What percentage of the total interstellar gas mass is concentrated within dense structures known as Molecular Clouds ($ ext{H}_2$)?
Answer
About 50%
Despite occupying only about 2% of the galaxy’s total volume, dense clouds, referred to as Molecular Clouds, are critical reservoirs of interstellar material. These coldest and densest structures hold a substantial portion of the galaxy's mass because their density is orders of magnitude greater than the diffuse gas. Specifically, these Molecular Clouds, where most hydrogen exists in molecular form ($ ext{H}_2$), contain approximately 50% of the entire mass of the interstellar gas available in the Milky Way plane. This makes them the primary sites for future star formation.
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