  The abundance patterns of metal-poor stars provide us a wealth of chemical
information about various stages of the chemical evolution of the Galaxy. In
particular, these stars allow us to study the formation and evolution of the
elements and the involved nucleosynthesis processes. Metal-poor stars are the
local equivalent of the high-redshift Universe, and thus offer crucial
observational constraints on a variety of issues regarding the early Universe.
This review presents an introduction to metal-poor stars and their role as
"cosmic lab" for the study of neutron-capture nucleosynthesis processes,
particularly that of the r-process. The metal-poor star HE 1523-0901 serves as
an example for this group of objects. It displays in its spectrum the strongest
overabundance of neutron-capture elements associated with the r-process. Heavy
neutron-capture elements such as Eu, Os, and Ir were measured, as well as the
radioactive elements Th and U. Abundance of Th and U, in conjunction with those
of stable elements make possible nucleo-chronomtery, i.e., the determination of
stellar ages. HE 1523-0901 appears to be ~13 Gyr old. Age uncertaintites range
from 2-5 Gyr for individual chronometers, and are largly due to theoretical
uncertainties in the initial production ratio of the employed chronometers. The
decay product of the radioactive elements, lead, can be used to constrain
r-process calculations. Only few such stars are currently known with detected
U. These objects, however, are crucial for the study of this nucleosynthesis
process.
