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

Nature volume 168, page 356 (1951)Cite this article

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 detect-ability of the hyperfine transition hinges on the question whether 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. If the spin temperature is lower than the temperature of the radiation field, the hyperfine line ought to appear in absorption; if it is higher, one would expect a 'bright' line ; while if the temperatures are the same no line could be detected. The total intensity within the line, per unit band-width, should depend only on the difference between these temperatures, providing the source is thick enough to be opaque.

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References

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  1. Lyman Laboratory, Harvard University, Cambridge, Mass. June 14. https://www.nature.com/nature

    H. I. Ewen & E. M. Purcell

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  1. H. I. Ewen
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  2. E. M. Purcell
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Ewen, H.I., Purcell, E.M. Observation of a Line in the Galactic Radio Spectrum: Radiation from Galactic Hydrogen at 1,420 Mc./sec.. Nature 168, 356 (1951). https://doi.org/10.1038/168356a0

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