(Box) 2. Tools of the trade
Astronomers' knowledge of the interstellar medium (ISM) is almost entirely the result of observations with radio and infrared (IR) telescopes. These devices are the only means that astronomers have of probing the depths of space, and of determining the molecular species and chemical processes that might occur there.
Both radio and IR telescopes detect the narrow electromagnetic frequency bands produced by the rotational transitions of molecular bonds of species in the ISM. Different molecules with different chemical bonds produce characteristic frequency band spectra; by matching those spectra with examples from known molecular species studied in the laboratory, identification can often be achieved.
Radio telescopes can only detect transitions involving a rotating dipole moment and so they are restricted to studying gaseous molecules. IR telescopes, meanwhile, can detect the vibrations of virtually all the molecules in the ISM, both gaseous and solid, although they are mostly used for detecting solids.
Dense molecular clouds are usually studied using IR telescopes and most of the infrared spectral features are due to the absorption (as opposed to emission) of infrared energy at specific frequencies by the constituent molecules of the cloud. The infrared source can either be a combination of an embedded protostar and the dust heated by that star, or a background star behind the cloud.
IR astronomical techniques do have their limitations and the data generated are by no means unequivocal. For instance, it is difficult to distinguish between molecules with similar molecular bonds, and thus molecular identifications based on only a couple of observed spectral features are often uncertain. In addition, any spectra produced from the study of a molecular cloud are actually an amalgamation of all absorptions along the line of sight from the infrared source. It is therefore difficult to assess whether the constituents identified within the cloud are in separate, distinct groups or all mixed together; and it is also not always clear which spectral features are the result of the cloud and which are intrinsic to the source.