Scientists simulate early cosmos, find gigantic stars

For much of the universe's first billion years, the searing brightness born of the big bang faded to black. This dark age represents the least-understood chapter in the history of the cosmos scientists have compiled.

On Friday, researchers report they have glimpsed – via computer simulations – the birth of the first small, stable clumps of gas that would have served as seeds for the first generation of stars. Within 10,000 years, the scientists say, these seeds would blossom into blazing orbs at least 100 times more massive than the sun.

The simulation is part of a broad effort to fill the dark-age gap. Astronomers worldwide are pushing ground-based optical telescopes to their limits, building vast radiotelescope arrays and looking to a new generation of space- and ground-based telescopes to probe this crucial period.

The nuclear furnaces in the first stars would have formed the first atoms of carbon, silicon, oxygen, and other heavy elements, researchers hold. These elements would become incorporated into later generations of stars, which in turn would add their contributions to the chemical inventory.

Over time, clusters of stars would form galaxies whose combined radiation would eventually shift the cosmos from opaque to transparent. The heavier elements the stars forged and launched into the cosmos would form basic organic and inorganic molecules, and become the raw material for planets.

"We have a good understanding of what the universe looked like shortly after it originated about 14 billion years ago. We also have a good idea of what the universe looks like now," says Lars Hernquist, a Harvard University astrophysicist and member of the team. "But there's a significant gap in our understanding of how the universe made this transition from what it looked like after the big bang to how it appears to us today."

Until new tools can peer more deeply into that gap, simulations remain the only vehicles for exploring the transition.

In a young, dark universe

The research team, led by Nagoya University's Naoki Yoshida, started with a universe dominated by recently discovered dark energy and by cold dark matter, which astronomers currently detect by its gravitational influence on matter they can see. Hydrogen dominates the small percentage of "normal" matter in this young, denser universe. It's in a form that renders it opaque to light.

The simulation picks up the story when the universe was roughly 300 million years old and 20 times more compact that it is today. The afterglow of the big bang had long since faded. Subtle variations in the density of dark matter across space led to regions where dark matter was more dense than others.

The simulation focuses on one of these denser areas, or halos. There, dark matter's enhanced gravity corrals hydrogen. The hydrogen cloud undergoes alternate periods of heating and cooling as it contracts due to gravitational collapse. It also shifts from cloud to flattened disk and finally to a stable, spherelike proto-star.

At this stage, with 1 percent of the sun's mass (or about 10 times Jupiter's mass), the proto-star's internal temperature has risen high enough to generate an outward pressure that prevents further collapse.

The stimulation stops there, but additional calculations suggest that within 1,000 years, the proto-star would have grown into a star 10 times more massive than the sun. By 10,000 years, the star would have topped 100 times the sun's mass.

Such huge stars are thought to have lived only for about 1 million years, compared with an expected lifetime of roughly 10 billion years for the sun.

The new results are important, says Tom Abel, a researcher at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University who published his own group's simulations in 2002. They confirm his team's results, which showed that these first stars probably grew in isolation from each other, and that they don't fragment along the way, as some have suggested.

But the new work also traces the formation of the first stable proto-stars. No star could form without these stable proto-stars, even in today's universe, Dr. Hernquist says.

New tools to see more stars

Now, cosmologists who focus on theory have their work cut out for them, Dr. Abel says in an e-mail. "We know exactly what happens until the first stars are one-hundredth the mass of the sun. We know they will attain approximately 100 times the solar value."

The task is to provide accurate pictures of what goes on in between, while a star's mass is burgeoning by a factor of 10,000, he adds. This will require far more complex programming and more computer horsepower.

Meanwhile, astronomers are pressing existing tools into action to try to push back the veil on the dark ages, notes Elizabeth Barton, an astronomer at the University of California at Irvine. For instance, astronomers have reported the discovery of large galaxies when the universe was roughly 780 million years old. Some tantalizing but disputed evidence has emerged for galaxies or galaxy-wannabes some 300 million years earlier.

Studies of second or third-generation stars will help, many of which may be orbiting in the halo of the Milky Way galaxy.

Astronomers in the US and China are also building an enormous radiotelescope array consisting of about 10,000 antennas in China's Xinjiang Province. They plan to use it to map the first glowing "bubbles" that appear as radiation from galaxies begins to ionize the dense fog of hydrogen between them and light it up.

"The only way we're ever going to understand this is when the theoretical work that's progressing rapidly in this field meets up with the observations work, which is coming from the other direction" of the timeline, says Dr. Barton.

"It's a big puzzle," she adds, and the new results, which appear in Friday's edition of the journal Science, represent a small but important piece.