We examine and compare the abundance levels in metal-poor halo stars of elements produced in the rapid neutron capture process (i.e. r-process) and the slow neutron capture process (i.e. the s-process). From ground-based high-resolution spectroscopy we have detected 20 elements, including several never seen before in low-metallicity stars, in the very metal-poor star CS 22892-052. The abundance distribution of elements in the range 56<Z<76 in this star is well matched by a solar system r-process abundance distribution, when scaled for the lower metallicity ([Fe/H] = -3.1) of CS 22892-052. There is no evidence of an s-process contribution to these abundances.
Using the Goddard High-Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST), we have detected the very heavy elements osmium and platinum (predominantly synthesized in the r-process) and lead (produced by a combination of the s-and r-processes) in the metal-poor ([Fe/H] = -1.7) giant HD 126238. These are the first such detections of "3 rd n-capture-peak" elements in a galactic halo star. We find that, within in the error limits the elemental abundances of Os and Pt are consistent with scaled solar system r-process abundances. Element by element comparisons of the data from HST and from ground-based data demonstrate, for the first time, the operation of the of the r-process for all of the elements from Ba to Pt in the progenitor (or progenitors) of HD 126238. Some elemental abundances observed in this star, particularly lead, can best be explained as resulting from a combination of s-and r-process nucleosynthesis contributions, prior to the formation of HD 126238.
Finally, we use the thorium abundance as a chronometer to estimate ages. In the case of CS 22892-052 we find a lower limit age of 15 5= 4 Gyrs.