s seventh and eighth grade teacher was Reber’s mother, Harriet Grote Grote, Harriet , who interested her son in astronomy by giving him a book by the famous astronomer.) Hubble used the Mount Wilson telescope to discover variable stars in the Andromeda nebula (a fuzzy patch of light in the night sky) that made it possible to calculate its distance and size. In 1924, he showed that the “nebula” was actually an independent galaxy far outside the Milky Way. By 1929, he had identified several galaxies and measured their velocities based on the shift in their spectral lines, most of which were toward the red end of the spectrum. These measurements indicated that the galaxies were traveling away from the Milky Way and that their recession speeds were proportional to the galaxies’ distances from the Milky Way. Hubble’s measurements thus led to the concept of an expanding universe.

In 1932, Karl G. Jansky reported his accidental discovery of radio waves from space. Using a rotating array of dipole antennae sensitive to 15-meter radio waves, he detected a steady hiss that appeared four minutes earlier each day. This disparity corresponded to the daily motion of the stars, so he concluded that he was receiving cosmic radio waves from beyond the solar system. Jansky was able to identify the source of the most intense radiation in the direction of Sagittarius, suggesting that it came from the center of the Milky Way galaxy. He also showed that weaker radio waves came from all directions in the Milky Way and suggested that their source was either the stars or the interstellar matter between the stars.

Jansky’s work was so unrelated to traditional astronomy that no professional astronomer followed up on it. As a young radio engineer at the Stewart-Warner Company in Chicago, however, Reber read Jansky’s papers and began to plan how he could measure the detailed distribution of the radiation intensity throughout the sky at different wavelengths. In 1937, he built a 9.5-meter parabolic reflecting dish in his side yard, mounted so that its elevation could be changed in a north-south direction (declination); scanning east and west (right ascension) would result from the earth’s rotation. He placed a dipole antenna at the focus and designed a receiver sensitive to the shortest wavelengths possible at the time. For ten years, he operated this radio telescope Telescopes;radio in Wheaton as the only active radio astronomer in the world.

After trying unsuccessfully to detect radiation at wavelengths of 9 centimeters and 33 centimeters, Reber finally changed his receiver to a new operating wavelength of 1.87 meters and began to get positive results by the spring of 1939. He did most of his work from midnight to dawn to avoid interference from automobile ignitions. As the Milky Way crossed the meridian late at night, Reber measured the increasing intensity of the cosmic radio waves by reading a microammeter at one-minute intervals. He published his initial results in the February, 1940, Proceedings of the Institute of Radio Engineers, where he noted an intensity too low to be caused by thermal emission, but suggested the possibility of “free-free” radiation from electrons interacting with positive ions (charged atoms) in interstellar gases.

In 1941, Reber began a complete sky survey with an automatic chart recorder and more sensitive receiving equipment. At 1.87 meters, his operating frequency was 160 megahertz (million vibrations per second), and his radio telescope had a beamwidth of about 12 Kelvins, making it possible to resolve some structure in the radio emissions from the Milky Way. The pen would slowly rise and fall as the reflecting dish rotated with the earth under the Milky Way. After collecting approximately two hundred chart recordings at increasing angles of elevation, he plotted the resulting radio contours as lines of constant intensity on the two hemispheres of the sky.

The resulting radio maps were published in the Astrophysical Journal in November, 1944. They revealed interesting details: The peak intensity was at the center of the galaxy in Sagittarius, with secondary maxima clearly evident in Cygnus and Cassiopeia. More important was his recognition that radio waves could penetrate the interstellar dust that obscures much visible light in the Milky Way.

Reber’s last observations in Wheaton were made from 1945 to 1947 at a wavelength of 62.5 centimeters and a frequency of 480 megahertz, giving a beamwidth of about 4 Kelvins, resulting in an improvement in resolution of nearly three times over his first radio maps. This second set of maps, published in the Proceedings of the Institute of Radio Engineers in October, 1948, revealed two noise peaks in the Cygnus region, later identified as a radio galaxy (Cygnus A) and a source associated with a spiral arm in the Milky Way (Cygnus X). An intensity peak in Taurus was later identified with a remnant of an eleventh century supernova in the Crab nebula, and another in Cassiopeia matched the position of a seventeenth century supernova. These results were the beginning of many important discoveries in the field of radio astronomy. Indeed, they were the beginning of the field itself.


Reber’s pioneering work and resulting radio maps led to growing interest in radio astronomy and many unexpected discoveries with radio telescopes of increasing sophistication and size. After Reber’s 1944 radio maps were published, the Dutch astronomer Jan Hendrik Oort Oort, Jan Hendrik asked a graduate student at the University of Leiden, Hendrik van de Hulst, to study the theory of interstellar radiation. In 1945, van de Hulst predicted that neutral hydrogen should emit 21-centimeter radio waves when its electron spin reverses in relation to its proton spin. By 1949, the Harvard physicist Edward Mills Purcell Purcell, Edward Mills began a search for such 21-centimeter radio waves with Harold Irving Ewen Ewen, Harold Irving , a graduate student who was sent to confer with Reber on techniques in radio astronomy. Ewen and Purcell developed special equipment and by 1951 succeeded in detecting the predicted 21-centimeter radio waves. Oort’s group then began a seven-year collaboration with Australian radio astronomers to map the spiral arms of the Milky Way galaxy, using the fact that 21-centimeter radiation could penetrate the interstellar dust that absorbed visible light.

In 1960, two radio sources were identified with what appeared to be stars, but each emitted much more radio energy than the Sun or any other known star. Four of these so-called quasars Quasars (quasi-stellar radio sources) had been discovered by 1963, when Maarten Schmidt Schmidt, Maarten at Mount Palomar Observatory recognized that their unusual spectral lines were caused by large redshifts that could be interpreted as rapid recession velocities and that they were thus likely to be extremely far away. At distances of billions of light years, these objects would have to be more than one hundred times brighter than entire galaxies to be observable from Earth. It was theorized that they constituted some kind of highly energetic stage in the early formation of a galaxy.

Another dramatic event in radio astronomy occurred in 1967, when Jocelyn Bell Bell, Jocelyn discovered pulsars Pulsars . She found rapidly recurring signals on recording charts from a huge array of 2,048 dipole antennae spread over four acres at the University of Cambridge. After she discussed this with the project leader Antony Hewish, they installed high-speed recorders and found sharp pulses at precise intervals of just over a second. More of these objects were soon found, some having even more rapid pulsations. They were believed to be fast-spinning neutron stars with high magnetic fields that produced a rotating beam of radio emission. A pulsar in the Crab nebula was later identified with the collapsed core of the supernova remnant that had appeared on Reber’s radio maps.

Perhaps the most important discovery in radio astronomy was the 1965 detection of microwave background radiation Cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson. Using a 6-meter horn antenna tuned to 7-centimeter waves at the Bell Telephone Laboratories in Holmdel, New Jersey, they found an unexpected excess of steady radiation with no directional variation, corresponding to about 3 Kelvins of thermal noise. This matched the predicted temperature of cosmic radiation from a primeval fireball in the “big bang” theory. Thus, radio astronomy provided evidence tending to confirm that theory of the creation and expansion of the universe. Radio astronomy;Milky Way galaxy
Milky Way galaxy

Further Reading

  • Burke, Bernard F., and Francis Graham-Smith. An Introduction to Radio Astronomy. 2d ed. New York: Cambridge University Press, 2002. Survey of the history, methodology, and discoveries of radio astronomy. Bibliographic references and index.
  • Hey, J. S. The Evolution of Radio Astronomy. New York: Science History, 1973. A good history of radio astronomy by one of the pioneers in its development. The first chapter describes the work of Jansky and Reber, followed by chapters on the growth of radio astronomy after World War II. A good discussion of radio mapping of the Milky Way, radio galaxies, quasars, and microwave background radiation is included with many good diagrams, photographs, and radio contour maps.
  • Lang, Kenneth R., and Owen Gingerich, eds. A Source Book in Astronomy and Astrophysics, 1900-1975. Cambridge, Mass.: Harvard University Press, 1979. Reproductions are given of many of the most important journal articles on radio astronomy, including the early papers of Jansky and Reber, and later papers on quasars, pulsars, and microwave background radiation. Introductory essays help explain the context and importance of these articles.
  • Spradley, Joseph L. “The First True Radio Telescope.” Sky and Telescope 76 (July, 1988): 28-30. An introductory article in a popular astronomy magazine on Reber’s background and work. Photographs of the first reflecting radio telescope and the original model used in its design are given, and Reber’s early results are discussed with reproductions of the radio contour maps he obtained.
  • Sullivan, W. T., III, ed. The Early Years of Radio Astronomy. New York: Cambridge University Press, 1984. A series of articles by the pioneers of radio astronomy on its early development. A 1958 article by Grote Reber entitled “Early Radio Astronomy at Wheaton, Illinois” includes about a dozen early photographs not in the original article. Other chapters describe the influence of radio astronomy on science and cosmology.
  • Verschuur, Gerrit L. The Invisible Universe Revealed. New York: Springer-Verlag, 1987. This book, subtitled “The Story of Radio Astronomy,” gives a good description of the results of radio astronomy with more than one hundred photographs and radio contour maps. Concluding chapters give a brief history of radio astronomy.
  • Zeilik, Michael, and John Gaustad. Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, 1990. A standard college textbook on astronomy with a good section on radio telescopes and results obtained with them. One of Reber’s radio contour maps is shown, and the meaning of such maps is discussed with good diagrams.

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