Donald G. York

There is much we do not understand about the interstellar medium, but progress is made in most areas (structure, cloud sizes, role of magnetic fields, properties of dust) on the timescale of decades. I will start with a brief summary of our picture of the interstellar medium in the Milky Way. However, the is one 90 year old mystery that refuses to yield to solution: the set of 600 absorption features known as the diffuse interstellar bands (DIBs, for short). They are thought to arise in molecules, but the sizes are very uncertain (five or 100 atoms?). The profiles are consistent in the 2 kpc volume over which they have been observed. One star, Herschel 36, shows and interstellar spectrum which is dramatic and peculiar. It probes a very small, warm, dense region sitting in a greatly enhanced radiation field from UV and IR radiation. The search is on for other regions with the same peculiar features, in hopes of better defining the quantum mechanical properties of the carriers of the DIBs.