Pauli exclusion principle
The Pauli exclusion principle is a fundamental principle of quantum mechanics according to which certain particles (fermions) cannot share the same quantum state. This principle allows to understand the shell structure of the distribution of electrons in atoms and the band structure of energy levels in solids. More surprisingly, it is also involved in the structure of certain stars (white dwarves) and leads to the electron degeneracy pressure phenomenon which prevents the star from collapsing under the effects of gravitation. However, when the star is too massive (1.4 to 9 times the mass of the Sun), the exclusion principle no longer holds and the star collapses into a neutron star and for larger masses, a black hole.