Abstract

The discovery and analysis of the most metal-poor stars lead to insight into conditions when the Universe and Galaxy were young. We present the rationale for studying such objects (which become progressively rarer at lowest abundance), with a description of their systematic discovery. Currently, some 130 stars in the Galaxy’s stellar halo are known which have [Fe/H] < -3.0, from high-resolution, high-S/N spectroscopic analyses. Four of them have [Fe/H] < -4.5. The Metallicity Distribution Function and the relative abundance patterns ([X/Fe] vs. [Fe/H]) of the sample will be discussed. As one proceeds to lowest abundance one finds astounding overabundances of some or all of the CNO group and of other light elements. While this diversity among the most metal-poor stars has yet to be fully understood, there exists a number of proposed models, which will be briefly presented.

Extremely metal-poor stars ([Fe/H] < -3.0) have recently been discovered in the Galaxy’s dwarf spheroidal and ultra-faint galaxy satellites. The similarity of these stars with those of similar [Fe/H] in the Galaxy’s halo suggests that objects not unlike the ultra-faint galaxies may have played a role in the formation of the Galaxy’s halo.