Final answer:
The end of the red supergiant phase occurs when a star has exhausted the hydrogen at its core, leading to core contraction and an increase in fusion, potentially resulting in a supernova or forming a white dwarf.
Step-by-step explanation:
The end of the red supergiant phase is marked by a star's core contracting and the outer layers expanding due to the continued fusion of hydrogen into helium. As the star exhausts the hydrogen fuel at its core, it moves off the main sequence and becomes larger, cooler, and more luminous, transforming into a red giant or supergiant. The core's contraction leads to increases in temperature and pressure until it can begin fusing helium into heavier elements like carbon and oxygen through the triple-alpha process, marking the end of the red supergiant phase. Eventually, the fate of a red supergiant will depend on its mass; those with sufficient mass may end their lives in a spectacular supernova explosion, while less massive stars will shed their outer layers and leave behind a dense core known as a white dwarf.