Abstract
Approximately 5 to 10% of the mass of the Milky Way is in the form of interstellar atomic hydrogen. The study of the physical properties of this matter is possible only because the ground state (12 S 1/2) can undergo a hyperfine transition, giving rise to radio emission (or absorption) at a wavelength of 21.1 cm. The transition occurs when the electron reverses its spin relative to that of the proton, the higher energy state being the one in which the two spins are parallel (the magnetic moments are parallel) and the lower state the one when they are anti-parallel. The prediction that this should be an observable transition was made by van de Hulst in 1944 well before receivers existed to make the measurements, and in 1951 two groups reported the detection of emission from interstellar neutral hydrogen (HI) in the September 1 issue of Nature (Ewen and Purcell, 1951 ; and Muller and Oort, 1951).
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Verschuur, G.L. (1974). Interstellar Neutral Hydrogen and its Small-Scale Structure. In: Verschuur, G.L., Kellermann, K.I. (eds) Galactic and Extra-Galactic Radio Astronomy. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-96178-6_2
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DOI: https://doi.org/10.1007/978-3-642-96178-6_2
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