Observation of a Line in the Galactic Radio Spectrum: Radiation from Galactic Hydrogen at 1,420 Mc./sec

The hydrogen 21-cm line In September 1951 Ewen and Purcell reported in Nature of the discovery of the hydrogen 21-cm line. In fact, they had seen the signal months before, but waited for corroboration by Dutch and Australian astronomers before publishing: Muller and Oort’s paper [Muller, C. A. & Oort, J. H. Nature 168 , 357–358 (1951)] in the pages following Ewen and Purcell's report includes the text of a cable sent by Pawsey from Australia. Oort had already realized the significance of the discovery - that detection of this spectral line, produced by transitions between hyperfine levels of the ground-state hydrogen atom, would permit measurements of velocities by the Doppler effect. The 21-cm line put radioastronomy on the map, and brought about a revolution in the study of galactic structure. A Study of Recent Views of its Causes. By G. S. Carter. Pp. xv + 368. (London: Sidgwick and Jackson, Ltd., 1951.) 30 s . net.