natural radionuclides may be conveniently subdivided into three types. first, those whose [[half-life|half-lives]] t 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) (). these are remnants of [[nucleosynthesis]] that occurred in stars before the formation of the [[solar system]]. for example, the isotope (t = ) of [[uranium]] is still fairly abundant in nature, but the shorter-lived isotope (t = ) is 138 times rarer. about 34 of these nuclides have been discovered (see [[list of nuclides]] and [[primordial nuclide]] for details).