A helium flash is a rapid and explosive fusion process that occurs in the cores of certain aging stars, particularly those that have exhausted their hydrogen fuel. When a star like a red giant runs out of hydrogen, its core contracts and heats up, eventually reaching temperatures high enough for helium to fuse into heavier elements. This sudden onset of helium fusion releases a tremendous amount of energy. During a helium flash, the energy output can increase dramatically, causing the star to expand and brighten significantly. This event is a crucial phase in the life cycle of stars, leading to the formation of heavier elements and influencing the star's evolution into later stages, such as the asymptotic giant branch.