Atlas of Astronomical Discoveries by Govert Schilling

Author:Govert Schilling
Language: eng
Format: epub, pdf
Publisher: Springer New York, New York, NY

After spending millions of years in the cold, dark Oort Cloud, cometary nuclei can experience orbital disturbances and fall towards the inner regions of the solar system, where they evaporate and disintegrate in a relatively short time, as happened to comet Schwassmann-Wachmann 3. (NASA/JPL)

1951 Hydrogen Hiss

Harold Ewen Discovers the 21-centimeter Line of Neutral Hydrogen

Harold Ewen thinks that he is competing with the Russians when, around 1950, he tries to find the line radiation of hydrogen. According to the Russian Iosif Shklovsky, the radiation should be relatively easy to detect, but it is not until he has discovered the 21-centimeter line that Ewen finds out that Dutch astronomers nearly beat him to it; if he had known that they were also searching for the line in the Netherlands, he would never have started looking.

The fact that neutral hydrogen emits radiation is deduced theoretically during the Second World War by astronomer Henk van de Hulst in Leiden, at the request of his mentor Jan Oort, who is one of the first to recognize the importance of the discoveries of Karl Jansky and Grote Reber and realizes that radio astronomy opens up a completely new window to the universe.

Twenty-five year old Van de Hulst – the son of teacher and author of children’s book W.G. van de Hulst – predicts that a hydrogen atom emits a small quantity of radiation at a wavelength of 21 centimeters when its electron starts spinning in the opposite direction. He presents his conclusions at a meeting of the Dutch Astronomers’ Club on April 15, 1944.

With the help of radio engineer Lex Muller, Oort starts looking for the faint line radiation of hydrogen. He uses a converted German Würzburg radar antenna with a diameter of 7.5 meters at Radio Kootwijk in the Veluwe national park. However, in the spring of 1950, the program is delayed by a fire in the receiver equipment.

At Harvard University, American Harold (‘Doc’) Ewen is also searching for the 21-centimeters line. As part of his Ph.D. research, he builds a horn antenna and a radio receiver but discovers in 1950 that the design needs to be radically modified. His supervisor, Edward Purcell, pays the $300 for the modifications out of his own pocket. Half a year later, on March 25, 1951, Ewen strikes lucky. The radiation of neutral hydrogen gas, predicted seven years previously by Van de Hulst, has finally been found.

Immediately after his discovery, Ewen hears that Van de Hulst is visiting Harvard College Observatory. Ewen informs him of his discovery personally, and also has a long telephone conversation with Oort during which he describes his method in detail. On May 11, the 21-centimeter radiation is also detected in Kootwijk. A few weeks later, the feat is repeated by Australian astronomers, and the three teams publish their results in the scientific weekly Nature on September 1.

Hydrogen is the most common element in the universe, and that makes the 21-centimeter line one of the most important aids for radio astronomers. By mapping the radiation of neutral hydrogen, they can study the structure and dynamics of the Milky Way and other galaxies.

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