By mass, what element constitutes nearly 24% of a molecular cloud's composition, second only to molecular hydrogen?
Answer
Helium
The elemental breakdown by mass within a molecular cloud reflects the general cosmic abundance profile. The dominant component is molecular hydrogen ($ ext{H}_2$), accounting for approximately 74% of the total mass. The second most abundant element by mass is helium, which constitutes nearly 24% of the cloud's total mass. All other elements, including the trace molecules and the critical solid interstellar dust grains that aid in shielding, make up the remaining small percentage. This composition profile establishes molecular clouds as fundamental reservoirs of stellar fuel.
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