We may never know for sure what the first stars in the universe looked like, but a new computer simulation has provided researchers with some initial brush strokes. In tomorrow's issue of Science, an international team reports that they have simulated the birth of a protostar--a nugget that could grow eventually into a supermassive star--starting with the basic ingredients and conditions that existed in the early universe.
Cosmologists probably know more about the first few minutes after the big bang--thought to have occurred about 13.7 billion years ago--than they do about the following billion years of the universe's existence, sometimes known as the cosmic dark ages. Within that interim, the first stars formed and began to light up the universe.
The problem with studying those very early stars is that they no longer exist. According to theory, all or nearly all of them began as supergiants hundreds of times more massive than the sun. Then they expended their nuclear fuel and exploded within a few million years. This early demise was good for the evolution of the universe, because the supergiants dispersed the heavy elements necessary to form smaller stars as well as planets and, eventually, people. But unfortunately for scientists, the primordial beasts also expunged all detectable evidence of themselves.
A team led by physicist Naoki Yoshida of Nagoya University in Japan set out to fill this cosmic evolutionary gap in the only way currently possible: They carried out a computer simulation that duplicates the process of star formation in the very young universe. The task actually proved easier than it would have been to simulate stellar evolution in the present day. That's because the current galactic environment is full of magnetic fields and turbulence, both of which greatly complicate the creation of accurate simulations. Not so in the first few million years after the big bang. Back then, all stars needed to form was a primordial soup of mostly hydrogen and some helium atoms, perturbed by the effects of gravity on minuscule differences in the density of the gases, and the mysterious substance known as dark matter.
Taking those elements, the researchers traced the collapse of a primordial gas cloud, from a halo roughly 1000 light-years in diameter, to a newborn but still unignited protostar with a radius about 25 times that of the sun. Astrophysicist and co-author Lars Eric Hernquist of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says that a protostar could grow quickly, accreting about 10 solar masses within 1000 years, and reaching its full size of 100 or more times the sun's mass in as little as 10,000 years. That's fast enough to crowd the young universe with supergiants whose explosions would provide the materials for future stars and galaxies. But it's still difficult to determine how many primordial stars there were, Hernquist says.
The simulation shows the creation of the first protostars in "great detail and physical realism," says astrophysicist Volker Bromm of the University of Texas, Austin. It "puts the finishing touches to the decade-old quest to build the first star from first principles," he says. But Bromm adds that the investigation needs to be carried forward to see how the primordial stars continued to evolve. It's possible, he says, that there might be some unforeseen process at work that could change current understanding of early star formation.
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