Big Bang Ripples Formed Universe's First Stars

Ker Than
for National Geographic News
July 31, 2008
Ripples in the early universe following the big bang 13.7 billion years ago caused gases to coalesce into the luminous seeds of the first stars, a new computer simulation reveals.

Such stellar embryos, or protostars, were the universe's first astronomical objects and its first sources of light.

Previous telescope observations have shown that very distant—and thus very old—cosmic objects contain heavy elements such as carbon and iron, which are formed only by the nuclear reactions inside full-grown stars.

This suggests that massive stars must have existed even earlier in the universe's history than telescopes can see. Until now, the earliest stages of primordial star formation had not been modeled in detail.

"Previous works probed up to only intermediate stages where only gas blobs or dark matter clumps are formed," said lead study author Naoki Yoshida of Nagoya University in Japan.

The new research brings astronomers a step closer to simulating the entire birth of an early star all the way up to nuclear ignition.

Understanding such processes is vital to figuring out how subsequent stars developed and seeded the cosmos with the elements that eventually gave rise to life.

(Related: "Dark Matter May Have Powered Universe's First Stars" [December 6, 2007].)

"If we want to understand how things came about and why they look the way they do now, we have to go back in time and understand how stars looked when they first began to form," said study co-author Lars Hernquist of the Harvard-Smithsonian Center for Astrophysics (CfA).

Simple Recipe

The first adult stars were bright, short-lived behemoths that ended the so-called cosmic dark ages—a 200-million-year period beginning shortly after the big bang when there was no visible light.

When some of these first stars died in explosive events called supernovae, they bequeathed a valuable inheritance of heavy elements to the rest of the universe.

The new model, described this week in the journal Science, traces the initial creation of these stellar firstborn based on the much simpler physics that existed in the early universe.

Back then there were only a few ingredients that could be used to create a star, and complicating factors such as stellar magnetic fields and energetic cosmic events did not yet exist.

Using supercomputers, Yoshida and colleagues were able to follow hydrogen and helium atoms and particles of dark matter through 200 million years of cosmic evolution.

When the simulation concluded, it had produced a churning knot of gas with a mass of just one percent that of our sun—the core of a first-generation star.

But the model stops short of simulating the moment the protostar achieves full nuclear fusion and transforms into a true star.

The physics involved in this next step is much more complicated and will require even more powerful computers to reproduce, the researchers say.

Cosmic Rosetta Stone

The new model is "very impressive work" that gives scientists a much more intimate view of the early stages of primeval star formation, commented Avi Loeb, who is also with the CfA but was not involved in the work.

"Previously what people did was stop the simulation when the gas was collecting within a scale of roughly the size of the solar system," Loeb said. "They were not able to zoom in on the central core."

Volker Bromm is an astrophysicist at the University of Texas, Austin, who wrote a review of the new work that also appears in Science.

Bromm called the new simulation a "cosmic Rosetta stone" that could help unlock the secrets of how modern stars form by providing a more detailed view of how stars took shape under much simpler conditions.

"It is like laying the foundation of a skyscraper," Bromm said.

(Related photo: "Colliding Galaxies Ignite Stellar Nurseries" [October 19, 2006].)

"In the simulations that we had before, we started at the first or second floor and we built a hundred-floor skyscraper. Now we can get in at the ground floor."

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