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Monday, August 4, 2008

First Stars Were Brutes, but Died Young, Astronomers Say


The first stars in the universe were short-lived brutish monsters, and they changed the nature of the cosmos forever, blazing away a dark fog that had smothered space for 300 million years and beginning to enrich the cosmos with the stuff of life.

P. A. Navratil, Texas Advanced Computing Center; J. L. Johnson, T. H. Greif, V. Bromm, University of Texas at Austin

An image from a computer simulation depicting the universe 300 million years after the Big Bang. The first stars blew bubbles of ionized radiation (blue) into the surrounding primordial gas (green).

That is the news from a new computer simulation of the early years of the universe, performed by a group of astronomers led by Naoki Yoshida of Nagoya University in Japan.

The calculations show how small lumps in the distribution of matter and energy could draw in more matter by gravity, heat up, shrink and become the first “cosmic objects” — tiny seeds or proto-stars one one-hundredth the mass of the Sun. In a mere 10,000 years or so, by sucking in surrounding clouds of gas, they probably grew into giant stars at least 100 times as massive as the Sun.

Poetically, those first stars would have blazed brightly and died young, burning out in only a million years, which means that such computer simulations are the only telescopes through which these original stars can be observed.

“The simulations offer a very clear picture of how the first stars formed,” Dr. Yoshida said, in a telephone news conference Wednesday. He and his colleagues reported their findings in a paper published in Science on Friday.

Volker Bromm, an astronomer at the University of Texas, Austin, who was not part of the team, said that Dr. Yoshida’s work had taken simulations of the early universe to a new level, although much work remained to be done. “The ultimate goal of predicting the mass and properties of the first stars is now within reach,” he wrote in a commentary that accompanied the Science paper.

Lars Hernquist of the Harvard-Smithsonian Center for Astrophysics, a member of Dr. Yoshida’s team, described the calculations as an attempt to fill a gap in cosmological knowledge.

Astronomers have a good idea of what the universe was like at an age of 400,000 years from studying a relict haze of microwaves left over from the Big Bang, and they know what it is like today. “This study is designed to understand how objects came into the universe,” he said, “and how they affected what came afterwards.”

The emergence of the first stars, about 300 million years after the Big Bang, was an epochal event for two reasons. First, they lighted up a universe that had been dark since shortly after the Big Bang fires had cooled. Through thermonuclear fusion, they also got the ball rolling on the alchemical transformation of the cosmos, from being composed essentially of pure hydrogen and helium to being littered today with heavier elements like carbon, oxygen, nitrogen and iron.

More massive stars burn hotter and faster and produce heavy elements more copiously than less massive ones. So Dr. Yoshida’s result would mean that this process of enrichment got off to a fast start, which astronomers could test by looking at the abundances of such elements in the lowest-mass and thus oldest stars around.

These massive stars would also have been prodigious producers of ultraviolet radiation needed to ionize hydrogen, which filled the universe like an opaque fog after the Big Bang cooled, and make it transparent to visible light, thus ending what cosmologists call the “dark ages.”

Astronomers have long reasoned that the first stars would have been massive, because without the heavy elements, which astronomers call metals, clouds of helium and hydrogen, the primordial gases, can’t easily cool off. So as the lumps compress under the pressure of incoming material, they heat up and push back. Only for very large amounts of gas can gravity overcome the pressure and the star start to form.

Astronomers have been using computers for decades to simulate the motions of cosmic particles coming together under gravity, but they typically have had to stop when the agglomerations became dense and hot enough for other forces — radiation, heat and gas dynamics — to complicate things. Dr. Yohsida said that his simulations were the first to be able to follow the complex interactions of gas and radiation that dominate evolution of the protostar.

Dr. Yoshida said his computer program, “like a piece of art,” had been seven or eight years in development. The simulations, performed on a network of 70 computer processors, begin with the universe as a nearly smooth mixture of hydrogen, helium and the mysterious dark matter— perhaps clouds of as-yet-unidentified elementary particles — whose gravity shapes the distribution of matter in the universe.

As time goes on, small ripples in the dark matter cause ordinary matter to puddle, heat up, lose energy by radiation and then shrink, eventually forming stable seeds about one one-hundredth the density of water and about one one-hundredth the mass of the Sun. For now that is as far as the rigorous calculations go.

The seeds, however, are surrounded by huge amounts of gas, from which they are likely to grow. By how much depends on further calculations.

Significantly, the computations did not show the gas fragmenting into smaller clumps on the way to becoming the protostar. If the material had fragmented, Dr. Hernquist explained, the first stars would have been closer to the Sun in their masses and life histories and how they died. “It is vitally important to decide in the end exactly how massive these stars are,” he said. That would help astronomers deduce what might have happened to them. Were they scattered to the heavens in supernova explosions or did they perhaps collapse into black holes?

“We don’t know how they die,” said Dr. Hernquist.

Original here

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