The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas and dust that exists between the stars (or their immediate "circumstellar" environment) within a galaxy. The matter normally consists of about 99% gas particles and usually 1% of dust. It fills interstellar space.

This compound is usually extremely tenuous, with typical densities ranging from a few single to a few hundreds of a particles 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 medium is also responsible for cosmic extinction, namely the decreasing light intensity of a star as the light travels through the medium. This extinction is caused by refraction and absorption of photons in certain wavelengths.

For example, a typical absorption wavelength of atomic hydrogen lies at ca. 121.5 nm, the Lyman-alpha transition. Therefore, it is nearly impossible to see light emitted at that wavelength from a star, because most of it is absorbed during the trip to Earth by Lyman-alpha absorption.

The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvins), warm (thousands of kelvins), and cold (tens of kelvins). 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.

Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.

See also

• List of molecules in interstellar space
• Outer space
• Intergalactic space
• Timeline of knowledge about the interstellar and intergalactic medium
• Solar space /Solar system/Stellar system
• Intergalactic medium
• Heliosphere