Natural radionuclides may be conveniently subdivided into three types. First, those whose half-lives t 1/2 are at least 2% as long as the age of the Earth (for practical purposes, these are difficult to detect with half-lives less than 10% of the age of the Earth) (4.6 × 10 years). These are remnants of nucleosynthesis that occurred in stars before the formation of the Solar System. For example, the isotope U (t 1/2 = 4.5 × 10 years) of uranium is still fairly abundant in nature, but the shorter-lived isotope U (t 1/2 = 0.7 × 10 years) is 138 times rarer. About 34 of these nuclides have been discovered (see List of nuclides and Primordial nuclide for details).