Most people are aware electrons can change energy levels and be found in excited states. An analogous process occurs in the atomic nucleus when protons or neutrons (the nucleons) become excited. The excited nucleon occupies a higher energy nuclear orbital. Most of the time, the excited nucleons return immediately to the ground state, but if the excited state has a half-life longer than 100 to 1000 times that of normal excited states, it is considered a metastable state. In other words, the half-life of an excited state is usually on the order of 10-12 seconds, while a metastable state has a half-life of 10-9 seconds or longer. Some sources define a metastable state as having a half-life greater than 5 x 10-9 seconds to avoid confusion with the half-life of gamma emission. While most metastable states decay quickly, some last for minutes, hours, years, or much longer. The reason metastable states form is because a larger nuclear spin change is needed in order for them to return to the ground state. High spin change makes the decays “forbidden transitions” and delays them. Decay half-life is also affected by how much decay energy is available. Most nuclear isomers return to the ground state via gamma decay. Sometimes gamma decay from a metastable state is named isomeric transition, but it’s essentially the same as normal short-lived gamma decay. In contrast, most excited atomic states (electrons) return to the ground state via fluorescence. Another way metastable isomers can decay is by internal conversion. In internal conversion, the energy that is released by the decay accelerates an inner electron, causing it to exit the atom with considerable energy and speed. Other decay modes exist for highly unstable nuclear isomers.