 Deviations from the (evolving) galactic average, locally sampled around the time that the sun's nuclear burning began, can generally be accounted for by mass fractionation (see the article on mass-independent fractionation) plus a limited number of nuclear decay and transmutation processes.There is also evidence for injection of short-lived (now extinct) isotopes from a nearby supernova explosion that may have triggered solar nebula collapse.Hence deviations from natural abundance on earth are often measured in parts per thousand (per mille or ‰) because they are less than one percent (%).