Nucleocosmochronology, or nuclear cosmochronology, is a technique used to determine timescales for astrophysical objects and events based on observed ratios of radioactive heavy elements and their decay products.
It has also been used to estimate the age of the Milky Way itself by studying Cayrel's Star in the Galactic halo, which due to its low metallicity, is believed to have formed early in the history of the Galaxy.
[7] Limiting factors in its precision are the quality of observations of faint stars and the uncertainty of the primordial abundances of r-process elements.
[3][clarification needed] He pioneered the idea that age could be calculated by the ratio of abundances of radioactive parent elements and their stable decay products.
[3] According to a tribute written by colleagues, a large part of the modern science of nuclear cosmochronology grew out of work by John Reynolds and his students.