Cayrel's Star
BPS CS31082-0001, named Cayrel's Star, is an old Population II star located in a distance of 4 kpc in the galactic halo. It belongs to the class of ultra-metal-poor stars, specifically the very rare subclass of neutron-capture enhanced stars. It was discovered by Tim C. Beers and collaborators with the Curtis Schmidt telescope at the Cerro Tololo Inter-American Observatory in Chile and analyzed by Roger Cayrel and collaborators. They used the Very Large Telescope at the European Southern Observatory in Paranal, Chile for high-resolution optical spectroscopy to determine elemental abundances. The thorium-232 to uranium-238 ratio was used to determine the age. It is estimated to be about 12.5 billion years old, making it one of the oldest known.
Compared to other ultra-metal-poor, r-process enriched stars CS31082-001 has higher abundances of the actinides, but a surprisingly low Pb abundance.
