spin

Spin (m_s, spin quantum number) is a quantum number that represents a type of angular momentum of a particle such as an electron. The angular momentum that spin represents is intrinsic to the electron: in some ways it is analogous to the electron actually rotating, but other factors show that is not actually the case. This is part of the mystery of quantum mechanics.

Any particular type of particle is confined either to integer spin numbers (such particles are termed a bosons) or to spins that differ from an integer by 1/2 (particles termed fermions). Some of the behavior-characteristics common to all fermions differ from those common to all bosons. Composite particles (i.e., hadrons) may act as fermions or as bosons according to the sum of the spins of their constituent elementary particles, e.g., a composite particle made up of two fermions behaves like a boson.

There are two possible spin values for an electron that is orbiting a nucleus and each electron shell (i.e., all the atom's electrons that share the same principal quantum number) cannot be more than half-filled with electrons of the same spin. An electron in the lowest shell of neutral hydrogen (in that sense, in the ground state) may still have either of two spins, +1/2 or -1/2, which give it slightly different energies (which underlie the atom's hyperfine structure). Such atoms that are at the slightly higher energy-level can (rarely) spontaneously flip their spin, producing a very low energy photon, the mechanism producing the 21-cm line.

Within astrophysics, the term spin is also often used referring to the rotation of astronomical objects or spacecraft, perhaps most often for rapid rotation, such as that of neutron stars.