The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas that exists between the stars (or their immediate "circumstellar" environment) within a galaxy. This gas is usually extremely tenuous, with typical densities ranging from a few tens to a few hundredths of a particle per cubic centimeter. Generally the gas is roughly 90% hydrogen and 10% helium, with additional elements ("metals", in astronomical parlance) present in trace amounts.
The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvin), warm (thousands of kelvin), and cold (tens of kelvin). This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases is still a matter of considerable contention in scientific circles.