 A particle spin is another key distinction that helps us categorize all these particles. You'll recall that electrons have spin one-half and follow Paul's exclusion principle. Photons have spin one and do not follow the exclusion principle. The statistics that describe spin one-half particle behavior in large groups was developed by Enrico Fermi and Paul Dirac. They are called fermions after Mr. Fermi. The statistics that describe spin one particle behavior in large groups was developed by Satyandronath Bose and Albert Einstein. They are called bosons after Mr. Bose. You can imagine that large groups of particles that can't fit into the same quantum state will behave differently than particles that can. In an energy well, the bosons all sit in a condensate at the bottom. The fermions arrange themselves in a hierarchy, like electrons and an atom. For example, a beam of photons can be made to have the same quantum state. This is how a laser works. On the other hand, the inability of electrons to fit into the same quantum state creates an outward pressure that halts a star's collapse and creates white dwarfs.