Observation of a Line in the Galactic Radio Spectrum

The ground-state of the hydrogen atom is a hyperfine doublet the splitting of which, determined by the method of atomic beams, is 1,420·405 Mc./sec.1. Transitions occur between the upper (F = 1) and lower (F = 0) components by magnetic dipole radiation or absorption. The possibility of detecting this transition in the spectrum of galactic radiation, first suggested by H. C. van de Hulst2, has remained one of the challenging problems of radio-astronomy. In interstellar regions not too near hot stars, hydrogen atoms are relatively abundant, there being, according to the usual estimate, about one atom per cm.3. Most of these atoms should be in the ground-state. The detectability of the hyperfine transition hinges on the question wether the temperature which characterizes the distribution of population over the hyperfine doublet—which for want of a better name we shall call the hydrogen ‘spin temperature’—is lower than, equal to, or greater than the temperature which characterizes the background radiation field in this part of the galactic radio spectrum.