Passage: The space between the stars in M87 is filled with a diffuse interstellar medium of gas that has been chemically enriched by the elements ejected from stars as they passed beyond their main sequence lifetime. Carbon and nitrogen are continuously supplied by stars of intermediate mass as they pass through the asymptotic giant branch. The heavier elements from oxygen to iron are produced largely by supernova explosions within the galaxy. Of the heavy elements, about 60% were produced by core-collapse supernovae, while the remainder came from type Ia supernovae. The distribution of oxygen is roughly uniform throughout, at about half of the solar value (i.e., oxygen abundance in the Sun), while iron distribution peaks near the center where it approaches the solar iron value. Since oxygen is produced mainly by core-collapse supernovae, which occur during the early stages of galaxies and mostly in outer star-forming regions, the distribution of these elements suggests an early enrichment of the interstellar medium from core-collapse supernovae and a continuous contribution from Type Ia supernovae throughout the history of M87. The contribution of elements from these sources was much lower than in the Milky Way.

Question: What type of medium takes up the space between the stars of M87?
gas