The most stable nuclear isomer occurring in nature is 73 Ta, which is present in all tantalum samples at about 1 part in 8,300. Its half-life is at least 10 years, markedly longer than the age of the universe. The low excitation energy of the isomeric state causes both gamma de-excitation to the Ta ground state (which itself is radioactive by beta decay, with a half-life of only 8 hours) and direct electron capture to hafnium or beta decay to tungsten to be suppressed due to spin mismatches. The origin of this isomer is mysterious, though it is believed to have been formed in supernovae (as are most other heavy elements). Were it to relax to its ground state, it would release a photon with a photon energy of 75 keV.