Brussels, May 2004
A team of German and Italian astrophysicists has discovered that some of the nuclear fusion reactions inside stars take place more slowly than was previously thought, meaning that stars, and in fact the entire universe, are older than we suspected.
The discovery was made by scientists from the national laboratories of Gran Sasso and the national institute for nuclear physics in Italy, and the Ruhr University in Bochum, Germany. They are partners in the Luna project (the laboratory for underground nuclear astrophysics), which aims to recreate some of the reactions that occur inside stars, particularly our own Sun, and measure the rate at which they occur - their so called cycle velocity.
'The [vast majority] of the energy emitted by our star derives from fusion reactions of four hydrogen nuclei that lead directly to the formation of a helium nucleus,' explains Carlo Broggini, the Luna project coordinator. Crucially, however, there are other, much slower reactions occurring simultaneously, albeit on a smaller scale, which together make up the carbon-nitrogen-oxygen cycle, adds Dr Broggini.
These reactions between the more complex structures found in stars occur at relatively low levels of energy, resulting in a slow overall process of just a few reactions a day. If this process were significantly faster, our Sun, for example, would have exhausted its reserves of hydrogen much earlier, making life on Earth an impossibility.
'[Overall] cycle velocity is determined by the slowest of the reactions that form it,' continues Dr Broggini, which in the case of our Sun is the formation of oxygen from the fusion of nitrogen and hydrogen nuclei. Reproducing such fusion reactions in a laboratory is not overly challenging in itself - the difficulty is doing so at the relatively low levels of energy with which they occur in stars, without interference from high energy cosmic radiation from the Sun.
'In an ordinary laboratory [located] on the surface [of Earth], the effects of the reaction studied by Luna would be totally hidden by similar, but much more abundant effects due to reactions caused by the cosmic rays [...] that crash into out planet without interruption,' explains Dr Broggini. For this reason, Luna's work has been carried out in the underground Gran Sasso laboratories: 'The laboratories are located under 1,400 metres of rock, which constitutes an impenetrable barrier for almost all the particles coming from space. Thanks to these particular conditions we could carry out our experiment.'
The results were surprising. The team discovered that the carbon-nitrogen-oxygen cycle progresses at around half the rate expected. Estimates of the age of the universe are based on its most ancient stars - those found in so called globular star clusters. Their ages are calculated through observations of the light spectra that they emit, and require a definition of the carbon-nitrogen-oxygen cycle velocity.
The Luna results have forced scientists to revise their estimates of the carbon-nitrogen-oxygen cycle velocity, and therefore they have also revised their calculation of the age of the universe, from around 13 to nearer 14 billion years old - a discovery of fundamental interest in itself, and one that will have a major impact in other areas of astrophysics.
For further information, please consult the Luna website: