There is a misconception that because fusion into elements beyond iron cannot liberate energy (true), fusion cannot proceed beyond iron (false). When exothermic fusion ceases because fuels are exhausted, fusion does NOT stop. End of exothermic fusion permits the core to lose heat and its temperature to decrease. As core's temperature decreases, it contracts under influence of gravity. This causes core's temperature to increase again. This huge rise in temperature forces post-iron elements to fuse into still heavier elements, despite the fact that such fusion absorbs energy and does not release it. Thermodynamics favors further fusion by the principle that excess heat causes exothermic reactions to reverse and higher mass products take up less volume. This cause cooling the core and permitting further contraction under the force of gravity. The result is a runaway collapse of the core into very heavy nuclei. Furthermore, a huge surplus of neutrons is created as electrons are squeezed into protons releasing neutrinos. This neutron-rich environment permits fusion into high neutron to proton ratio nuclei such as gold, lead, ..., even uranium, plutonium and beyond. The core continues to contract until it reaches a density and pressure that is sufficient to counteract (briefly) the compression of gravity due to the exclusion principle as it applies to the neutron fluid. The infalling outer core bounces off of the super-dense and super-hot inner core. Now the inner and outer core proceed in their own separate directions. The inner core continue to contract as more and more neutrons are created from protons and electrons, releasing neutrinos. The neutrinos pass through the surrounding material, permitting further escape of energy and more cooling of the inner core until the inner core becomes basically a soup of neutrons. Meanwhile, the outer core that bounced up continues to expand with its load of very heavy elements well beyond iron. This explosion continues, scattering the post-iron elements into the surrounding space outer layers of the star, resulting in a Type II super-nova. The post-iron elements along with the pre-iron elements that remained in the outer layers of the star enrich interstellar space, providing material for future stars and planets. Elements beyond uranium do not survive into the next phase of star formation because their lifetimes are far too short.
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