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For twenty-five years, George Preston has been deciphering the chemical evolution of stars in our Milky Way. He and Steve Shectman started this quest using a special technique to conduct a needle-in-the-haystack search for the few, first-generation stars, whose chemical compositions sketch the history of element formation in the Galaxy. These earliest stars are very rare and they are characteristically low in heavy metals because of their age. They were made of Big Bang material, mostly hydrogen and helium. It was only later that heavier elements were formed in the nuclear furnaces of newer stars.
In their first study, Preston and Shectman compiled a list of hundreds of candidate low-metal stars. They followed it with a more detailed analysis confirming their status, one by one. Over the years, others joined the effort including Carnegie’s Andy McWilliam and Ian Thompson, Tim Beers of Michigan State University, Chris Sneden of the University of Texas, John Cowan of the University of Oklahoma, and most recently Inese Ivans, the new Carnegie-Princeton Fellow.
Preston is still exploring the consequences of the very first survey using the facilities at Las Campanas. He is tracing the rates of atomic enrichment of different atomic species produced by various nuclear mechanisms. He uses the decay rate of radioactive thorium in some of the oldest stars to measure their ages. By discovering traces of the heaviest stable elements, lead and bismuth, he is also looking into the processes in other stars to refine theories of special nucleosynthesis—a process that creates and expels elements in certain dying stars. He also explores the mysteries of mass exchange between members of old binary star systems that contain these dying stars. Most recently he has turned his attention to a surprising find—a recently-discovered pulsating (RR Lyrae) star highly enriched in carbon, a characteristic that defies experience and prior expectations. Preston just wishes that he could be thirty years old again to take full advantage of the wonderful telescopes and instruments with which the Observatories are now endowed.
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