We are all made of stardust. Almost all of the chemical elements were produced by nuclear reactions in the interiors of stars. When a star dies a fraction of the elements is released into the inter-stellar gas clouds, out of which successive generations of stars form.
Astronomers have a basic understanding of this chemical enrichment cycle, but chemical evolution and nulceosynthesis are still not fully understood. Andrew McWilliam measures the detailed chemical composition of Red Giant stars, which are about as old as the galaxy and retain their original chemical composition. He is seeking answer to questions such as: What are the sites of nucleosynthesis? What modulates element production? What can we learn about galactic history by reading this fossil record?
McWilliam tests nucleosynthesis and chemical evolution theory by studying the composition of Red Giant stars in different systems. The central bulge of our galaxy, for instance, probably evolved quickly and was the destination for infalling gas. Dwarf galaxies likely evolved slowly and lost much of their initial gas. McWilliam and colleagues verified theory by studying these different systems; although the two systems displayed more complexity than anticipated.
Approximately 100 million supernova events—giant stellar explosions—have occurred in our galaxy. The ejecta were mixed and averaged, resulting in an homogeneous composition. This homogeneity makes it very difficult to determine the range of element ratios produced by supernovae. McWilliam and colleagues studied the composition of a sample of very old stars, with few elements (dubbed metal-poor) and found that they possess an enormous range in certain element abundance ratios indicating that not all supernovae are alike. Their results indicate that certain elements in the extreme metal-poor stars were dominated by the ejecta from very few supernovae, in some cases from just one. These very rare stars are ideal for testing supernova nucleosynthesis predictions, and to probe the early evolution of the galaxy.
McWilliam received his B.Sc. from London University and his M.A. and Ph.D. from the University of Texas–Austin. Before joining the Carnegie staff as staff astronomer he was a research associate at Cerro Tololo Inter American Observatory, a visiting assistant professor at New Mexico State University, a Carnegie postdoctoral associate and the first Barbara McClintock Fellow at Carnegie. For more information see http://obs.carnegiescience.edu/users/andy
