Published online 31 October 2002 | Nature | doi:10.1038/news021028-6


Astronomers test star's metal

Galactic fossil holds answers to origins of elements.

The Milky Way houses relics from its youth.The Milky Way houses relics from its youth.© NASA/2Mass

Astronomers have discovered a star on the outskirts of the Milky Way that may represent those that filled our Galaxy, and perhaps the Universe, with the chemical elements around us1. This fossil from the early days of our Galaxy contains virtually no metal.

Stars churned out everything in the Universe except hydrogen, helium and a small amount of lithium, which were created in the Big Bang. Stellar nuclear furnaces converted the light elements into heavier ones such as carbon, phosphorus and lead, all of which astronomers, somewhat disconcertingly, refer to as metals.

The newfound star is a dim pinprick 36,000 light years away in the southern constellation Phoenix. It will enable astronomers to perform "stellar archaeology", says Norbert Christlieb of the University of Hamburg in Germany, who led the team of discoverers.

"The gas cloud from which the star formed is mostly preserved," he says. The cloud is representative of the Galaxy just one billion years after the Big Bang, when the heavy elements were beginning to form.

Search for a star

Astronomers predicted that such low-metal objects existed. But 20 years of fruitless searching prompted many to think that they'd got something wrong. Our Sun is rich in metal - mainly iron. There are many stars with 10,000 times less metal, but none with levels as low as those thought to have existed in the early days of the Galaxy.

"We've broken this apparent barrier," says Christlieb. The team sifted through one million candidate stars to find one that fits the theoretical bill perfectly. Their stellar relic, named HE0107-5240, contains 200,000 times less metal than the Sun.

"To find a star with 20 times less metal than any previously found is astounding," says Catherine Pilachowski, who studies star formation at Indiana University in Bloomington. HE0107-5240 also raises new questions, she says. It contains a "striking excess" of nitrogen and carbon. These 'metals' are never usually abundant in stars of the cool, giant family to which HE0107-5240 belongs.

“>To find a star with 20 times less metal than any previously found is astounding.”

Catherine Pilachowski
Indiana University Bloomington

This could explain why extremely metal-poor stars are so rare. They could have very different origins from better-studied stars. To discover whether this is so, more low-metal stars must be found. "We have to look harder," says Pilachowski.

Christlieb's group is hoping to find stars containing just hydrogen, helium and tiny amounts of lithium. "Such a star would allow us to directly study the pristine Big Bang material," he says.

As more powerful telescopes are built, we should extend the search to our neighbouring galaxy, Andromeda, says Pilachowski. To explain the origins of stars and therefore metals in the Universe, it's necessary to look at stars in other galaxies to make sure that HE0107-5240 is not a freak. "We need to be sure we're not being misled by one pathological case," she says. 

Indiana University Bloomington

  • References

    1. Christlieb, N. et al. A stellar relic from the early Milky Way. Nature, 419, 904 - 906, (2002). | Article | ISI | ChemPort |