When a massive star undergoes nuclear fusion in its core, it initially fuses hydrogen into helium through a process called the proton-proton chain or the CNO cycle, depending on the star's mass. This fusion process releases a tremendous amount of energy, which counterbalances the gravitational forces that would otherwise cause the star to collapse.
As the core of the star heats up and the pressure increases, heavier elements can undergo fusion. Helium can fuse into heavier elements like carbon and oxygen, and those elements can further fuse to create even heavier elements like neon, magnesium, silicon, and iron.
However, iron is a particularly significant element when it comes to fusion in stars. Iron has the highest binding energy per nucleon among all elements, which means it requires more energy to fuse iron nuclei together than is released in the fusion process. Therefore, fusing iron or elements heavier than iron actually requires an input of energy rather than releasing energy.
At the same time, the fusion reactions that create elements heavier than helium require higher temperatures and pressures to overcome the increased electrostatic repulsion between the positively charged atomic nuclei. The fusion processes become progressively more difficult and less efficient as the atomic nuclei become larger and more positively charged.
As a result, when a massive star exhausts its nuclear fuel and reaches the stage of iron fusion in its core, it becomes unstable. The core collapses under the influence of gravity, leading to a supernova explosion. This explosion releases an enormous amount of energy and allows for the synthesis of heavier elements beyond iron through processes like rapid neutron capture (r-process) or slow neutron capture (s-process) in the intense conditions of the supernova explosion or during the subsequent evolution of the remnant.
In summary, iron acts as a fusion endpoint in massive stars because fusion processes involving iron require an input of energy instead of releasing energy. Elements heavier than iron are typically formed through processes occurring during supernova explosions or in the later stages of stellar evolution, when the conditions are extremely intense and different nucleosynthesis mechanisms come into play.