From newly-obtained high-resolution , high signal-to-noise ratio spectra the abundances of the elements La and Eu have been determined over the stellar metallicity range -3 < [ Fe/H ] < +0.3 in 159 giant and dwarf stars .
Lanthanum is predominantly made by the s -process in the solar system , while Eu owes most of its solar system abundance to the r -process .
The changing ratio of these elements in stars over a wide metallicity range traces the changing contributions of these two processes to the Galactic abundance mix .
Large s -process abundances can be the result of mass transfer from very evolved stars , so to identify these cases , we also report carbon abundances in our metal-poor stars .
Results indicate that the s -process may be active as early as [ Fe/H ] = -2.6 , alalthough we also find that some stars as metal-rich as [ Fe/H ] = -1 show no strong indication of s -process enrichment .
There is a significant spread in the level of s -process enrichment even at solar metallicity .