Answer:
When the star runs out of hydrogen the core's temperature decreases so the thermal pressure decreases as well. With a result of the core becoming less efficient to hold up the rest of the star's weight. The core and the gas around it starts shrinking As this happens, the gas in the core and shell around it heats up. Then the star is becomes a supernova.
gravity reversing on its self because the energy/fusion was keeping it from collapsing
