Some 20 to 30 percent of the mass of our galaxy is in the form of the interstellar medium (ISM), i.e., the material between the stars. The ISM consists primarily of gas, with atomic or molecular hydrogen and helium contributing approximately two-thirds and one-third of the total mass, respectively. The next most abundant atoms, oxygen, carbon, and nitrogen, collectively account for about 1 percent of the ISM’s mass. The remaining elements are present only in trace amounts. Approximately 1 percent of the mass of the ISM is in the form of micron-size dust particles. Astronomical observations, combined with studies of interstellar grains preserved in meteorites, suggest that the dust might consist variously of amorphous carbon, complex fullerenes, polycyclic aromatic hydrocarbons, diamond, silicon carbide, silicates, carbonates, and a host of other candidates, all with or without mantles of ices and/or organic compounds.1
Important components of the ISM are molecular clouds, which are dense, massive objects found throughout the Milky Way and in many external galaxies. In these molecular clouds—also known as dense clouds—the gas density is 103 to 106 particles/cm3, which is very high by interstellar standards, and their masses can be as large as a million times the mass of the Sun. They are also usually very cold objects, with temperatures typically in the range from 10 to 100 K. Because of their high masses, these objects are the sites of star and planet formation, and also where complex gas-phase chemistry occurs.
Astronomical observations of the ISM have revealed the presence of numerous organic compounds. More than 125 different chemical species have been identified in interstellar and circumstellar regions, some containing 10 or more carbon atoms (Table 2.1). Assuming that the carbon in the ISM is present in cosmic abundance, then only 0.04 percent (by number) of the material there is carbon, even though approximately 80 percent of the observed species in the ISM are organic, including almost all of the larger molecules, many of which are relatively complex.2 Organic compounds are, however, only a trace constituent of the ISM and account for less than 1 percent of its total mass. Inorganic compounds abound, with CO, for example, accounting for some 20 percent of the carbon in dense interstellar clouds. CO is, itself, outnumbered by the most common molecular species, H2, by a factor of approximately 10,000. The majority of the molecular species identified in the ISM have been discovered using high-resolution (1 part in 106 to 108) spectroscopic techniques of radio and millimeter astronomy. This