Why is there an upper limit to the mass of a white dwarf star?
When atomic nuclei are squeezed very close together, the low-energy orbits that electrons would normally be able to occupy around them overlap with similar orbits in neighboring nuclei – so the orbits become indistinguishable. The rules of quantum mechanics tell us that no two electrons can occupy the same orbit, so the electrons are forced into higher-energy orbits (pushed to higher speeds) just because of the density of the matter. This quantum pressure can oppose gravity as long as the density doesn’t get too high. If a white dwarf has more than 1.4 times the mass of the Sun squeezing the nuclei, there will be too few orbits available to the electrons (since they cannot travel faster than the speed of light) and the star will collapse – causing a supernova explosion.
