Introduction

Proof of the existence of interstellar gas was first provided by observations of narrow absorption lines in the visible spectra of distant stars. In 1904, J. Hartmann identified lines of Ca II that did not share the periodic variations in Doppler shift exhibited by the principal stellar absorption lines in a spectroscopic binary star, and attributed these ‘stationary lines’ to foreground material outside the stellar system. Somewhat more than three decades later, the first interstellar molecules, CH, CH+, and CN, were discovered in similar ways. There is a similarly long history of related investigations in laboratory spectroscopy and in theoretical interpretation.

The interstellar absorption lines tend to be very narrow compared with the photospheric absorption features in the spectra of the hot stars used as background light sources. In terms of line broadening by thermal motions and frequent atomic collisions, this suggests low densities and low temperatures for the absorbing material. In most cases, the observed lines arise only in the lowest states of atoms and molecules, indicating also that the densities and temperatures are too low to maintain significant excited state populations. As we will see, it is possible to infer from such observations quite a lot of information about element abundances, temperatures, densities, cosmic-ray fluxes, and intensities of radiation inside particular clouds.

The term ‘diffuse interstellar cloud’ has no precise denotation and distinctions between different kinds of interstellar clouds – e.g., diffuse, dark, and giant molecular – are somewhat poorly defined.