Radio observations of molecules in the interstellar gas

Radio astronomers have succeeded since 1968 in identifying nearly 50 molecules in the dense concentrations of the interstellar gas now generally termed molecular clouds. Most interstellar molecules are stable compounds familiar to the terrestrial chemist, but nearly one-fifth are ions, radicals and acetylenic carbon chains so reactive in the laboratory that before being detected in Space they had rarely been observed or were entirely unknown. The heavy atom backbone of the known interstellar molecules is a linear chain of C, N, O or S (Si is found in two diatomic molecules) ; rings and branched chains are missing. The most readily observed spectral lines of most interstellar molecules are rotational transitions at millimetre wavelengths. These are generally excited by H 2 collisions, and depending on the H 2 number density, the levels can be either in rotational equilibrium, or far from it. Maser line emission from OH, H 2 0 , SiO and CH 3 OH - extremely intense, small sources typically much less than 1" in angular size, often polarized and sometimes time-dependent - are the most striking examples of nonequilibrium excitation. A number of rare isotopic species are observed in interstellar molecules, those ol C, N and O having been studied the most intensively. Isotopic ratios differing from those on Earth by two- or threefold apparently exist, and in all but one case can be attributed to stellar nucleosynthesis since the formation of the Solar System. Molecular clouds apparently constitute an appreciable fraction of the interstellar medium by mass and are the largest reservoir of matter in Nature subject to the chemical bond. They are of great astronomical interest because of their central role in star formation and galactic structure: it is possible that all stars form in molecular clouds, and as molecular clouds are largely restricted to the spiral arms, they provide a new and highly specific tracer of the large-scale structure of the galactic system.