| interstellar medium (ISM) |
|
The diffuse material in the space within galaxies between individual stars, which are typically separated by several light years. In our own Galaxy, the mass of material in the interstellar medium is estimated to be at least one-tenth that in the stars. It is concentrated in the central region and in four spiral arms. In general, spiral galaxies have substantial amounts of interstellar material and elliptical galaxies little or none. There is a continuous interaction between stars and the interstellar medium, which is not uniform but consists of a number of diverse components: dark clouds of gas and dust, regions of ionized hydrogen and neutral hydrogen, molecular clouds, globules, a very hot dilute gas and high-energy cosmic ray particles. Interstellar clouds are the sites of star formation but are also enriched by material ejected by supernovae and other processes of mass loss from stars. Over distance scales of thousands of light years, the structure of the interstellar medium is probably dominated by the coalescing of supernova remnants. The thick shells surrounding them ultimately cool and condense into small clouds. Such clouds can interact, either coalescing or fragmenting on collision. See also: astration. Local Bubble. |