Neutron Degeneracy Pressure: Quantum mechanics restricts the number of neutrons that can have low energy. Each neutron must occupy its own energy state. When neutrons are packed together, as they are in a neutron star, the number of available low energy states is too small and many neutrons are forced into high energy states. These high energy neutrons make up the entire pressure supporting the neutron star. Because the pressure arises from this quantum mechanical effect, it is insensitive to temperature, i.e., the pressure doesn't go down as the star cools. Similar to electron degeneracy pressure but, because the neutron is much more massive than the electron, neutron degeneracy pressure is much larger and can support stars more massive than the Chandrasekhar mass limit.