What are the primary elements making up about 99% of a molecular cloud's mass?
Answer
Hydrogen and Helium
The fundamental composition of the giant stellar nurseries, known as molecular clouds or nebulae, is dominated by the universe's two lightest elements. The text specifies that the ingredients are overwhelmingly simple, consisting of about 75% hydrogen and 24% helium. This means that together, these two elements account for approximately 99% of the cloud's total mass. The remaining 1% consists of all the heavier elements, which are collectively termed 'metals' in astronomical parlance, and which include the materials necessary for rocky planet formation.
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