Studies of interstellar matter on scales from 10AU to 10 kpc

This thesis presents four optical spectroscopic studies of absorption by matter in the diffuse interstellar medium on scales ranging from 10 AU to 10 kpc. The observations investigate two current problems in interstellar medium (ISM) research: small-scale structure (SSS), and the diffuse interstellar bands (DIBs). Very high spectral resolution observations of interstellar Na I, Ca I, Ca II, K I and CH absorption towards kappa Vel are presented. Combined with observations over the last 15 years taken from the literature, the small-scale structure in front of this star is probed on scales of ~ 10 AU. The high resolution and signal-to-noise of the new observations allow detailed modelling of the absorption profiles and the identification of multiple absorption components. For the two narrowest components, the line profile models are used to constrain the temperature, depletion, electron density and total number density within the structures. Diffuse interstellar bands are used as probes of SSS in long-slit observations of lines-of-sight towards three binary/multiple star systems: HD 168075/6, HD 176269/70 and four members of the mu Sgr system, one of which is identified as a member for the first time. The results show clear variations in DIB absorption in the HD 168075/6 and mu Sgr systems over scales of ~ 50,000-200,000 AU, and demonstrate the efficacy of medium-resolution observations of DIBs for identification of small-scale structure in the ISM. Multi-object spectroscopy of 452 stars in the omega Cen globular cluster is also presented, which probes interstellar absorption by Na I, Ca II and DIBs in two dimensions, on scales of ~ 1 pc. The first detections of diffuse interstellar bands in the M33 galaxy are reported. Multi-object spectroscopy of 43 stars is used to derive spectral types and reddenings, and measure DIB absorption across the disk of the galaxy (~ 10 kpc). Very strong DIB absorption per unit E(B-V) is found for one star in the sample, towards which a total of seven DIBs are detected