Questions: Stellar Nucleosynthesis

Question 1

Why is iron (Fe) the endpoint of energy-releasing fusion in a star rather than some heavier element?

• A. Iron has the highest number of protons and cannot absorb more
• B. Iron has the highest binding energy per nucleon, so fusing it requires energy input rather than releasing it
• C. Iron is too dense for the star's gravity to compress further
• D. The CNO cycle naturally terminates at iron

Answer: (B)

Binding energy per nucleon peaks at iron-56. Elements lighter than iron release energy when fused (exothermic) because the product is more tightly bound. Elements heavier than iron would require energy input to fuse (endothermic) because the product is less tightly bound per nucleon. When an iron core accumulates, the star has no energy source to resist gravitational collapse.

Question 2

The Sun is currently fusing helium into carbon in its core.

• T. True
• F. False

Answer: False

The Sun is currently in the main-sequence phase, fusing hydrogen into helium via the proton-proton chain. Helium fusion (the triple-alpha process, producing carbon) requires much higher core temperatures (~100 million K) and will only occur when the Sun expands into a red giant and compresses its helium core sufficiently — roughly 5 billion years from now.

Question 3

What is the key difference between the s-process and r-process of neutron capture, and where does each occur?

Model answer: The s-process (slow neutron capture) occurs in AGB stars over thousands of years, adding neutrons one at a time with enough time for beta decay between captures; it produces elements up to bismuth. The r-process (rapid neutron capture) occurs in extremely neutron-rich, energetic environments — neutron star mergers and core-collapse supernovae — adding many neutrons faster than beta decay can occur, producing the heaviest elements including gold and uranium.

The distinction is whether beta decay (neutron → proton + electron) has time to occur between neutron captures. In the s-process, it does, so the nucleus stays near the valley of stability. In the r-process, it does not, so extremely neutron-rich isotopes are built up before decaying back toward stability — reaching much heavier nuclei. The 2017 gravitational wave observation of a neutron star merger (GW170817) confirmed the r-process occurs in such mergers.