News of the WeekNUCLEAR ASTROPHYSICS

New Measurement of Stellar Fusion Makes Old Stars Even Older

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Science  28 May 2004:
Vol. 304, Issue 5675, pp. 1226
DOI: 10.1126/science.304.5675.1226b

A key nuclear reaction inside stars takes significantly longer than standard models assume, European researchers have discovered. The result, which nuclear physicists at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Gran Sasso, Italy, report in a pair of online papers, implies that the most ancient star clusters are at least 700 million years older than previously believed.

“The LUNA experiment is beautiful,” says John Bahcall, an astrophysicist at the Institute for Advanced Study in Princeton, New Jersey, praising the group as “magically gifted experimentalists.”

The LUNA team used an underground particle accelerator at Gran Sasso to measure the speed of the carbon-nitrogen-oxygen (CNO) cycle, one of the pathways by which stars fuse hydrogen into helium, releasing energy (see diagram). The cycle determines how long it takes a youthful hydrogen-burning star to turn into a giant helium burner. Astrophysicists can estimate the age of a star on the cusp of that transition by measuring its mass and then calculating how long it took to reach its current state.

Brakedown.

A nuclear reaction in stars takes longer than believed.

ILLUSTRATION: P. HUEY/SCIENCE

The CNO cycle, however, is only as fast as its slowest step: a nuclear reaction in which the isotope nitrogen-14 absorbs a proton from hydrogen and turns into oxygen-15. Researchers had estimated the rate of the reaction by shooting protons at nitrogen-14 in particle accelerators. But the measurements were marred by noise from cosmic rays, and astrophysicists suspected they erred on the speedy side.

In papers scheduled to be published in Physics Letters B and Astronomy and Astrophysics, the LUNA researchers report that the limiting step is indeed only half as rapid as previously assumed. Working 1400 meters underground to shield their detectors from cosmic radiation, they smashed protons into a nitrogen-14 target and then measured the gamma rays the nitrogen released as it became oxygen-15. The results push the age of the oldest stars to almost 14 billion years. That's close to the figure of 13.7 billion years for the age of the universe that physicists derived from measurements by the Wilkinson Microwave Anisotropy Probe (Science, 14 February 2003, p. 991), although both still have significant uncertainties, Bahcall says.

The team plans to repeat the experiment at more realistic collision energies, says Carlo Broggini, spokesperson for the LUNA project. The first set of experiments was run at energies above 140 kilo-electron volts (KeV), Broggini says. A new gamma ray detector should allow researchers to study collisions at close to 25 KeV, the peak energy level at which the reaction occurs in stars.

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