hydrogen line(redirected from 21 cm line)
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hydrogen line(21 cm line) See H I region.
hydrogen line[′hī·drə·jən ‚līn]
A spectral line emitted by neutral hydrogen having a frequency of 1420 megahertz and a wavelength of 21 centimeters; radiation from this line is used in radio astronomy to study the amount and velocity of hydrogen in the Galaxy.