All of my internet research comes up with the same answer which is that when the core fuses all of its fuel, there is no more outward pressure, so the gravitational attraction takes over. What isn't explained is WHY it will explode outwards. I realize that the temperature will increase, but wouldn't conservation of energy say that the star would bounce back out to its original size, almost like a harmonic oscillator? There has to be another source of energy other than the Coulombic and gravitational repulsion/attraction of the atoms? My only guess would be that the gravitation potential energy is converted into such a high temperature when compressing that the nuclei fuse into heavier elements than iron. But that wouldn't give off energy, it would remove it.