A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state (higher energy) levels. "Metastable" describes nuclei whose excited states have half-lives 100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10 seconds). The term "metastable" is usually restricted to isomers with half-lives of 10 seconds or longer. Some references recommend 5 × 10 seconds to distinguish the metastable half life from the normal "prompt" gamma-emission half-life. Occasionally the half-lives are far longer than this and can last minutes, hours, or years. For example, the 73 Ta nuclear isomer survives so long (at least 10 years) that it has never been observed to decay spontaneously. The half-life of a nuclear isomer can even exceed that of the ground state of the same nuclide, as shown by 73 Ta as well as 75 Re, 77 Ir, 83 Bi, 84 Po, 95 Am and multiple holmium isomers.