hydrogen line(redirected from 21-cm radiation)
hydrogen line(21 cm line) See H I region.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.