As fusion slows down during the later stages of a star’s life the radiation pressure of the fusion reaction at the center of the star lowers, allowing gravity to further compress the core of the star. If further fusion reactions do not yield enough energy to resist this collapse, gravity will force the atoms within the star closer and closer together. At this point, one of the only forces that are still able to resist the inward force of gravity is electron degeneracy pressure. Electron degeneracy pressure is an outward force produced when various subatomic particles are forced together to the point where the rules of the Pauli Exclusion Principle forces particles apart. The Pauli Exclusion Principle states that two discrete particles (or energy states) cannot exist simultaneously within the same quantum position. The inability for multiple particles to exist within the same position creates an outward force on the particles, forcing them apart.