Ryle’s Radio Telescope Locates the First Known Radio Galaxy

Martin Ryle’s interferometric radio telescope detected and provided details on the structure of the first identifiable radio galaxy, located in the constellation Cygnus.

Summary of Event

The initial measurements of cosmic radio emission made by Karl G. Jansky Jansky, Karl G. and later Grote Reber Reber, Grote between 1932 and 1940 showed reasonably close relations between the gross contours of patches of radio intensity and the positions and structures of galaxies observable by optical telescopes. This led many astronomers to conclude that most, if not all, celestial radio emissions derived from continuously distributed sources, such as interstellar gas. Up to the immediate post-World War II period, probably the greatest weakness of the new subdiscipline of radio astronomy was the very limited accuracy it provided in determining the absolute celestial position and structural details of detected radio sources. High location accuracy was necessary to reduce sufficiently the margin of error in designating a portion of the sky that optical astronomers could search for the visual counterparts (if any) of a given radio source. This portion of the sky to be searched was known as an “error box.” [kw]Ryle’s Radio Telescope Locates the First Known Radio Galaxy (1948-1951)[Ryles Radio Telescope Locates the First Known Radio Galaxy]
[kw]Radio Telescope Locates the First Known Radio Galaxy, Ryle’s (1948-1951)
[kw]Telescope Locates the First Known Radio Galaxy, Ryle’s Radio (1948-1951)
[kw]Galaxy, Ryle’s Radio Telescope Locates the First Known Radio (1948-1951)[Galaxy, Ryles Radio Telescope Locates the First Known Radio]
Radio astronomy;Cygnus A
Cygnus A
Telescopes;radio interferometers
Radio astronomy;Cygnus A
Cygnus A
Telescopes;radio interferometers
[g]Europe;1948-1951: Ryle’s Radio Telescope Locates the First Known Radio Galaxy[02330]
[g]United Kingdom;1948-1951: Ryle’s Radio Telescope Locates the First Known Radio Galaxy[02330]
[c]Astronomy;1948-1951: Ryle’s Radio Telescope Locates the First Known Radio Galaxy[02330]
[c]Science and technology;1948-1951: Ryle’s Radio Telescope Locates the First Known Radio Galaxy[02330]
Ryle, Martin
Smith, E. Graham
Baade, Walter
Minkowski, Rudolf

Immediately following World War II, J. S. Hey Hey, J. S. used receivers from the Army Operational Radar Unit to perform initial experiments on some of the extraterrestrial radio emissions reported earlier by Jansky and Reber. Subsequently, in 1946, Hey and others published the first paper on apparent fluctuations in received radio noise from the direction of the constellation Cygnus. Hey and his colleagues reported an observational discovery of particular import: that the radio source in Cygnus varied significantly in strength over very short time periods. In contrast to Reber, who had concluded that interstellar hydrogen between the stars was the source of all celestial radio signals, Hey argued that the spatial localization and temporal periodicity of emissions strongly suggested a localized or starlike object.

In Australia, a similar post-World War II radio astronomy group was formed under J. L. Pawsey Pawsey, J. L. . In 1946, the Australian group verified Hey’s observations of a localized radio source in Cygnus, using one of the earliest radio interferometers of the “Lloyd’s mirror” Lloyd’s mirror[Lloyds mirror] type. An interferometer is essentially a multielement receiving array relying on receiver spacings matching the constructive interference wavelengths of the emitting source. Further “radio stars” were discovered by Pawsey’s group in June, 1947, using an improved Lloyd’s mirror, or image-method, interferometer developed by L. McCready McCready, L. , Pawsey, and R. Payne-Scott Payne-Scott, R. . This interferometer included a multisensor aerial mounted atop a high cliff, overlooking the ocean, which specifically made use of the Lloyd’s mirror effect of optics and acoustics. The Lloyd’s mirror, or image interference, effect comprises the constructive and destructive interference patterns resulting from interference between direct and multiply reflected radio waves reflected at the ocean’s surface.

Almost simultaneously, using a different type of interferometer, Martin Ryle at Cambridge found another intense localized radio source in the constellation Cassiopeia. Ryle, together with J. Ratcliffe Ratcliffe, J. , J. Finlay Finlay, J. , and others, brought extensive wartime experience in developing airborne radar detectors, radar countermeasures, underwater sonar arrays, and signal detection and localization equipment. They were joined in 1946 by E. Graham Smith. Ryle’s new interferometer was a multielement array whose elements could be moved horizontally to different separation distances. The receivers included two groups of four yagi-type antennae separated by five hundred meters or more and operating at a receive frequency of 80 megahertz. A yagi antenna includes a dipole connected by a transmission line to a number of equispaced but unconnected dipoles mounted plane-parallel to the first. Ryle’s cosmic radio “pyrometer” was used successfully in July, 1946, to resolve the angular diameters of a large sunspot.

Hey and his colleagues remained unable to determine the accurate position of their radio source to better than 2 degrees because of resolution limitations of their radio telescope. J. G. Bolton Bolton, J. G. and G. J. Stanley Stanley, G. J. in 1948 also used the Lloyd’s mirror technique to show that the Cygnus radio source was a discrete, not a distributed, source. The successful resolution of solar sunspots of small diameter suggested to Ryle additional radio telescope improvements to improve upon the Hey, Bolton, and Stanley measurements for Cygnus, as well as his own object in Cassiopeia.

In 1948, Ryle, Smith, and others made the first detailed radio observations of Cygnus A, using an improved version of their pyrometric radio telescope. Ryle and Smith subsequently published an improved position for Cygnus A in 1948 and showed that, unlike the sun, the Cygnus A and Cassiopeia sources were unpolarized. Ryle and Smith’s measurements included the discovery of short-period radio bursts (of less than 20 seconds duration), which they (incorrectly) used to argue that the ultimate radio source must be a radiating star of some as yet unrecognized type.

Eventually, it was decided that the most direct way to determine more about these radio sources would be to measure their angular size radiometrically with the greatest resolution possible: If the objects were, in fact, stars, their expected angular size would be fractions of a second of arc. An angular size in the minutes of arc would indicate that they were galaxies. Earlier efforts to improve position estimates for radio stars largely depended on observing a very small part of the object’s celestial track after it had risen above the horizon. Almost all early measurements were therefore affected severely by near-horizon atmospheric refraction not amenable to easy correction.

In 1949, it was proposed to carry out these measurements for Cygnus A and Cassiopeia by constructing a very large variable baseline interferometric radio telescope, with a maximum possible baseline separation of 160 kilometers receiving on 124 megahertz, the so-called long Michelson interferometer of 1950. Because of the critical importance of relative phase in determining details of the object’s spatial distribution of radiated energy, in 1951, Ryle developed a new “phase-switching” receiver based on 1944 sonar detection efforts. Phase switching permits the radio receiver to discriminate accurately and reject sources with large angular size and, thus, better emphasize and locate weaker sources using larger receiver gains.

After Ryle completed the phase-switching prototypes and tested them again on the Sun, in 1951 R. C. Jennison Jennison, R. C. and M. K. Das Gupta Gupta, M. K. Das attempted to take measurements for Cygnus and Cassiopeia. Because both sources were resolved clearly with baselines of only a few thousand meters, the radio objects were clearly not stars. These same researchers shortly thereafter discovered that the Cygnus A source was actually two distinct sources. Ryle’s phase-switched records were such improvements that his colleagues compiled the first radio-object catalog containing more than fifty sources.

Smith recollected later that even the confirmation of the extragalactic nature of the Cygnus radio source had little immediate effect in changing the radio “star” concept. Unable to measure a parallax for these objects, by late 1951, Smith had further localized the coordinates of Ryle’s two radio stars to better than one minute of arc in right ascension and 101.6 centimeters in declination, reducing the original error boxes of Hey and others by a factor of sixty. Smith then approached the director of the Cambridge Observatory to seek visual identification of the two radio sources.

While D. Dewiest Dewiest, D. in 1951 did, in fact, find part of the Cassiopeia supernova remnant on a photographic plate made by a 101.6 centimeter reflector telescope, the poor atmospheric observing conditions in England precluded definitive identification. Shortly thereafter, Smith sent his data and his new positions to Walter Baade and Rudolf Minkowski of the Palomar Observatory in Southern California. Baade and Minkowski’s visual objects were discovered only after many difficulties, being in a field rich in many other stars and faint galaxies. In a letter of April 29, 1952, Baade wrote to Smith that the result of the photograph was puzzling. He found a rich cluster of galaxies, and the radio position coincided closely with one of the brightest members of the cluster. In his letter dated May 26, 1952, Baade wrote to Smith that Minkowski had obtained the spectrum of the nebula with the new spectrograph. He believed they had encountered an extragalactic object because of the large redshift of the emission lines.


At first, there was notable skepticism in the optical astronomy and cosmology communities over the notion of extragalactic radio sources. This climate of disbelief was responsible for the fact that Baade and Minkowski’s results could not be published until 1954. Subsequently, it required additional confirmation for these and the other purportedly extragalactic objects, to foster the leap of the imagination necessary to place these sources in a cosmological category. Perhaps the most decisive radio data came from Ryle and P. A. G. Scheuer Scheuer, P. A. G. in 1955. From their 1C and 2C surveys, they found that the absolute radio luminosity of “normal” spiral galaxies is comparable to that radiated by the Milky Way galaxy. Plotting optical versus radio emission intensity for 1C-2C radio sources showed that most of the normal galaxies were grouped in a specific region, where optical equals radio energy. Nevertheless, there were many other galaxies—many not different in their optical appearance from normal galaxies—which were much more powerful sources (termed radio galaxies).

By the 1958 Solvay Conference on the Structure and Evolution of the Universe, the existence of at least eighteen then-confirmed extragalactic radio objects was accepted as opening a new era in cosmology. The experiment to look for parallactic motion of assumed radio stars had become the identification of the first extragalactic radio sources. Radio astronomy;Cygnus A
Cygnus A
Telescopes;radio interferometers

Further Reading

  • Baade, W., and R. Minkowski. “Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A.” Astrophysical Journal 119 (1954): 206-2 14. The final refinement of the radio position and the establishment of the optical galaxy group are discussed.
  • Bolton, J. G., G. J. Stanley, and O. B. Slee. “Positions of Three Discrete Sources of Galactic Radio-Frequency Radiation.” Nature 164 (1951): 101. The first confirmation of the discrete nature of the Cygnus radio object is discussed.
  • 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. Includes a chapter on radio galaxies and quasars. Bibliographic references and index.
  • Pawsey, J. L., and R. N. Bracewell. Radio Astronomy. Oxford, England: Clarendon Press, 1955. Contains a good introduction to key concepts and mathematics of aperture synthesis in astronomy.
  • Ryle, M., and F. G. Smith. “A New Intense Source of Radio Frequency Radiation in the Constellation Cassiopeia.” Nature 162 (1948): 462. The original publication documenting the discovery of intense radio emission.
  • Sullivan, Woodruff T., ed. Classics in Radio Astronomy. Boston: D. Reidel, 1982. Contains reprints of many key journal articles on radio telescope improvements and applications.
  • _______. The Early Years of Radio Astronomy: Reflections Fifty Years After Jansky’s Discovery. New York: Cambridge University Press, 1984. A comprehensive collection of the early history of radio astronomy. Contains the Smith-Baade correspondence on the discovery of the optical counterpart to Cygnus A radio source.
  • Verschuur, Gerrit. The Invisible Universe: The Story of Radio Astronomy. New York: Springer-Verlag, 1974. Although an excellent source for contemporary material on radio astronomy, its historical recounting is limited and biased.

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