During the last several decades , there have been a number of advances in understanding the rapid neutron-capture process ( i.e. , the r -process ) .
These advances include large quantities of high-resolution spectroscopic abundance data of neutron-capture elements , improved astrophysical models , and increasingly more precise nuclear and atomic physics data .
The elemental abundances of the heavy neutron-capture elements , from Ba through the third r -process peak , in low-metallicity ( [ Fe/H ] \mathrel { \vbox { \offinterlineskip \hbox { $ < $ } \kern 1.29 pt \hbox { $ \sim$ } } } –2.5 ) Galactic halo stars are consistent with the scaled ( i.e. , relative ) solar system r -process abundance distribution .
These abundance comparisons suggest that for elements with Z \geq 56 the r -process is robust—appearing to operate in a relatively consistent manner over the history of the Galaxy—and place stringent constraints on r -process models .
While not yet identified , neutron-rich ejecta outside of the core in a collapsing ( Type II , Ib ) supernova continues to be a promising site for the r -process .
Neutron star binary mergers might also be a possible alternative site .
Abundance comparisons of lighter n -capture elements in halo stars show variations with the scaled solar r -process curve and might suggest either multiple r -process sites , or , at least , different synthesis conditions in the same astrophysical site .
Constraints on r -process models and clues to the progenitors of the halo stars—the earliest generations of Galactic stars—are also provided by the star-to-star abundance scatter of [ Eu/Fe ] at low metallicities in the early Galaxy .
Finally , abundance observations of long-lived radioactive elements ( such as Th and U ) produced in the r -process can be used to determine the chronometric ages of the oldest stars , placing constraints on the lower limit age estimates of the Galaxy and the Universe .