H I region
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H I regionA diffuse region of neutral, predominantly monatomic, hydrogen in interstellar space. A typical H I region is about 5 parsecs across, contains 50 solar masses of gas, and is at a temperature of 70 K. Between these clouds is more tenuous neutral hydrogen gas at a temperature of about 8000 K. Although the temperature is too low for optical emission, neutral hydrogen emits radio radiation at the spot frequency of 1.420 405 751 786 gigahertz, which corresponds to a wavelength of about 21 cm. This emission, termed the hydrogen line or 21 cm line emission , is associated with a forbidden transition between two closely spaced energy levels of the ground state, related to the relative electron and proton spin orientation in the hydrogen atom. Predicted in 1944 by van der Hulst, it was first detected in 1951.
This radio emission has allowed the distribution and relative velocity of neutral hydrogen to be studied both in our own and in nearby spiral galaxies, using line receivers. Motions within the galaxy cause the observed frequency (1.4204 GHz) to be displaced by the Doppler effect. The velocity is determined from the frequency shift. See also H II region; interstellar medium.