Studying Neutral Hydrogen Shells in the Interstellar Medium Elizabeth M. Tennyson, Dr. Shauna Sallmen Department of Physics, University of Wisconsin - La Crosse, La Crosse, WI 54601 Dr. Eric J. Korpela Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA, 94720 ABSTRACT One of the many processes that occur in our Milky Way Galaxy is the in- teraction of Neutral Hydrogen (H I) shells with the Interstellar Medium (ISM). H I shells are formed when material from stellar winds and supernova explosions expands outward and mixes with the ISM. Without this phenomenon the heavy elements made in stars would not be recycled and reused for future star and planet systems. The goal of this project is to better understand how interstellar shells evolve. To acquire a better grasp of a shell’s characteristics and develop- ment, we compared images for 55 shells that were discovered in the Search for Extraterrestrial Neutral Hydrogen (SETHi) radio database. The SETHi images show cold, neutral gas, the H-Alpha Composite Survey images show warm, ion- ized gas, the IRIS (infrared) images show warm dust, and the RASS (X-ray) images reveal hot gas. The initial comparison identified nine potentially interest- ing shells. Of these, four are most suitable for detailed follow-up study, based on information available from their kinematic distances. We present a catalog of the shells we classified, including details of those that are likely targets. Follow-up detailed multi-wavelength comparisons can give us estimates on a shells’s age and evolutionary status. Subject headings: ISM: general — ISM: bubbles, ISM: supernova remnants 1. Introduction Neutral Hydrogen (H I) shells are found throughout the Interstellar Medium (ISM) of our Milky Way Galaxy. These interstellar shells are formed by supernova explosions and stellar winds, which eject hot, ionized material. Over time, the ionized gases cool and become neutral gases. The materials that are dispersed from these events mix with the