048.2 Central Heating Restored

During the red giant stage, the central helium core continues to contract and grow hotter until it reaches the ignition temperature for fusing helium to make carbon.

Because ignition occurs in a highly compressed core, there is an explosion, called the Helium Flash.

The new equilibrium, with energy coming from the center again, is more like a normal star, so the red giant contracts and its surface temperature rises.