What two light elements account for approximately 98% of the total mass within the ISM gas component?
Answer
Hydrogen and helium
The gaseous portion of the Interstellar Medium (ISM), which itself constitutes about 99% of the ISM's mass, is predominantly composed of the two lightest elements forged in the Big Bang. These elements are hydrogen and helium. Together, these two components make up approximately 98% of the mass within the gaseous phase of the ISM. The remaining 2% of the gas is comprised of heavier elements, such as carbon and oxygen, which are synthesized later through nuclear fusion processes inside stars before being dispersed back into space upon stellar death.
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