Although most of the mass of the Milky Way Galaxy is condensed into stars, interstellar space is not completely empty. It contains gas and dust in the form both of individual clouds and of a diffuse medium. Interstellar space typically contains about one gas atom per cubic centimetre and 100 dust particles per cubic kilometre.
KeywordsInterstellar Medium Molecular Cloud Galactic Plane Supernova Remnant Planetary Nebula
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