Final answer:
When a high-mass star runs out of hydrogen, its core collapses and heats up, leading to helium fusion through the triple-alpha process, temporarily energizing the star.
Step-by-step explanation:
After a high-mass star exhausts its hydrogen fuel, the core undergoes significant changes. It begins to collapse under its own gravity, leading to an increase in density and temperature. Subsequently, the heat generated causes the surrounding helium to reach temperatures where helium fusion can commence, known as the triple-alpha process. This new fusion process provides a temporary source of energy for the star. Following helium burning, the star goes through a series of stages that can lead to the formation of a planetary nebula and finally, for the most massive stars, a supernova explosion.