Which stage of a star's life cycle is characterized by the explosion of a supernova?

The stage of a star's life cycle that is characterized by the explosion of a supernova is indeed when a massive star transitions through the red giant phase. During this phase, the star exhausts the nuclear fuel in its core, leading to the outward pressure from nuclear fusion being overpowered by gravitational forces. As the core collapses, it can reach extremely high temperatures and densities, prompting the outer layers of the star to be expelled violently in a cataclysmic event known as a supernova.

By contrast, when a star becomes a neutron star, it has already experienced this explosive phase. A white dwarf is the remnant left after a less massive star has shed its outer layers and does not undergo a supernova. In the case of a black hole, this occurs after a supernova event if the remaining core is massive enough to collapse further, but it is not directly associated with the explosion itself. The red giant phase is critical, as it represents the turning point leading to this dramatic and complex transformation of a star.