When did nucleosynthesis first occur?

• A. In the solar system formation
• B. During the Big Bang
• C. In present-day stellar cores
• D. During supernovae

Answer: (B)

Nucleosynthesis is the formation of new atomic nuclei. The earliest time this happens is during the Big Bang, when the universe was extremely hot and dense. In the first few minutes after the Big Bang, protons and neutrons fused to form the lightest nuclei—mostly hydrogen and helium, with small amounts of lithium. This Big Bang nucleosynthesis set the primordial chemical makeup of the universe and occurred long before stars and planets existed. Later on, stars fuse hydrogen into helium and, in later stages, heavier elements; supernovae also forge heavier elements, but those events happen after the initial burst of synthesis.