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The world we know, including our bodies, is constructed from the chemical elements, most of which were produced by nuclear reactions in the interiors of stars. When a star dies a fraction of the elements synthesized during the star's lifetime 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. Red Giant stars preserve much of their original chemical composition in their outer envelopes, and can have ages up to the age of the Galaxy; thus their compositions comprise a fossil record of chemical evolution. Andrew McWilliam measures the detailed chemical composition of Red Giant stars to answer questions such as: What are the sites of nucleosynthesis? What physical conditions modulate element production? What can be learned about galactic history by reading the fossil record?
To test nucleosynthesis and chemical evolution theory, McWilliam is studying the composition of Red Giant stars in systems with a variety of characteristics. 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. Thus, the bulge composition should reflect nucleosynthesis by short-lived stars, whilst the Sagittarius dwarf galaxy should show abundance patterns characteristic of long-lived stars.These expectations have been verified by McWilliam and colleagues; although these two systems displayed more complexity than anticipated.
Approximately 100 million supernova events 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, extremely metal-poor, stars and found that they possess an enormous range in certain element abundance ratios, which showed that supernovae are not all alike; rather, they produce a variety of element ratios. The dispersion indicates that certain elements in the extreme metal-poor stars were dominated by the ejecta from very few supernovae, in some cases a single supernova event is indicated. These very rare stars offer the potential to test supernova nucleosynthesis predictions, and to probe the early evolution of the Galaxy.
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