Ultraviolet investigations of the interstellar medium from astrospheres to the local cavity

Abstract Ultraviolet spectroscopy plays a critical role in our understanding of the interstellar medium. This chapter describes how the analysis of resonance lines of abundant neutrals and ions leads to physical and morphological models of interstellar gas facilitated by innovative satellites and instruments. The neutral hydrogen Lyman-α line probes hydrogen walls in the heliosphere and astrospheres where stellar winds interact with inflowing neutral hydrogen from the local ISM. Doppler shifts and line widths of interstellar gas in the lines of sight to nearby stars identify warm neutral clouds within 15 pc of the Sun, their vector velocities, and column densities. The Sun is likely at the edge of one of these clouds, the Local Interstellar Cloud (LIC). Surrounding the warm clouds is the Local Cavity, which is proposed to be an irregularly shaped Stromgren sphere produced by the extreme-ultraviolet radiation from the star e CMa and nearby hot white dwarfs. The strengths and weaknesses of very different models of the ISM are discussed in terms of insights obtained from the analysis of UV interstellar spectra.