Weekly Science Update

Wednesday, August 20, 2008
The First Stars
New stars are continually forming in clouds of gas and dust in our galaxy. Astronomers at the CfA and elsewhere who watch these births have a very good (though not perfect) understanding of how and why they happen. The natal regions, however, are all rich in elements like carbon and oxygen whose properties facilitate the birth process. These elements, however, did not exist in the early universe - only hydrogen and helium (and a trace of some others) were made in the big bang processes of creation. All the other elements in our world were fabricated in the fusion furnaces of stars, and later ejected into space in supernovae or winds. How, then, did the very first generation of stars in the cosmos come to be without this facilitating material? Answering this fundamental question has long been a goal of astronomy, yet the first stars -- however they came to be -- must have existed so long ago and be so far away that observing any individual ones is out of the question for today's technology.

Theoretical models of the first stars, however, designed using complex computer simulations, have made considerable progress in answering the question. They show that because the first stars could contain only hydrogen and helium, they must have been much more massive than our sun, perhaps one hundred or more times bigger, in order for gravitational collapse to lead to nuclear ignition.

CfA astronomer Lars Hernquist, together with two of his colleagues, has published a new paper in last week's journal of Science that carries these simulations to new levels of precision. Starting with basic cosmological information about the distribution of matter after the big bang, the new computations track the evolution of the primordial clumps of material on spatial scales from hundreds of thousands of light-years down to a small fraction of a stellar radius, a remarkable dynamic range of about ten trillion, and the first time computations have been so detailed. The computations also follow the steadily increasing density of the material, and do so over an even broader range, a factor of nearly a billion trillion, from its state in the diffuse gas until stellar ignition is imminent.

The team's results show that the earliest onset of stellar behavior can occur even when these protostars are much less massive than our sun (as little as 1% of a solar mass). These objects can then act as seeds onto which additional material accretes to create more mature, and much more massive stars. Apparently these small protostars can form in part because disk-like structures develop that allow such small masses to emerge, in contrast to purely spherical collapse that results in much larger protostars. The results are an important advance in our understanding of the nature of the very first stars and how they develop from the minute density ripples left a few hundred million years after the big bang.

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