Old stars: even older: determining a new age for the universe.

Using particle accelerators to mimic the conditions inside stars, two independent research groups have found evidence that the most-ancient known stars are about a billion years older than astronomers had estimated. This provides new evidence that the universe is about 14 billion years old.

The age recalibration rests on a nuclear reaction that prevails in old stars that have nearly exhausted the hydrogen at their cores. For most of its life, a star produces energy by fusing hydrogen nuclei to make helium. The fusion occurs at a leisurely rate, but when little hydrogen is left, the star activates an alternative helium-making process, which is based on collisions between protons and nuclei of carbon, nitrogen, and oxygen.

The speed of this process depends on the slowest reaction in the chain: the collision between a proton and a nitrogen-14 nucleus. By studying this collision at energies approaching those of the interior of stars--a feat never before accomplished--the two teams have found that the reaction proceeds only half as fast as had been estimated. The reaction's sluggishness enables gravity to shrink a star more than it otherwise would, which makes the star brighter. Because astronomers use brightness to determine an elderly star's age, the newly determined reaction rate adds another billion years to the senior citizens of the cosmos.

In one of the experiments, Carlo Broggini of the National Institute for Nuclear Physics in Padova, Italy, and his colleagues fired protons at nitrogen-14 nuclei in a laboratory 1,400 meters beneath the Gran Sasso Mountain near L'Aquila, Italy. The underground chamber is shielded from the rain of cosmic rays, which otherwise would generate so many high-energy signals that they could overwhelm the subtle signs of the reaction between protons and nitrogen-14. Broggini's team reports its findings in the June 17 Physics Letters B and in an upcoming Astronomy & Astrophysics.

Science News has learned that another team has similar results. Art Champagne of the University of North Carolina at Chapel Hill and his collaborators used an above-ground particle accelerator at the Laboratory for Experimental Nuclear Astrophysics in Durham, N.C., to study the same nuclear reaction. This team used two sets of detectors to distinguish spurious gamma rays produced by cosmic rays from those produced by proton-nitrogen-14 collisions.

Champagne and his colleagues haven't yet published their results. But when the two teams compared data and realized how closely their findings matched, "there were sighs of relief on both sides," Champague says.

Broggini and his coworkers observed the reaction at slightly lower energies than Champagne's team did, so they came closer to the conditions inside stars. Other researchers, including John Bahcall of the Institute for Advanced Study in Princeton, used earlier data to argue that the proton-nitrogen-14 reaction rate is slower than the rate that had been assumed. Bahcall notes that Broggini and his colleagues "have much better experimental data."

The finding is important because it pushes back the formation of the first stars, comments Brian Chaboyer of Dartmouth College in Hanover, N.H. The new age determination is in line with observations from the Wilkinson Microwave Anisotropy Probe, which recently found that the universe is 13.7 billion years old (SN: 2/15/03, p. 99).