The universe's first stars began as tiny seeds that snowballed into behemoths weighing 100 Suns
(Illustration: David A Aguilar/CfA)
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The very first stars were indeed giants, according to a new simulation. It shows that huge stars coalesced out of the primordial gas about 300 million years after the big bang, ending the cosmic dark ages and spawning later generations of stars, including our Sun.
Other groups have simulated various parts of the process, but a team led by Naoki Yoshida of Nagoya University in Japan has built a much more complete picture of how the first stars formed.
The universe was much simpler then than today, with matter distributed very smoothly and made up mostly hydrogen and helium.
Yoshida's team followed a patch of the early universe that started out slightly denser than its surroundings. Gradually, it collapsed under its own gravity and slowly warmed up.
Eventually hydrogen atoms in the collapsing cloud formed molecules, which can radiate away the building heat. That reduced the thermal pressure and allowed the core of the cloud to collapse further, until it formed a dense "protostar".
"The first object to be formed in the universe is a very tiny protostar with mass just 1% of the Sun," says Yoshida.
Stellar seed
That protostar acts as a seed, he says. It is surrounded by a huge amount of gas, which it quickly pulls in, growing into a star with about 100 times the mass of the Sun.
At that size it would burn very brightly, casting the first light since the big bang and thus ending what is known as the cosmic 'dark ages'. It would also generate the heavier elements that are a vital ingredient of later generations of stars.
One question the team wanted to answer was whether the shrinking gas cloud would become unstable and break up into smaller pieces, leading to much smaller primordial stars. The group saw no sign of that fragmentation.
This study gives us a firm foundation for our understanding of how these stars formed, says Volker Bromm of the University of Texas at Austin, who was not part of the study.
"For the first time one simulation, in one go, has followed the evolution from cosmological initial conditions on scales of millions of light years all the way to the initial protostar on scales of a few solar radii," Bromm told New Scientist.
Violent deaths
After a brilliant million-year lifetime, it is not clear how one of these stars would have died. It depends in detail on how massive it is, says Yoshida's co-author Lars Hernquist of Harvard University. "They might explode as a very bright supernova, disrupting the entire star."
Such a violent end would scatter the star's load of heavy elements into space, and stir up other gas clouds to trigger more star formation.
And such powerful explosions might be visible from Earth, says Hernquist, perhaps showing up very faintly in images from NASA's planned James Webb Space Telescope. If these stars end as gamma-ray bursts – fleeting but powerful blasts of high-energy radiation thought to be caused by jets of matter exploding from a dying star – they might be easier to spot.
Yoshida's simulations reveal one incidental detail. A little while before the true protostar seed is formed, the collapsing core of the gas cloud goes through a stage in which it becomes a flattened disc, with two trailing spiral arms of gas. So, on a small scale, the first objects in the universe may have foreshadowed the great spiral galaxies they would help create.
Journal reference: Science (vol 321, p 647 and 669)
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