Closing in on the birth of the first stars. (Iron-Poor Star).

For decades, astronomers have been searching for stars born soon after the Big Bang, around the time the Milky Way began forming. Researchers now report that they've found one of these ancient stars.

According to a widely accepted theory, the Big Bang forged nearly all of the hydrogen and helium in the universe but only trace amounts of a few other heavier elements. In time, as star formation began, the gas that condensed to form galaxies became increasingly enriched with heavier elements. All stars synthesize heavy elements, but the largest supply has come from massive stars exploding as supernovas. The very oldest stars, born before most supernova explosions had a chance to pop off, should therefore contain only a minuscule supply of iron and other metals.

In the Oct. 31 Nature, astronomers report finding a Milky Way star that has just one-two-hundred-thousandth the iron content of the sun, an amount that's just one-twentieth that of the previous record holder among iron-poor stars. The discovery team includes Norbert Christlieb of Hamburg University in Germany and Timothy C. Beers of Michigan State University in East Lansing.

"This is the closest astronomers have come to having direct knowledge of the chemistry of the universe shortly after the Big Bang," says Beers.

That's not to say that the star, which lies within the galaxy's ancient halo, belongs to the very first generation of stars, adds Beers. That the star contains small amounts of iron and other metals, rather than none at all, is evidence that it must have been preceded by a generation of yet older, massive stars that exploded as supernovas, he notes. Dubbed HE01017-5240, the star "may be the first example of a truly second-generation star," Beers says.

Several theorists have calculated that the very first stars were 50 to 300 times more massive than the sun (SN: 6/8/02, p. 362). These behemoths would have died in supernova explosions just a few million years after their birth.

In contrast, the newly discovered star, which lies 36,000 light-years from Earth, is about four-fifths the mass of the sun. Such low-mass stars not only live far longer than heavier stars but are much more common. By measuring the chemical composition of HE0107-5240, "we're starting to get the recipe for how [the more typical] low-mass stars formed," says Beers.

"When I first heard about these results, I was jumping up and down," says Tom Abel of Pennsylvania State University in State College, whose research focuses on simulating the birth of the first stars. He speculates that the star might have formed as long as 200 million years after the Big Bang.

Like several other iron-deficient stars, HE0101-5240 has an abnormally high abundance of carbon and nitrogen. Beers and his colleagues say that the presence of those heavy elements doesn't negate the star being ancient. They suggest that the star wasn't born with these elements but produced them over its lifetime. Alternatively, the star may have been born with a more massive partner that transferred the two heavier elements to it, suggests Beers.

The variations in the abundance of elements heavier than helium, which astronomers sweepingly refer to as metals, is further evidence that HE0101-5240 formed at a time so early in the universe that heavy elements weren't yet evenly distributed, says Abel.

The discovery team analyzed millions of stellar spectra in search of the most ancient stars, narrowing the candidates to about 8,000 metal-poor stars. The scientists then took high-resolution spectra of several stars, including HE0101-5240, with one of the quartet of 8-meter telescopes collectively known as the Very Large Telescope, in Paranal, Chile. If more such stars are found, it could lead to a more refined estimate for the age of the universe.