Interstellar medium

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Eagle nebula. Pillars made of dense cold gas are visible.

In astronomy, the interstellar medium (sometimes called the ISM) is the mix of plasma, gas, and dust in outer space between the stars of a galaxy. It is made mostly of ions, atoms, molecules, cosmic dust, and cosmic rays.

Various gases exist in outer space. Most common are hydrogen and helium. Interstellar cosmic dust makes nebulae.