Content of the region between the stars, including vast, diffuse clouds of gases and minute solid particles.
Such tenuous matter in the Milky Way Galaxy accounts for about 5% of its total mass. By no means a complete vacuum, the interstellar medium contains mainly hydrogen gas, with a smaller amount of helium and sizable quantities of dust particles of uncertain composition. Primary cosmic ray s also travel through interstellar space, and magnetic fields extend across much of it. Most interstellar matter occurs in cloudlike concentrations, which can condense to form star s. Stars, in turn, continually lose mass through stellar winds (see solar wind ). Supernova s and planetary nebula e also feed mass back to the interstellar medium, where it mixes with matter that has not yet formed stars (see Populations I and II ).