Relatively soon after the Big Bang, things started percolating in the infant cosmos, the scientists wrote Thursday in an online article in the journal Science.

From the uniform mixture of sub-atomic particles and radiation present at the dawn of the universe, clouds of gases began forming alongside invisible dark matter, whose presence is known only due to its gravitational impact on celestial objects.

Tom Abel of the Harvard-Smithsonian Center for Astrophysics and Britain's Cambridge Institute of Astronomy said these clouds of gases at some point reached a critical mass and collapsed in on themselves. This gravitational collapse resulted in the formation of huge, bright stars unlike any that exist today, added Abel, the study's senior author.

These original stars were huge, with a size range of about 30 to 300 times the mass of the sun, with 100 solar masses the most likely size, Abel said. But none of them still exist, having blown apart eons ago in supernovae -- star explosions.

"The first star formed a few hundred million years after the Big Bang," Abel said in a telephone interview.

"They only formed out of hydrogen and helium gas, and that is actually what influences their properties substantially. So they are brighter than the regular star ... and also hotter and larger," Abel added.

The scientists could only speculate about the number of these original stars that populated the universe. "The exact number, of course, we don't know. Potentially, there could have been about 10,000 or so of these objects in the region that later became our Milky Way," Abel said.

Abel's co-authors were Michael Norman of the University of California at San Diego and Greg Bryan of the Massachusetts Institute of Technology.

Best evidence to date Because these theoretical original stars are long gone, scientists resorted to simulations to study how they formed.

The scientists believe their cosmological simulation provides the best evidence ever gathered about the formation of the original stars and could help settle a lively debate over the nature of the first large celestial objects.

"The nature of the first objects was not known -- whether they were stars, super-massive black holes, people even speculated about brown dwarfs (objects slightly too small to qualify as stars) or Jupiter-sized objects (even smaller). Our study finds that they were massive stars," Abel said.

Another surprising result, he added, was that the collapse of these large clouds of helium and hydrogen formed a single huge star rather than a bunch of smaller ones.

"The surprising thing that we find is that these massive stars form in isolation," Abel said. "In our own galaxy, most of the stars actually form in clusters of stars, so they always come as multiples. But we showed they form alone in their own little protogalaxy or pre-galactic object."

Co-author Norman said the thermonuclear furnaces at the cores of these stars probably forged the first elements heavier than helium and hydrogen. Scientists believe it has taken many generations of stars -- each processing the debris left behind by earlier ones and then belching it out through supernovae -- to produce the abundance of elements now found in stars and elsewhere in the universe.

Norman said the first heavy elements had to have been synthesized, released and distributed through star explosions within the first billion or so years after the Big Bang.

The results reported in Science were carried out on a supercomputer at the University of Illinois. New calculations aimed at determining more about these first stars, including their size, are running on a machine at Norman's university.

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