Which of the following best describes the approximate chemical composition by mass with which all stars are born?

• A. About 70% helium and 30% hydrogen
• B. About 90% hydrogen and 10% helium
• C. About 75% hydrogen and 25% helium
• D. About 50% hydrogen and 50% helium

Answer: (C)

Verified step by step guidance

1. Understand that the question is about the typical elemental composition of stars at their formation, which is primarily hydrogen and helium by mass.
2. Recall that stars form from clouds of gas and dust in space, and the primordial composition of these clouds reflects the elemental abundances produced by the Big Bang.
3. Recognize that the Big Bang nucleosynthesis resulted in a universe composed mostly of hydrogen (~75% by mass) and helium (~25% by mass), with trace amounts of heavier elements.
4. Note that this composition is consistent across most stars when they are born, as heavier elements (metals) are present only in small quantities.
5. Therefore, the best description of the chemical composition by mass with which all stars are born is approximately 75% hydrogen and 25% helium.