Stars get their energy by converting hydrogen into helium in one of two ways. In the proton-proton chain, four protons essentially combine to from a helium nucleus and two electrons (although the chain is quite complex, involving the creation of deuterium and helium-3 nuclei along the way). Alternatively, protons can fuse with light nuclei, such as carbon, nitrogen and oxygen, and produce helium as part of the CNO cycle.

The slowest reaction in the CNO cycle is the fusion of a proton with a nitrogen-14 nucleus to produce oxygen-15 and a gamma-ray. The rate for this reaction can be used to determine the rate at which energy is released by the CNO process and, consequently, the brightness and lifetime of a star. The LUNA team reproduced this reaction by colliding high-energy protons with nitrogen-14 nuclei at the Gran Sasso underground laboratory in northern Italy. The team measured the reaction rate at energies down to 140 keV and then extrapolated their results to the region below 80 keV that is important in stars.

The team calculated that the CNO cycle occurs two times slower than previous estimates. “The most fascinating implication of this study is that it can be used to estimate a new age for the universe,” Gran Sasso director Eugenio Coccia told PhysicsWeb.

The age of the oldest stars in the universe — those that form globular star clusters — is calculated by analysing their light spectra using current values for the rate of the CNO cycle. “Since this rate seems to be slower than thought, the age of the star clusters has also been newly calculated and found to be older by about one billion years,” said Coccia. “Therefore, the age of our universe passes from the previous estimate of about 13 billion years to around 14 billion years.”

The LUNA collaboration, which includes physicists from Italy, Germany, Belgium and Portugal, is now repeating the experiment with new detectors that are about 100 times more sensitive than the previous detectors.