The Growth of Radio Astronomy ( Major Trends In The History of Astronomy)

In the midst of the early turmoil over the distance scale and the internal structure of stars, a few vague attempts were made to examine a whole new aspect of astronomy. Until this time, astronomers had been observing only in the optical part of the spectrum, that part ranging from about 300 to 1000 nm. Then, suddenly in 1931-2. an accidental discovery was made: Karl G.Jansky of the Bell Telephone Laboratories, while in the process of studying high-frequency radio disturbances found a strange source of interference. At first he linked it with the Sun, but on further examination he found that it came from a fixed direction in space, somewhere near the constellation Sagittarius.

Although Jansky discovered cosmic radio waves he did not follow up the findings. The first radio telescope was built during the middle 1930s by Grote Reber, an American radio engineer and amateur astronomer. Reber was impressed by Jansky’s work and built a parabolic radio antenna to examine more closely the peculiar cosmic’ radio noise. Interaction with astronomers began when he took his results to the Yerkes Observatory to discuss the implications of this noise from the Milky Way. His research paper in the ‘Astrophysical Journal’ in 1940 stirred more interest, With his measures of intensity he produced the first radio map which indicated strong sources in Sagittarius, Cassiopeia and Cygnus.

Development of radar techniques resulted in great advances in receiver technology. With the possibility of more precise radio-astronomical instruments the stage of data collection began. In addition to wartime technical developments, the discovery of a radio source, the Sun, occurred on the British early-warning radar system in February, 1942, although it was only publicly announced after the war. J.S.Hey, an English radio astronomer, concluded that an unusually intense outburst coincided with the appearance of a large solar flare, and he thus found a connection between radio emission and solar activity. Soon, systematic observations began in Britain and Australia, and later other countries, to examine more closely the radio Sun and the cosmic noise. The construction of bigger, better and more expensive instruments began, partly stimulated by the discovery of the first discrete source, Cygnus A, by the English astronomers J.S.Hey, S.J.Parsons and J.W.Phillips. More accurate telescopes were needed to pinpoint the location of such radio objects to enable them to be linked with optically-visible objects.

Practical astronomers in England and the Netherlands turned eagerly to radio astronomy since, unlike optical astronomy, it is not interfered with by clouds and fog. In 1947, L.S.McCready, J.L.Pawsey and R.Payne-Scott designed a radio interferometer for achieving moderate resolving power. A variety of designs were completed in the next few years: two separate arrays at Cambridge. two 27 -m paraboloid dishes in the Owens Valley built by the California Institute of Technology, and the Mills Cross near Sydney, Australia. Most of the designs favoured interferometers since they were easier to build and less expensive than the single great dishes. while providing the necessary resolution. However, a large steer-able paraboloid reflector has the advantage of great versatility and a number of these were also constructed. The largest was the 75-m dish at Jodrell Bank, England, completed in 1957 under the direction of Sir Bernard Lovell. The results obtained by these great instruments quickly influenced astronomical research. Using interferometric techniques E.G.Smith, then working at Cambridge, and B.Y.Mills, in Australia, were able to establish the positions of the strongest radio sources accurately enough for W.Baade and R.Minkowski to use the Hale Observatory 5-m telescope to make optical identifications in 1952-4. About the same time, H.I.Ewen and E.M.Purcell of Harvard detected the first radio line, the 21-centimetre line of neutral hydrogen. Independent discoveries in the Netherlands and Australia were announced simultaneously. The line had been predicted by the Dutchman H.C. van de Hulst in 1944. It provided a new and impressive way of mapping the structure of the Galaxy, a technique unencumbered by the dust and gas that interferes much with optical studies. In 1953-4, the Dutch radio scientists succeeded in mapping the outer spiral structure of the Galaxy.

In the decade spanning the late 1950s and early 1960s, the great catalogues of radio sources, among them the famous Third Cam¬bridge (3C) catalogue, were produced. By the mid-1960s, electronic developments and increased funding for big science placed radio astronomers in the position where they could compete on equal terms with optical astronomers. With the opening of this new window on the heavens, previously invisible objects could be studied. The discoveries of radio galaxies, quasars, pulsars, the solar wind. and interstellar molecules influenced profoundly the future direction of astronomical research. Theoretical developments in the nature and origin of the radio emission, and clues to the origin of the Universe from the extensive counts of radio sources developed alongside. Objects requiring a whole new look at the physics of the energy mechanism revealed themselves for the first time.

Astronomy entered the age of big science from about 1955 on-wards. The most searching questions became dependent upon an observatory equipped with the latest developments technology could offer and with a budget to match.

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