Iron nuclei are the most tightly bound nuclei,which is why their production leads to type II supernovae.

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Iron nuclei have the highest binding energy per nucleon, which means it takes more energy to break them apart into their individual nucleons. This makes the production of iron nuclei a key factor in the energy balance of a star. When a massive star runs out of fuel and can no longer generate enough energy to counteract the force of gravity, it collapses in on itself and triggers a type II supernova. During this supernova, the star's core becomes hot and dense enough to fuse lighter elements into heavier ones, including iron. This process releases energy and causes the star to explode.

Iron Nuclei Are the Most Tightly Bound Nuclei,which Is Why