Extremely Metal-Poor Stars: The Local High Redshift Universe

Extremely metal-poor (EMP) stars can only have formed early in the history of the Galaxy, and represent the local equivalent of the high redshift universe. With them, we can study the early supernovae, the early chemical evolution of the Galaxy, and the history of star formation in the Milky Way. By analogy we can learn about those epochs of galaxy formation in the distant past that are currently at such high redshifts that they are beyond the reach of even the largest existing telescopes, a technique some call "near-field cosmology". While H, He, and some Li came out of the Big Bang, all other elements were formed in stars, and were dispersed by supernovae and stellar winds into the gas from which subsequent stellar generations formed. The ejecta from supernovae played the most important role in the early Universe. SN models have many parameters, including the history of the progenitor star (initial mass, mass loss history, internal nucleosynthesis history prior to the explosion, etc), the details of the explosion (energy, ejected mass, mixing) etc. There are vigorous groups pursuing the details of these models both theoretically and computationally in the US and abroad. But there are so many free or poorly known parameters that these efforts are best guided by observations of metal-poor stars.