Slow reaction. Nuclear physicists now say a key step in a nuclear reaction inside stars happens slower than thought.

Oldest Stars Get Even Older

A key nuclear reaction inside stars takes significantly longer than standard models assume, European researchers have discovered. The result implies that stars burn more slowly than astrophysicists have thought, making the most ancient star clusters at least 700 million years older than previously believed.

Nuclear physicists at the Laboratory for Underground Nuclear Astrophysics (LUNA) experiment in Gran Sasso, Italy, used an underground particle accelerator to measure the speed of the carbon-nitrogen-oxygen (CNO) cycle, one of the pathways by which stars fuse hydrogen into helium, releasing energy. The cycle determines how long it takes a youthful hydrogen burning star to consume its original fuel and transmogrify 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.

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 step 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 the 17 June issue of Physics Letters B and an upcoming issue of Astronomy and Astrophysics, the LUNA researchers report that the crucial, limiting step in the CNO reaction is indeed only half as likely to occur as previously assumed. Working more than a mile underground to shield their gamma ray 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 gamma rays detected were far too few for the standard assumption about the CNO reaction rate to be correct.The results push the age of the oldest stars to almost 14 billion years. This rolls back the lower limit on the age of the universe as well. That's a good match with the figure of 13.7 billion years for the age of the universe that physicists derived from measurements of the cosmic microwave background by the Wilkinson Microwave Anisotropy Probe, though both still have significant uncertainties, says John Bahcall, an astrophysicist at the Institute for Advanced Study in Princeton, New Jersey. ”The LUNA experiment is beautiful,” he says, praising the group as “magically gifted experimentalists.”