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.

Describing interstellar matter in our galaxy in all of its various forms, this book also considers the physical and chemical processes that are occurring within this matter. The first seven chapters present the various components making up the interstellar matter and detail the ways that we are able to study them. The following seven chapters are devoted to the physical, chemical and dynamical processes that control the behaviour of interstellar matter. These include the instabilities and cloud collapse processes that lead to the formation of stars. The last chapter summarizes the transformations that can occur between the different phases of the interstellar medium. Emphasizing methods over results, The Interstellar Medium is written for graduate students, for young astronomers, and also for any researchers who have developed an interest in the interstellar medium.

Two planetary nebulae (PNe) were studied in order to investigate small scale molecular structures in planetary nebulae, the survival of which affects the structure and composition of the interstellar medium (ISM) from which the next generation of stars is born. Molecular hydrogen (H[subscript 2]) was used a tracer for molecular emission and its excitation properties used as a probe of shock physics. Previous studies of several planetary nebulae have indicated a relationship between molecular hydrogen and cometary knots as a shielding mechanism allowing for continued survival and formation of the molecule. Our first PN studied, the Dumbbell nebula (NGC 6853), revealed not only H[subscript 2] bearing knots (similar to the Helix and Ring nebulae), but also regions of H[subscript 2] emission which were poorly sculpted or unstructured diffuse regions, unlike previously studied nebulae. The second PN, NGC 2392 has well documented cometary knots, however we find no detection of molecular hydrogen emission from the knots or otherwise making it an outlier among cometary knot bearing planetary nebulae. Both nebula studied in this research challenge current models of H[subscript 2] bearing small scale structures in PNe and indicate a need to expand our rather small sample of such objects so that we may begin to understand the processes behind H[subscript 2] formation, excitation, and its role in cometary knots of the PN as a whole.