How a star is born

New simulation accurately tracks seeds of first stars

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Out of the darkness came the light. About 400,000 years after the Big Bang, the radiation from that fireball cooled and faded, plunging the cosmos into blackness. A few hundred million years later, the first stars emerged and relit the universe. And now scientists have a better idea how it happened.

WHEN GAS MET DUST This computer simulation reveals, starting clockwise from top left, how gas and dust came together about 300 million years after the Big Bang to form the first stars in the universe. The new simulation is the first to accurately track this process’s initial stage, when a stellar embryo, or protostar, forms at the center of a halo of dark matter. Each frame zooms in on the emergence of the protostar, at increasingly smaller scales. Click on image for full story. Yoshida et al., Science

STAR SEEDS The first stars began as tiny seeds, or protostars, that grew rapidly into luminous bodies about 100 hundred times the mass of the sun. In this artist’s impression, swirling clouds of hydrogen and helium gases are illuminated by the first starlight in the universe. In the lower portion, one of these massive stars has exploded as a supernova, ejecting heavier elements that will someday be incorporated into new stars and planets. Click on the image for a full story. D. Aguilar/CFA

Astronomers have long struggled to accurately model this milestone event. Simulations suggested that clouds of dark matter — invisible material making up more than 80 percent of the mass of the universe — gathered and compressed pockets of the hydrogen and helium gases forged during the Big Bang. When the compressed gases achieved densities high enough to ignite nuclear reactions, a star was born. And according to these models it was a whopper — 100 to 300 times the mass of the sun.

But such simulations arrive at the final mass estimate by leapfrogging over some of the trickier astrophysics, making the models uncertain. A new model, described by Naoki Yoshida of Nagoya University in Japan and his colleagues in the August 1 Science, for the first time simulates the formation of a primordial star without having to rely on an array of approximations.

A key to their success, the researchers say, was a painstaking accounting of the complex interactions between the gases and radiation. Those interactions determine how efficiently the gases can radiate away heat and resist gravitational collapse by exerting an outward pressure. Unless those interactions are incorporated, “you can’t get to the point where you can evaluate how the first stars formed,” notes coauthor Lars Hernquist of the Harvard-SmithsonianCenter for Astrophysics in Cambridge, Mass.

After nearly two years of testing, it took a month of computing time to run the simulation. It’s the first model that connects the universe’s primordial fluctuations in density to the growth of a bona fide fledgling star, Yoshida says.

The simulation tracks the gases that pack together inside a sufficiently massive dark-matter cloud, or halo, about 300 million years after the Big Bang. Over the course of about 100,000 years, according to the model, the compressed gases reach densities roughly equivalent to that of liquid water on Earth.

At that point, the gases inside the halo have formed a protostar, about one-hundredth the mass of the sun.

Although the simulation stops at this point, Yoshida and his colleagues estimate that in about 10,000 years — an eye blink in astronomical terms — the protostar will pack on enough additional material to become a star about 100 times as heavy as the sun.

Yoshida and his colleagues “have reached a crucial halfway point by pushing their simulation all the way toward the formation of a primordial protostar,” notes Volker Bromm of the University of Texas at Austin. The next step, he says, is to track precisely how the light protostar becomes a heavyweight. Reaching that final frontier will require a careful accounting of the rapid and complex interactions involved as the protostar piles on more material.

The birth of the first stars fundamentally changed the universe. Their emergence not only ended the cosmic dark ages — the murky period after the fading of the radiation from the Big Bang — but set the stage for the formation of galaxies and galaxy clusters as they appear in the universe today, Bromm says. For instance, although these heavyweights lasted for only about a million years, their death in supernova explosions seeded the universe with the first elements heavier than lithium.

In the July 1 Monthly Notices of the Royal Astronomical Society, Bromm, Thomas Greif of UT–Austin and the Institute for Theoretical Astrophysics in Heidelberg, Germany and their colleagues pick up the storyline after the death of the very first stars, about two millions years after the simulations by Yoshida and his team end. That’s when gravity collects the black-hole remnants of the first stars and the heavy elements they produced into young galaxies that reached temperatures high enough to excite atomic hydrogen, an extremely efficient way to radiate away heat and promote further star formation. The newer mode of cooling would also cause fledgling stars to fragment, resulting in stars that have much lower masses — and last much longer — than the very first batch, which was studied by Yoshida’s team.

In their simulation, Bromm and his collaborators tracked filaments of cool gas that, driven by gravity, stream at high speeds into the young galaxies from their parent dark-matter halos. The turbulence created when different high-speed streams barrel into the galaxy’s center is likely to trigger the first generation of lower-mass stars, Bromm and his colleagues suggest. Some of the ancient stars observed in the outskirts of the Milky Way may have formed in this way, the researchers propose. Studying these stellar fossils may therefore reveal the conditions that prevailed at the end of the dark ages, Bromm says.

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