- the matter occurring between the stars of our Galaxy, largely in the spiral arms, and consisting mainly of huge clouds of ionized, neutral, or molecular hydrogenAbbreviation: ISM
- Material, mostly hydrogen gas, other gases, and dust, occupying the space between the stars and providing the raw material for the formation of new stars. Nebulae are the most distinct areas of the interstellar medium; they appear when the clouds of gas and dust cluster due to interaction with nearby stars or star remnants.
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