A spiral galaxy like the Milky Way contains stars, the remains of stars (stellar remnants) and a diffuse interstellar medium (ISM) of gas and dust. The interstellar medium consists of 10 to 10 particles per cm usually of about 70% hydrogen by mass, with most of the remaining gas consisting of helium. This medium is chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. 