What distinguishing characteristic defines Molecular Clouds compared to other ISM phases?

Answer

They are the coldest and densest types of clouds, allowing for molecular hydrogen formation.

Molecular Clouds are the coldest and densest phases of the ISM, often having temperatures below 100 K, which permits hydrogen atoms to combine into molecular hydrogen ($ ext{H}_2$).

What distinguishing characteristic defines Molecular Clouds compared to other ISM phases?
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