The first stars formed when the universe finally cooled enough and expanded enough to allow gravity to go to work. Hydrogen and helium were drawn together to form "gravity-bound puddles of gas," as Norman, a senior research scientist at NCSA and a professor of astronomy at the University of Illinois at Urbana-Champaign, calls them. These puddles, also called primordial star-forming clouds, then condensed to a star's density relatively quickly.

"It took 100 million years for those clouds to form, but then it only took about 100,000 years for the first stars to form from them," Norman says.

The computer simulation of this process was also relatively quick. In about two days, using NCSA's SGI Origin2000 supercomputer, Norman and his collaborators recreated this pooling of cosmic gases into star-forming clouds.




The simulation started with a vast hypothetical section of the young universe, a cube 18,000 light years on each side. It then tracked the formation of a cloud, zooming in on interesting areas using a technique known as adaptive mesh refinement, until the simulation was focused on an area of space about one-third of a light year on each side.

In effect, the simulation started in a space the size of a large gymnasium and focused in on a single atom within that gymnasium.



That sort of resolution - like setting your telescope to a power of one million without losing any of the clarity - was simply unprecedented. And, it allowed for a great glimpse into the size and characteristics of those first stars.

Before these simulations, some scientists said the first stars would have been 1,000 times smaller than our sun; others said they would have been one million times larger. The Alliance team's simulations suggest the clouds would have formed stars about 10 to 100 times the mass of our sun.

"We've hemmed the speculation in dramatically," Norman says.

Because of their size and density, these stars died quickly, burning out in a few million years. But they also served a purpose.

Heavy elements such as carbon, oxygen, and iron formed in their cores, and these elements were blown back into circulation when the stars exploded. New stars developed with these elements as part of their cores. Born of the material that their precursors created, the new stars burned longer and more efficiently.

"We simulated that first little fish that's eaten by the bigger fish and then a bigger fish," Norman says.

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