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The Discovery of Cygnus A

January 31, 2001 ::

Grote Reber constructed his telescope in 1937, at his own expense, in his back yard in a Chicago suburb.
(Photo: NRAO)

Grote Reber spent long hours every night scanning the skies with his telescope.
(Photo: NRAO)

Cygnus A was an early target of the Chandra X-ray Observatory. It has been the focus of attention and controversy among astronomers since it was discovered more than fifty years ago. The history of scientists' attempts to come to grips with what is happening in Cygnus A shows the value of observing a source with different types of telescopes, and serves as a cautionary tale to all who would leap to conclusions on the basis of limited data. In 1946, many engineers and scientists in the United Kingdom and Australia used the talents and technology acquired during World War II to explore the sky with radio telescopes. They were especially interested in a region in the constellation Cygnus which had been identified as a strong source of radio waves by radio astronomy pioneer Grote Reber, who had used a $2,000 homemade radio telescope in his back yard in Wheaton, Illinois to make a radio map of the Milky Way.

Grote Reber's radio map for the two hemispheres of the sky. Cygnus A is in the region around +40 degrees declination on the map A.
(Credit: G. Reber, The Astrophysical Journal)

In 1946, British physicists Stanley Hey and his two colleagues S. J. Parsons and J. W. Phillips, used modified anti-aircraft radar antennae to study the Cygnus region in detail. They found a strong, rapidly fluctuating radio source that they named Cygnus A, and concluded: "it appears probable that such marked variations could only originate from a small number of discrete sources."

The idea is that, if the radiation from a source is observed to vary over a certain period of time, say 20 seconds, then the size of the source must be less than the distance a shock wave or other disturbance that causes the variability can travel in 20 seconds. Otherwise, the variation would be weak and undetectable. The fastest speed that a disturbance can travel is the speed of light -- 300,000 kilometers per second. The maximum size of a source that varies in 20 seconds is then 20 X 300,000 = 6 million kilometers -- about the size of a large star with a diameter 4 times that of the Sun. This led some astronomers, most notably Martin Ryle, to propose that Cygnus A and similar sources were a new type of star that shone at radio wavelengths but was invisible at optical wavelengths. He called these objects radio stars.

Martin Ryle was among those to propose that Cyg A and similar sources were a new type of star.
(Photo: University of Frankfurt)

By 1950 more detailed observations by Ryle and others in England and Australia showed that Cygnus A radiation was in fact quite steady; its apparent variability was due to the bending of radio waves by clouds of ionized gas in the Earth's ionosphere. A similar effect causes stars to twinkle.

Paradoxically, even though the radio star hypothesis was invented to explain the variations of Cygnus A and other sources, the discovery that these variations were due to the Earth's atmosphere and not Cygnus A did not cause the supporters of this hypothesis to abandon it. They pointed out, correctly, that stars twinkle and planets do not, because stars are point-like and planets are disk-like (so the twinkling is washed out for an Earth-bound observer). Therefore, since Cygnus A twinkles, it must be point-like, or least have a small angular size.

Thomas Gold argued against the radio star hypothesis.

At a conference in London in 1951, the radio star hypothesis was debated by some of the leading astronomers of the day. Martin Ryle and George McVittie defended the radio star hypothesis, while Thomas Gold and Fred Hoyle argued that the sources of the radio emission were beyond the Milky Way galaxy.

"Why . . . does not one find any identifiable visual object where those very near radio stars are supposed to be?" asked Gold.

Gold went on to point out that the fifty or so radio sources known at that time were not concentrated like the stars in our galaxy, but rather far-flung, like galaxies which are much more distant, and proposed that the sources were radio galaxies.

"It cannot be ruled out that other galaxies may behave quite differently from our own, for it is known that there are very different types."

"It does not seem to me that an extragalactic nebula (meaning a galaxy) can do the trick," responded McVittie, who pointed out that very few of the galaxies were known to be radio emitters, and that most of the "radio stars" could not be associated with a known galaxy.

"I think the theoreticians have miSunderstood the experimental data," Ryle added, emphasizing along with McVittie that "there is as yet no evidence to suggest that other extragalactic nebulae emit radio waves having a much greater intensity than our own galaxy." He went on to make the important point that the normal mechanisms for producing radio waves, such as radiation from a gas, would not work very well for galaxies.

Hoyle, in response to Ryle's slam at the theoreticians, responded that "the boot is really on the other foot, for Professor McVittie and Mr. Ryle have dogmatically asserted that the discrete sources cannot be of extragalactic origin, although . . . five have been found to correspond to nearby extragalactic nebulae."

Walter Baade focused the powerful 200 inch telescope on Palomar Mountain on Cyg A. What would he find?
(Photo: The Archives, California Institute of Tech.)

The whole argument was settled before the year was out. Francis Graham Smith used an improved radio telescope in Cambridge, England to get a much more accurate position of Cygnus A. Smith airmailed his results at once to Walter Baade at the California Institute of Technology in Pasadena. Within a few weeks, Baade was in the observing cage of the 200 inch telescope on Palomar Mountain. He focused the powerful telescope on the position given by Smith, and took two photographs, one in blue and one in yellow light. The next afternoon, he developed the photographs.

"I knew something was unusual the moment I examined the negatives," Baade recalled. "There were galaxies all over the plate, more than two hundred of them, and the brightest was at the center. It showed signs of tidal distortion, gravitational pull between the two nuclei -- I had never seen anything like it before. It was so much on my mind that while I was driving home for supper, I had to stop the car and think."

The above image, taken from the original article by Baade and Minkowski, shows a negative of Cygnus A taken at different wavelengths
(Credit: W. Baade & R. Minkowski, The Astrophysical Journal)

What Baade concluded was that he was seeing two galaxies in collision. This was especially exciting to Baade, because he and Lyman Spitzer, Jr. had published a paper earlier in the year in which they discussed some of the observable effects of such cosmic train wrecks. Baade's colleague Rudolph Minkowski, noted the coincidence.

"Baade and Spitzer invented the collision theory," Minkowski is reported to have said, "and now Baade finds evidence for it in Cygnus A."

Baade, angered by Minkowski's remark, challenged him to a bet of a thousand dollars that Cygnus A was a collision. They settled on a bottle of whiskey, and the proof: emission lines of high excitation. Such lines are produced by atoms and ions in a low density gas that has a temperature of ten thousand degrees or more. The spectra of stars, in contrast shows predominantly absorption lines due to relatively cool material in the atmosphere of the star. The basic idea is that a collision should produce a cloud of hot, low density gas that would produce emission lines in the spectrum of Cygnus A.

Rudolph Minkowski took up Baade 's bet for a bottle of whiskey.
(Photo: The Archives, California Institute of Tech.)

Within weeks Minkowski had taken the spectrum of Cygnus A with the Palomar telescope and conceded the bet, although the payoff was not exactly what Baade had anticipated.

"For me, a bottle is a quart," Baade said, "but what Minkowski brought was a hip flask . . . Two days later, it was a Monday, Minkowski visited me in order to show me something -- he saw the flask and emptied it."

The ironic part of this story is that, a dozen years later, most experts would agree that Minkowski might have been justified in drinking the whiskey. A series of startling new discoveries had shown that the spectrum of Cygnus A was not due to colliding galaxies!

Next Time: Cygnus A, Quasars, and Quandaries.

References:

Baade quotes from J. Pfeiffer 1956, The Changing universe (London: Victor Gollancz Ltd) and I. Robinson et al. 1965 Quasi-Stellar Sources and Gravitational Collapse (Chicago: U. Chicago Press)
Discussion of the radio observations of Cygnus A and the quotes from Gold, Hoyle, Ryle and McVittie are from a collection by K. Lang and O. Gingerich, 1979 A Source Book in Astronomy and Astrophysics, 1900-1975 (Cambridge: Harvard U. Press) p.774-785

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