The existence of subparsec structure in the cold diffuse interstellar medium has been the subject of some debate. After a review of the literature, we will present the first survey of Na I absorption lines seen toward binary stars, and so demonstrate that small scale structure is ubiquitous in the interstellar medium. We will explain details of the observations, reduction of the data and the various analyses which were applied to the data. We explore several models of the ISM and discuss them in the light of the data. Some thought is given to the possibility that some of the observed structure might be attributable to circumstellar gas, rather than interstellar gas. We conclude that the gas is most likely interstellar in nature. Finally, we outline several areas of study which could further the understanding of small scale structure in the interstellar medium and the physical processes which govern it.

Publisher Description

A comprehensive yet accessible textbook introducing the nature of the rarefied matter that pervades the space between stars.