Glossary term: Interstellar Medium

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Description: The interstellar medium (ISM) is a term to describe all of the gas and dust that lies between star systems in a galaxy. Our Solar System lies in the disk of the Milky Way where most of the ISM is atomic hydrogen mixed with atomic helium and dust. The interstellar medium has a very low density compared to planetary atmospheres. This density varies across the galaxy and is highest in the spiral arms where there are many clouds of gas and dust. A small volume of the ISM is in the form of denser, cold clouds of molecular hydrogen. Some of these molecular clouds will collapse under their own gravity, leading to the formation of new stars. Stars put gas and dust back in to the ISM through stellar winds, supernovae and planetary nebulae. The gas and dust in the interstellar medium is the main cause of interstellar extinction.

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Term and definition status: This term and its definition is still awaiting approval

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