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.
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