Final answer:
Iron is the main product of silicon fusion in the core of a massive star, representing the end of energy-producing fusion reactions before the star's eventual supernova or other end-of-life phenomena.
Step-by-step explanation:
The main product of silicon fusion reactions in the core of a massive star is iron. As a massive star evolves and grows hotter, its core fuses lighter elements into heavier ones. Lighter nuclei, up through silicon, release energy when they fuse because the products are more stable than the reactants. However, when silicon fuses into iron, the process reaches an endpoint. Iron is the most stable of the nuclei known, and unlike the fusion of lighter elements, fusing iron requires additional energy rather than releasing it.
Therefore, the production of iron through silicon fusion represents the last step of energy-producing nuclear reactions within a star before it reaches the end of its life cycle, leading to either a supernova or the formation of a dense stellar remnant like a neutron star.