The matter between the stars-the interstellar medium-manifests itself in many interesting and unexpected ways, and, as the detritus of stars, its properties and behavior hold clues to the history and future evolution of both stars and the galaxies that contain them. Stars are accompanied by diffuse matter all through their lifetimes, from their birthplaces in dense molecular clouds, to the stellar winds they eject with varying ferocity as they evolve, to their final fates as they shed their outer layers, whether as planetary nebulae or dazzling supernovae. As these processes go on, they enrich the interstellar medium with the products of the stars' nuclear fusion. The existence of life on Earth is eloquent evidence of this chemical enrichment. In this course we will study the interstellar medium in its various forms. We will discuss many of the physical mechanisms that produce the radiation we observe from diffuse matter, including radiative ionization and recombination, collisional excitation of "forbidden" lines, collisional ionization, and synchrotron radiation. This course will be both writing- and observing-intensive. Weekly short papers will be assigned; these papers will be thoroughly edited by the professor and returned so that comments can be absorbed for future submissions. In class, students will present key ideas from the assigned reading and will solve relevant problems. Throughout the semester students will also work in small groups to design, carry out, analyze, and critique their own observations of the interstellar medium using the equipment on our observing deck. Format: seminar and discussion, 3 hours per week, plus computer work and observing. Evaluation will be based on weekly papers, class presentations/problem-solving, and observing projects. Prerequisite: Physics 201. Enrollment limit: 10. This course is writing intensive. (This course is part of the Critical Reasoning and Analytical Skills Initiative. )