May 14, 2004 Some nuclear fusion reactions inside stars occur more slowly than we thought and, as a consequence, stars themselves, as well as galaxies and the entire universe are a bit older than expected. This is what comes out from the last results of Luna experiment (Laboratory for Underground Nuclear astrophysics), settled by National Laboratories of Gran Sasso and realized in cooperation by Infn and Ruhr University in Bochum (Germany). The study, that will be published on the review Physics Letters B next June 17, has been published today on the website of the review. A second article has been accepted by the review Astronomy and Astrophysics.
Luna's aim is the production of some reactions that occur inside stars, in particular in the Sun, and the measure of their velocity. In Luna, protons (which means hydrogen nuclei) are made collide against nitrogen nuclei: a reaction that leads to the formation of an oxygen nucleus with the contemporary emission of energy. "The great part 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. But there is another process in consequence of that helium nuclei are produced and this process passes through the so-called carbon-nitrogen-oxygen cycle. Cycle velocity is determined by the slowest of the reactions that form it, the one that leads to an oxygen nucleus as a result of the fusion between an azote nucleus and a proton. That is to say the reaction studied by Luna", explains Carlo Broggini, Luna coordinator.
Producing fusion between the nitrogen nucleus and a proton is not difficult itself, but a difficulty lies in obtaining it at the same energy it occurs in stars: a relatively low energy, thanks to which the phenomena is quite slow, corresponding to a very few reactions a day (a lucky case for our planet, because if these phenomena occurred rapidly, the Sun would have burned up its 'fuel' in a few time and this would have made life –as we know it -impossible). "In an ordinary laboratory settled on surface, 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 rain that crashes into our planet without interruption. The Gran Sasso Laboratories are on the contrary located under 1.400 meters of rocks, which constitute an impenetrable barrier for almost all the particles coming from Space. Thanks to these particular conditions we could carry out our experiment", says Carlo Broggini.
The result has been surprising: the carbon–nitrogen –oxygen cycle occurs two times more slowly than expected. "The most fascinating aspect of this study is that another estimate of Universe age flows from it. Actually, the age of the most ancient stars, those that form the so-called globular star clusters, is calculated on the base of the light spectra they emit, supposing we know the carbon–nitrogen–oxygen cycle's velocity As this last one seems to be slower than previewed, the age of the globular star clusters' has also been newly calculated and grown-up of about one billion years. As a consequence, in the light of Luna's new data, the age of our Universe passes from the previous estimate of about 13 billions years to that of about 14 billions years" explains Eugenio Coccia, director of Gran Sasso National Laboratories.
Luna's results offer also another information: neutrinos provided with high energy produced by carbon – nitrogen – oxygen cycle are the half part of what expected, because this last one is responsible of only the 0,8% of the energy emitted by the Sun (and not the 1,6%, as believed). This data is of great interest for astroparticles physicists engaged in experiments that are specifically focused on neutrinos of relatively high energy, as it is for example the experiment Borexino, which is in preparation at Gran Sasso National Laboratories, or the Japanese Kamland.
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