In astronomy, the interstellar medium (or ISM) is the gas and dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.
The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The matter consists of about 99% gas and 1% dust by mass. Densities range from a few thousand to a few hundred million particles[clarification needed] per cubic meter with an average value in the Milky Way Galaxy of a million particles per cubic meter (1 atom per cubic centimeter). The Sun, for example, is presently traveling through the Local Interstellar Cloud (0.1 atoms/cc), within the Local Bubble (0.05 atoms/cc). As a result of primordial nucleosynthesis, gas in the ISM is roughly 89% hydrogen and 9% helium and 2% elements heavier than hydrogen or helium by number of protons, with additional heavier elements ("metals" in astronomical parlance) present in trace amounts.
The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.
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