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The Purest Star

30 October 2002 (All day)
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Clean slate. Ultraviolet spectral lines of iron and nickel reveal that a newly found ancient star (third from top) contains the lowest proportion of heavy elements yet seen.

Astronomers have found a primitive star that preserves a chemical record of the infant cosmos. The little star, just now facing the end of its long life, dates to an era immediately after the formation of the very first stars.

Ever since stars began to shine, they have been relentless element factories. Stars transform the raw products of the big bang--mostly hydrogen and helium--into heavier elements such as carbon, oxygen, silicon, and iron. When stars explode as supernovas, they forge massive elements and spray the rich mixtures into space. When astronomers come across a star that doesn't incorporate many of these heavy elements, they know it must predate most supernovas. The oldest star found in the outskirts of the Milky Way, for example, contained about one ten-thousandth as much iron as the sun.

Now, an ambitious survey of more remote parts of the galaxy has uncovered a star 20 times as anemic. Astronomer Norbert Christlieb of the University of Hamburg, Germany, and his colleagues scrutinized the star, called HE0107-5240, in December 2001 with the European Southern Observatory's Very Large Telescope array in Paranal, Chile. Analysis of its light shows that the atmosphere is a strikingly unspoiled broth of hydrogen and helium with the barest dash of heavy elements: just one iron atom for every 7 billion atoms of hydrogen. The star's near-purity suggests that just a few earlier supernovas--and perhaps only one--polluted its primordial composition. "This may be the first example of a true second-generation star," says co-author Timothy Beers, an astronomer at Michigan State University in East Lansing. "It's our best look at the starting recipe that led to the rest of the periodic table [of the elements]."

The team's results, which appear in the 31 October issue of Nature, impress other astronomers. "It's astounding that we can glimpse such an early stage of the universe through the composition of this star," says Catherine Pilachowski of Indiana University in Bloomington. Tiny stars such as HE0107-5240 might have formed as companions to the first short-lived stars, which probably were gigantic and unstable, Pilachowski notes.

Related sites
Hamburg/European Southern Observatory telescope survey of ancient stars
Science article on the first stars in the universe

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