New cosmological simulations performed on a supercomputer have provided astrophysicists with the best indication to date of how the first star in the universe formed. The simulations, detailed in a paper in the November 16 issue of Science, suggest that the first star resulted from the gravitational collapse of a cloud of hydrogen and helium some 100 times more massive than the sun.
“Our modeling suggests that the first star may have condensed from a protogalactic cloud into a self-gravitating, fully molecular mass of hydrogen and helium at least 100 times that of our sun,” says Michael L. Norman, a professor of physics at the University of California, San Diego and a senior fellow at UCSD’s San Diego Supercomputer Center. “We think that such objects, one per protogalactic mass, may have become the first stars to shine in the universe.”
In addition to Norman, who is also an astrophysicist at UCSD’s Center for Astrophysics and Space Sciences, the other authors of the paper include Tom Abel of the Harvard-Smithsonian Center for Astrophysics, now working at England’s Cambridge Institute of Astronomy, and Greg L. Bryan of the Massachusetts Institute of Technology.
One important consequence of star formation are the chemical elements produced in stars. Elements heavier than lithium, which astronomers call metals, that occur naturally throughout universe, are the result of nucleosynthesis, the process by which stars forge heavier elements from lighter ones within the nuclear fires of their cores. Astrophysicists believe that it has taken many generations of stars, each processing the debris left by earlier ones and then distributing them through massive star explosions, or supernovae, to produce the elemental abundances revealed by spectroscopic observations of the stars, gas and dust found in the universe.
Elements heavier than hydrogen and helium are found even in stars so far away that the epoch of their formation corresponds to a time when the universe was only about 10 percent of its current age.
“Thus the first heavy elements had to be not only synthesized, but also released and distributed through the intergalactic medium, within the first billion or so years after the Big Bang,” says Norman. Only supernovae of sufficiently short-lived massive stars can provide such an enrichment mechanism.
While astronomers agree that the first generation of cosmic structures formed massive stars, there has been no general agreement on the nature of the first large-scale structures. Globular clusters, super-massive black holes and Jupiter-size bodies all have been proposed. Yet the evolution of the large-scale structure of the universe depends very much on the details of the very first structures to form.Norman and his former students have worked for a number of years to increase the predictive ability of their model of the early universe, "basically waiting for the size of the supercomputers to catch up to the spatial dynamic range that we need," he explains.
The group's latest calculations extend its previous calculations by some five orders of magnitude. “We follow length scales from a few kiloparsecs down to 100 solar radii,” Abel says, “while calculating gravity, hydrodynamics, primordial gas chemistry, and radiative processes accurately.”
“The formation of the first star takes place in a simpler environment than any other: the gas is just hydrogen and helium and the initial conditions can be precisely specified,” Norman says.
What was the first star like? “The picture we get from our simulations suggests that all metal-free stars are very massive and form in isolation,” Norman says. While their supernovae occurred so long ago that we have seen no remnants, measurements of stars in some galactic halos suggest that these stars were enriched by a single population of massive stars, which would support the picture derived from the calculations. With a few more orders of magnitude in dynamic range, it will be possible to close in on the actual mass and fate of the very first stars.
The results reported in Science were carried out on a 16-processor SGI Origin 2000 supercomputer at the National Center for Supercomputing Applications (NCSA) in Illinois; current calculations are running on the 1100-processor IBM Blue Horizon machine at UCSD’s San Diego Supercomputer Center.
Norman is eager to extend the calculations to a new, multi-center, networked TeraGrid of computers at NCSA, SDSC, Caltech and Argonne National Laboratory. The $53-million configuration, financed by the National Science Foundation, will have a peak speed of about 14 trillion calculations per second.
It will allow Norman and his group to extend the dynamic range of its calculations on both the high end (to start with a larger portion of the universe) and the low (to follow the condensing protostellar cloud to the point of ignition).
Norman compares the range of the computer code written by the team of astrophysicists to perform their new calculations to the dynamic range of a great orchestra. “Bruno Walter was famous for being able to deliver, with equal clarity, the tinkling of a triangle or the roaring of a cannon of kettledrums and brass,” he says. “We hope to be thought of as the 'Bruno Walters' of astrophysics.”
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