The interstellar medium (ISM) is the material that exists between the stars which, in our own Galaxy, is concentrated strongly in the galactic disk and consists of about 99% gas and 1% dust grains. The gas, mostly hydrogen, is a complex soufflé of hot (10,000 to over 1 million K) and cool (3 to 100 K) components. A pressure balance is maintained, however, because cool regions have a higher density (up to 10¹⁰ particles per cm³ in some molecular clouds) while hot regions have a density well below the average density of the ISM of about 1 particle per cm³. The source of the diversity of the ISM is the continual release of matter and energy from stars in the form of stellar winds, planetary nebulae and supernovae. This creates thermal instabilities in the ISM resulting in clouds of various temperatures and composition. In elliptical galaxies the ISM consists almost entirely of hot gas (at roughly ten million K) that extends seamlessly into a gaseous halo surrounding the galaxy.

Small quantities of heavy elements in the ISM are crucial in the formation of planets and, ultimately, of life. These elements were formed in the cores of earlier generations of stars and have recycled through the ISM to become the raw material from which new stars and their retinues of worlds are formed.