Final answer:
When a star's core is composed of iron, it leads to a supernova explosion due to the inability of iron to undergo nuclear fusion, resulting in a core collapse.
Step-by-step explanation:
When a star collapses to the point that its core is composed of iron, it explodes in a supernova. This catastrophic explosion occurs because iron cannot release energy through nuclear fusion, which is the process that powers a star.
Without the release of energy from fusion to counteract gravity, the core collapses under its own weight. The collapse happens suddenly and leads to a violent explosion that blasts the outer layers of the star into space. Following the supernova, the core might become a neutron star or, if massive enough, a black hole.