Spin-flip Transition

Origin of the 21-cm emission line that originates with a neutral 1H atom. The proton and the electron each have a quantum “spin,” which points either “up” or “down.” Spins can be parallel (both of them “up” or “down”), or antiparallel (opposite states). The antiparallel state has slightly less energy than the parallel state, so if an atom in the parallel state changes to antiparallel, a 21-cm radio photon is emitted. This transition occurs every ~10 million years. Although the probability of detecting cold hydrogen gas through this mechanism is very small, the relative rarity of the transition is more than compensated for by the superabundance of neutral 1H in the interstellar medium; at any one time some fraction will be in the slightly excited state.

The spin-flip transition can only be used to trace the distribution of neutral hydrogen in the Universe. For regions rich in molecular hydrogen (e.g. molecular clouds), astronomers must use a different tracer. This is generally the carbon monoxide molecule (CO) which has a characteristic emission at the shorter wavelength of 2.6 mm.