Very Metal Poor Stars in the Milky Way: Constraints on Stellar Nucleosynthesis

The living record of early Galactic nucleosynthesis is written in the chemical compositions of metal-poor stars. For stars with metallicities -1.0 ≥ [Fe/H] ≥ -2.5, several decades of spectroscopic studies have delineated the abundance trends of elements that are synthesized by major nuclear fusion reaction chains. There is very strong observational evidence that the r-process isotopes identified in metal-poor stars and the solar system matter are in fact the product of two distinct types of r-process events. The observed pattern beyond Z ⩾ 40 up to Th-U should most likely be produced by only one (or a few) r-process event(s) in a unique stellar site. This "main" r-process then produces the "low-Z" elements (40 ⩽ Z ⩽ 48) under-abundant compared to solar, and reaches the full solar values presumably around Te.