Interstellar Atoms and MoleculesA very important feature of the galactic radio emission is the strong radiation on specific
wavelengths, or so-called spectral lines, that are emitted by atoms and molecules in the
interstellar gas. The most prominent of these is the
radiation of neutral atomic hydrogen, the most abundant element in the universe, at the wavelength of 21 cm. The atoms in this case and others radiate at a specific frequency but may arrive at Earth at a different frequency due to the Doppler effect associated with the motion of the atoms toward or away from the Earth. By studying this Doppler shift in the frequencies of the observed spectral lines, the astronomer can deduce the temperatures and motions of the gas clouds that the radiation comes from. In this way the velocities and arrangements of gas clouds in our galaxy and others have been determined.In addition to atomic hydrogen, spectral lines have been discovered, surprisingly, from more than 40 different molecules. Those found so far are primarily organic molecules. The largest contain as many as 13 atoms and have molecular weights of more than 100. Carbon monoxide is found to be very abundant in space, as is formaldehyde. The molecules that are most effective in producing molecules important to biology in laboratory experiments are also abundant in space. This discovery suggests that interstellar chemistry perhaps played a role in the development of life on Earth and elsewhere, or else that the process was mimicked here. Extraterrestrial CivilizationsAt several observatories in the United States, Canada, and the Soviet Union, astronomers are searching for radio signals from other civilizations. Much more elaborate coverage over long time periods is called for, however, before such searches could in any way be considered statistically meaningful (see life, extraterrestrial).