What is the primary elemental makeup of the Interstellar Medium (ISM)?
Answer
Roughly 70% hydrogen and 28% helium
The interstellar medium (ISM), which comprises the diffuse material throughout space, has a characteristic elemental composition derived from cosmic history. This mixture is dominated by the lightest elements created in the Big Bang. Specifically, it consists of approximately 70% hydrogen, which forms the bulk of the gas, and about 28% helium. The remaining fraction accounts for all heavier elements, often referred to as 'metals,' which are synthesized later in the interiors and through the deaths of prior generations of stars. This specific ratio dictates the initial conditions for the formation of molecular clouds where subsequent star birth occurs.
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