Researchers may not be able to spot the universe's first stars in their telescopes yet, but that hasn't stopped them from taking a close look at how those fireballs emerged from the cold, dark days when the universe was young.

New three-dimensional simulations published in Science show the series of steps by which the uneven fog of hydrogen atoms present before stars took shape would have clumped into protostars—dense balls of hydrogen just 1 percent the mass of the sun.

These stellar seeds would eventually grow into full-fledged stars larger than the sun, fusing hydrogen into helium and then the rest of the elements that make up modern stars, planets and life itself. The results help illuminate the "cosmic dark ages" during the first billion years after the big bang that kicked off the universe some 13.7 billion years ago.

Variations in the cosmic microwave background radiation tell researchers that matter resulting from the big bang was unevenly distributed. Stars and galaxies formed in areas where gas was concentrated. Sounds simple enough, but the huge distances involved have made simulations challenging.

In the new study, researchers from Nagoya University in Aichi, Japan, the National Astronomical Observatory of Japan in Tokyo and Harvard University computed the temperature and density of gas across thousands of light-years (much greater than the average distance between stars) at a resolution of about 50,000 miles (80,500 kilometers), 10 times smaller than the radius of the sun.

They focused on the time when the universe was about 1.3 billion years old. The simulated gas condensed under its own gravity into clouds of hydrogen molecules measuring tens of light-years across but containing only as much mass as the sun. The cloud cooled by emitting radiation, which allowed its core to form a flattened, rotating spiral.

After it could cool no more, the center of the spiral congealed into a relatively stiff ball measuring roughly three million miles (five million kilometers) in size and reaching a temperature hotter than 18,000 degrees Fahrenheit (10,000 kelvins) in its center. The simulation stops short of true star formation, however, because shock waves emanating from the protostar made its behavior too complicated.

The results confirm earlier, more simplified simulations, researcher Abraham Loeb of the Harvard–Smithsonian Center for Astrophysics says. What remains unclear, he adds, is how much gas the core accumulates after it begins to burn hydrogen, which determines the final mass of the star and ultimately its fate—supernova or black hole.

Loeb says it is widely believed that the star tops off at a few tens of solar masses, which when it died would yield a supernova and blow heavy elements into space and seed the formation of smaller stars. But there's a chance the star could keep growing until it reached several hundred solar masses. In that case, it would probably collapse into a black hole and take its heavy elements with it.

One major goal of the James Webb Space Telescope (JWST), the successor to Hubble scheduled for launch in 2013, is to glimpse faint light from the first galaxies to confirm researchers' understanding of the earliest stars.

Astrophysicist Volker Bromm of the University of Texas at Austin wrote in an editorial accompanying the study that the combination of JWST with other experiments and improved simulations "promises to close the final gap in our cosmic worldview in the decade ahead."