George Preston

George Preston, with Carnegie collaborators Andrew McWilliam, Stephen Shectman, and Ian Thompson, is engaged in the astronomical equivalent of looking for radium in pitchblende. He is searching for extremely rare astronomical relics -- the earliest generations of stars formed in the Milky Way. The signature of these relics is the virtual absence of elements heavier than helium in their atmospheres. They are identified by wholesale examination of spectra from the myriad of stars that inhabit the dense central bulge of our galaxy.

The very first generation of stars was made of Big Bang material, that is, hydrogen and helium with a trace of lithium. All the heavier elements have been produced by nucleosynthesis in massive stars that explode as supernovae after they exhaust their fuel supplies. Debris from the first supernova explosions polluted the galactic gas. Out of this gas, future generations of stars formed. Continual repetition of this process for the past 15 billion years or so has gradually increased the heavy-element content of the universe to its present level.

The second generation of stars contained no more than one-tenth of a percent of the metal content of the Sun. These stars comprise no more than one-tenth of a percent of the stars formed since the beginning of time. These are the objects sought by the Carnegie team. Because they are made of gas enriched by single, or at most a few, supernova events, their chemical compositions tell what particular combinations of heavy elements are produced in various supernova explosions. What little was learned about the "yields" of supernovae from an earlier Carnegie survey reveals that these explosions are not all alike. Their debris exhibits enormous variety, which means that the accumulation of all the chemical elements to their present levels is a complex process of which we have only a dim comprehension.

The Carnegie team conducted the first phase of their new search for extremely metal-poor stars in 1997 and 1998 at Las Campanas using the Swope and du Pont telescopes. They performed CCD photometry in 50 star fields in the galactic bulge with a special set of filters that identify metal-poor candidates among the 100,000 red giants present. Unfortunately, the metal-poor signature can be forged by a few other exotic groups of stars -- those with very high velocities, powerful chromospheric emission, or large carbon excesses. GRISM¹ spectroscopy of the metal-poor candidates initiated at the du Pont telescope in 1999, however, eliminated the forgers to produce a list of bona fide metal-poor stars. "We don't even know where the carbon in our bones came from," says Preston. He expects that in the next decade spectral analysis of these extremely metal-poor stars at the Magellan telescopes will shed light on this and many other questions about the creation and evolution of the chemical elements in the universe -- and in our bones.

Fig. 9. This image shows spectra (plots of light intensity versus color progressing from blue to red) for two stars. The spectrum of the Sun with present-day levels of heavy elements (above) looks like a picket fence. The spectrum of CD -38:245, a star of similar temperature but with 10,000 times fewer heavy elements (below), looks almost featureless. Such stars are rare but unmistakable.

¹ A GRISM is a special combination of diffraction Grating and pRISM used to analyze starlight.