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Two old stars in our Galaxy are posing a challenge to the standard model
of the big bang. Although they formed from material nearly unchanged since
the big bang, they have far less of the element lithium than the big bang
theory predicts.

Cosmologists believe that hydrogen, helium and lithium were produced
in the big bang. In 1981, the French astronomers Monique Spite and Francois
Spite found that the ratio of lithium to hydrogen in old stars was about
10 ^-10 to 1, in agreement with predictions of the big bang theory
(Nature, vol 297, p 483). But last year astronomers discovered that an old
star named G186-26 had no detectable lithium at all (Astrophysical Journal
Letters, vol 373, p L47).

Now Julie Thorburn of the University of Chicago’s Yerkes Observatory
in Wisconsin has found two more such stars, named G122-69 and G139-8. Because
they have ratios of iron to hydrogen that are only 1/350th the Sun’s, the
stars must have formed 10 to 15 billion years ago, when the Galaxy itself
had little iron.

Thorburn used the 4-metre telescope at Kitt Peak National Observatory
in Arizona to obtain spectra of the stars. Lithium absorbs red light at
a wavelength of 6707 Angstroms, but Thorburn could find no sign of such
absorption in the stars’ spectra. This means that G122-69’s lithium-to-hydrogen
ratio is less than 10 ^-11 and that G139-8’s ratio is less than
1.8 x 10 ^-11; these numbers are between a fifth and a tenth of
that predicted by the big bang theory. Thorburn also confirmed the low lithium
abundance of G186-26.

Of other stars as old and iron-deficient as these, only 11 have been
studied for lithium, so the three lithium-deficient stars constitute 20
per cent of the sample. The other stars all have higher levels of lithium,
as the big bang theory predicts.

If the amount of lithium in the three lithium-deficient stars accurately
reflects the quantity produced by the big bang, the entire theory would
be in jeopardy, because it predicts a much higher abundance. But Thorburn
makes no such claim. She says that the three stars may have destroyed their
initial supply of lithium, in which case the observed abundances of the
element in these stars would not reflect the primordial abundance of the
element.

‘As interesting as these results are, I don’t think they tell us anything
about primordial lithium,’ says Gary Steigman of Ohio State University.
He believes that the higher lithium abundance in the other old stars is
the amount produced by the big bang, and that the three lithium-deficient
stars have destroyed their lithium, as Thorburn suggests. Nonetheless, he
says, ‘When an issue this important is raised, you have to consider all
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Thorburn plans to expand the sample by measuring the lithium abundances
of 70 other old stars. She will publish her study concerning the two lithium-deficient
stars in the 1 November issue of Astrophysical Journal Letters.