For my research, I am searching for the oldest stars in our Milky Way Galaxy and in small dwarf galaxies that surround the Milky Way. By studying these stars, we can learn details about the nature of the early Universe shortly after the Big Bang, the era of the very first stars and when the first galaxies began to form. Chemical elements heavier than hydrogen and helium were first forged in these first massive stars and then ejected in the enormous supernova explosions that followed. The next generation of stars then formed from this enriched gas, thus inheriting and preserving the chemical signature of the first explosions. Hence, the oldest stars in the Milky Way are these 13 billion year old stellar fossils that allow us to reconstruct the chemical enrichment and chemical evolution processes that shaped the Galaxy from the earliest times until today. No other astrophysical probe can provide such detailed information on this early time.

With my group at MIT and jointly with many collaborators from around the globe, I’m employing some of the world's largest optical telescopes, most notably the Magellan Telescopes in Chile. We identify old stars by means of their chemically primitive nature which implies that they must have formed early on, before the Universe was significantly enriched in chemical elements as is the case today. These so-called “metal-poor” stars then contain only tiny amounts of heavy elements (“metals”) that are heavier than hydrogen and helium. We measure the abundances of various chemical elements to establish their abundance signature which in turn tells us about the physical and chemical conditions of the early gas from which these stars formed.