Most of what is known about the distribution of cold gas in the Galaxy, including the mapping of the nearby spiral arms, has come from detailed studies of the variation of 21-cm emission across the sky. The existence of the 21-cm line coming from cold clouds atomic hydrogen was predicted in 1944 by Hendrik van de Hulst. In 1959, the famous Morrison-Cocconi conjecture, concerning the possibility of detecting artificial signals at this wavelength, heralded the birth of SETI in its modern form.
Related category• SPECTRA AND SPECTROSCOPY
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