As the core of a star that is 1.4 or more times as massive as the Sun begins to yield to gravity and starts to shrink, it grows hotter and denser, and a new series of nuclear reactions begins, temporarily halting it's collapse. When the core becomes essentially just iron, fusion ceases, and, in less than a second, the star begins gravitational collapse, the core temperature rises to over 100 billion degrees, and the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in an explosive shock wave called a .