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When a massive star exhausts its nuclear fuel, it undergoes a supernova explosion. This is a dramatic stellar event that occurs at the end of the life cycle of a massive star (typically more than eight times the mass of the Sun). As nuclear fusion in the star's core ceases, the outward pressure that counters the gravitational force diminishes, causing the core to collapse under its own gravity. This collapse leads to extremely high temperatures and pressures, allowing the outer layers of the star to be expelled violently into space, resulting in the bright and energetic explosion known as a supernova.
The significance of a supernova extends beyond just the explosion itself; it contributes to the cosmic lifecycle by enriching the interstellar medium with heavy elements formed during the star's life and nucleosynthesis. This process not only allows for the formation of new stars and planets but also contributes to the overall evolution of galaxies.
The other choices correspond to different stages or outcomes in the life cycles of stars but are not accurate descriptions of the event that occurs specifically when a massive star exhausts its nuclear fuel. For example, a red giant phase occurs during the later stages of a star's life when its core contracts and outer layers expand, while a white dwarf represents the remaining core