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The study of the chemical composition of the most primitive s tars of the galactic halo has been made possible with the help of large surveys aimed at finding s uch tars, and by powerful new instruments, as the Keck telescopes, the Subaru telescope, and the ESO Very Large Telescope. The atmospheres of these primitive stars possess, per hydro gen atom, from 1/1000th to 1/10000th less supernovae-made elements than the Sun, and reflect the y ields of the first supernovae. It was once expected that these yields would show a larger scatter t han those in the more metal-rich Population II stars, which have been enriched by many more su p rnovae explosions than the earlier generations. If we leave aside one class of objects, the Carbon-Enhanced Metal-Poor (CEMP) stars, which is the topic of another talk at this confe rence, a rather well-defined set of abundance ratios emerge for C to Zn amongst the most primitiv e population, with a scatter that is surprisingly small. The quality of the high-resolution spe ctroscopic data is such that the observed level of scatter in the measured elemental abundances for th ese species is no longer limited by accuracy of the observations, nor by other errors inherent t o the analysis of the data. By way of contrast, amongst the neutron-capture elements produced b y ther-process, at a given metallicity a spread reaching a factor of over 1000 exists for elements su ch as Ba. The stable portion of the r-process pattern observed in such stars is the second peak (Z = 56 to 72), in which the relative abundances of these elements in very metal-poor stars are al most indistinguishable from their inferred proportions in solar-system material.

Constraints on the yields of the first supernovae in the Universe