University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Colloquium

Wednesday, November 4th 2009
3:35 pm:
Speaker: Alexander Heger, University of Minnesota
Subject: The Deaths of the Biggest Stars
Refreshments served in Room 216 Physics after colloquium

A few hundred thousand years after the Big Bang the primordial gas recombined, became transparent - the last light from that we now see as Cosmic Microwave Background. There was very little structure in the universe at that time, no source of light - we call it now the Cosmic Dark Ages. It would take several hundred million years before the first stars in the universe would form, making the first source of light after the big bang, when the "dark" matter could clump, collecting up baryonic matter that could cool down, and condense into the first stars. Still, the gas was quite warm, so it would take massive, big clouds to collapse under their own gravity, making big stars. At least, this is what out best theories tell us. But no one actually has ever observed this to date. So, is that story true? How big were the first star actually? What can we do to find out? Looking at the current universe, we do see that some quite big stars are still formed today, but they shed mass in massive winds and in giant eruptions and will die not quite as big as they were born. Is the same true for the first stars if born as big as the biggest stars we see today? Or were there even bigger stars, and how would the evolve and die? Could these latter once be the predecessors of the supermassive black holes harbored in the centers of even some of the earliest galaxies we see? So, how can we find out? At least part of the story we may be able to uncover now by looking at ashes of the first stars, the pattern of elements that made and that were incorporated in subsequent generations of stars. Observes now have found some very old stars in the halo of our galaxy, one of its oldest constituents, that likely have formed very early in the universe. These stars have only miniscule traces of heavy elements, they almost exclusively consist of the matter made by the big bang. We now believe that many of those likely have been "polluted" only by a few, maybe a single star. But the ratio of heavy elements that a star makes depends a lot on how massive is was, and how it died. So looking at the ashes of these first stars, as incorporated in the old stars we have found, may tell the story of the lives of those first stars. What will we discover?

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