The Galactic Interstellar Medium
Galaxies are formed mainly of stars, but these stars are immersed in a relatively diffuse and cold gaseous medium. Its density is on average of the order of 1 particle per cm3, 10 cm−3 in clouds of atomic hydrogen, and 1000 cm−3 in molecular clouds. Its temperature goes from 5 K in these latter regions up to 104 K in ionized regions heated by young stars. Hydrogen forms the bulk of the interstellar gas, helium around 25%. Other elements are present in trace amounts. The interstellar gas is enriched by heavy elements ejected by stars (in supernova explosions and stellar winds).
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