Discovery
During the 1930s, it was noticed that there was a radio 'hiss' that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to be coming from the centre of the Galaxy. These discoveries were published in 1940 and were seen by Professor J.H. Oort who knew that significant advances could be made in astronomy if there were emission lines in the radio part of the spectrum. He referred this to Dr Hendrik van de Hulst who, in 1944, predicted that neutral hydrogen could produce radiation at a frequency of 1420.4058 MHz due to two closely spaced energy levels in the ground state of the hydrogen atom.
The 21 cm line (1420.4 MHz) was first detected in 1951 by Ewen and Purcell at Harvard University, and published after their data was corroborated by Dutch astronomers Muller and Oort, and by Christiansen and Hindman in Australia. After 1952 the first maps of the neutral hydrogen in the Galaxy were made and revealed, for the first time, the spiral structure of the Milky Way.
Read more about this topic: Hydrogen Line/history1
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