The utility of CO for observing distant molecular clouds lies in the fact that the obscuring interstellar dust is transparent to millimeter photons (see § 2.4 below). To trace the HI gas, it is fortunate that there is another detectable, long-wavelength transition, the hydrogen line at 21.1 cm. Since its discovery in 1951, the 21 cm line has been of paramount importance for revealing the interstellar medium in our own and external galaxies.
We can understand the origin of this radiation by recalling the quantum mechanics of the hydrogen atom. In the nonrelativistic model based on Schrodinger's equation, the energy of the system, consisting of a single electron of mass me and charge e bound to a proton of mass mp, is given by
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