exclusion principle

In physics, a principle of atomic structure originated by Austrian-born US physicist Wolfgang Pauli. It states that no two electrons in a single atom may have the same set of quantum numbers.

Hence, it is impossible to pack together certain elementary particles, such as electrons, beyond a certain critical density, otherwise they would share the same location and quantum number. A white dwarf star, which consists of electrons and other elementary particles, is thus prevented from contracting further by the exclusion principle and never collapses. Elementary particles in the class fermions obey the exclusion principle whilst those in the class bosons do not.