In the core of a star, fusion can build up heavier and heavier nuclei, for example carbon from helium. Nuclei much heavier than iron, however, cannot be produced in this way. This is because
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → fusion into nuclei beyond the peak of the binding energy per nucleon curve does not release energy
Fusion releases energy only when the nucleus formed is more tightly bound. That is, its binding energy per nucleon must be greater than that of the nuclei that fuse. On the binding energy curve this quantity rises for light nuclei and reaches its peak near iron (\( A=56 \)). It then falls slowly for heavier nuclei.
Option (1) fusion into nuclei beyond the peak of the binding energy per nucleon curve does not release energy — Correct. A nucleus beyond the peak is less tightly bound per nucleon than the nuclei that would form it. So such a fusion absorbs energy instead of releasing it.
Option (2) the nuclear force does not act between nucleons in nuclei heavier than iron — Incorrect. The nuclear force binds the nucleons of every nucleus. Heavy nuclei such as uranium are held together by the same force.
Option (3) the core temperature of a star cannot rise above about \( 10^{8}\,\mathrm{K} \) — Incorrect. About \( 10^{8}\,\mathrm{K} \) is only the stage at which helium fuses into carbon. Later stages of collapse heat the core further and build heavier nuclei. So temperature is not what stops the process at iron.
Option (4) all the hydrogen in the core is used up before such nuclei can be formed — Incorrect. The core hydrogen runs out even before helium fuses into carbon. Yet heavier nuclei still form after that. The loss of hydrogen is what makes the core collapse and heat up for these later stages.
NCERT (2026-27):
Nuclear Fusion – Energy Generation in Stars – “When two light nuclei fuse to form a larger nucleus, energy is released, since the larger nucleus is more tightly bound, as seen from the binding energy curve in Fig.13.1. …”
Nuclear Fusion – Energy Generation in Stars – “Helium is not the only element that can be synthesized in the interior of a star. As the hydrogen in the core gets depleted and becomes helium, the core starts to cool. The star begins to collapse under its own gravity which increases the temperature of the core. If this temperature increases to about 108 K, fusion takes place again, this time of helium nuclei into carbon. This kind of process can generate through fusion higher and higher mass number elements. But elements more massive than those near the peak of the binding energy curve in Fig. 13.1 cannot be so produced.”
Fig. 13.1: The binding energy per nucleon as a function of mass number.