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Iron Deficiency Reveals Nearly Pristine Star

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Science  01 Nov 2002:
Vol. 298, Issue 5595, pp. 939b-941b
DOI: 10.1126/science.298.5595.939b

Astronomers have found an ancient star that preserves a chemical record of the infant cosmos. The little star, just now facing the end of its long life, suggests that the first stars in the universe might not all have been the colossi that models predict. “It's astounding that we can glimpse such an early stage of the universe through the composition of this star,” says astronomer Catherine Pilachowski of Indiana University, Bloomington.

Stars are relentless element factories, transforming hydrogen and helium (products of the big bang) into heavier elements such as carbon, oxygen, silicon, and iron. Massive elements also form in the fires of supernova explosions, which spray the rich mixtures into space. Generations of stars have seeded our Milky Way galaxy in this way, altering its primordial composition into a potpourri more conducive to rocky planets and computer chips.

Ancestral stars might persist, burning slowly on the Milky Way's sparse outskirts where new stars no longer arise. Astronomers have scoured space for those objects for more than 2 decades. Previously, the most primitive star found in such searches contained about one 10-thousandth as much iron as the sun. Some researchers speculated that they would never come closer to the so-called Population III—the first stars, born with no heavy elements (Science, 4 January, p. 66).

However, 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 in December 2001 with one of the four 8-meter telescopes in the European Southern Observatory's Very Large Telescope array in Paranal, Chile. Analysis of the light from the star, called HE0107-5240, shows that its 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 team's results appear in the 31 October issue of Nature.

HE0107-5240 might record an imprint of the first supernovas, says co-author Timothy Beers, an astronomer at Michigan State University in East Lansing. For instance, dollops of nickel are evident in the previous most iron-poor star, but HE0107-5240 is nearly nickel-free (see figure). That absence might reflect a basic difference in how the earliest supernovas forged elements, because even a single modern supernova would have supplied enough nickel to pollute the star. “This may be the first example of a true second-generation star,” Beers says. “It's our best look at the starting recipe that led to the rest of the periodic table [of the elements].”

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.

CREDIT: N. CHRISTLIEB ET AL./ESO/VLT-UVES

The star is now a red giant: the bloated end stage of a star that has fused most of its hydrogen fuel. However, Beers notes, it lived for at least 12 billion years as a small star just 80% as massive as our sun. Current models maintain that primitive gas clouds with almost no heavy elements could not have formed tiny stars, because hydrogen alone can't cool clouds to the frigid temperatures needed for small clumps of gas to collapse. Rather, theories hold, the first stars were enormous—perhaps 100 to 1000 times larger than our sun. HE0107-5240 suggests that little stars were in the initial mix as well or were born soon thereafter. Some tiny stars might have formed as companions to gigantic ones and survive as relics to this day, Pilachowski notes.

The Hamburg survey might reveal more primitive stars to help fill in the tale. Christlieb's team has analyzed just one-quarter of its most promising candidates so far.

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