What phenomenon marks the end stage of massive stars, resulting in gravitational collapse?

The end stage of massive stars is marked by a supernova explosion, which occurs as a result of gravitational collapse. In massive stars, nuclear fusion processes in their cores create heavier elements until iron is formed. Iron does not release energy through fusion, leading to a situation where the outward pressure from the fusion reactions can no longer counterbalance the force of gravity pulling inward.

Once fusion ceases, the core collapses under its own gravity, leading to temperatures and densities so high that electrons and protons combine to form neutrons, producing a neutron star. If the core's mass is sufficient, a black hole may form instead. The sudden and violent release of energy when the outer layers of the star are expelled during the explosion is what we observe as a supernova.

The other options represent different processes that do not specifically signify the final collapse of a massive star. Stellar fusion refers to the ongoing nuclear reactions that occur during a star's life, nuclear decay involves the transformation of one element into another, and planetary nebula formation occurs in stars with lower masses as they shed their outer layers, not during the collapse of massive stars. Thus, the supernova explosion is the key event that indicates the end of a massive star's lifecycle due to gravitational collapse