Thirty years of research have led to the discovery of one of the most intriguing mysteries of the stars. Finally, by studying the neutrinos launched into the cosmos by the Sun, an international group of about eighty physicists found an answer explaining why stars shine without collapsing. Leading the company described in the scientific magazine Nature Professor Gianpaolo Bellini of the State University of Milan and the National Institute of Nuclear Physics (Infn) also dedicated the cover to the exceptional result. The major contributions also came from researchers from the University of Princeton and the University of Genoa (Marco Pallavicini) to which British and German participations were added.
Under the Gran Sasso
The heart of the discovery was the Borexino experiment in the underground laboratories of the Gran Sasso of the Infn. I note that the Sun and the stars in the sky are continuously ignited by a nuclear fusion reaction where four hydrogen nuclei fuse to generate a helium nucleus. The Sun is a medium-small star and the pressure generated by its internal temperature allows to balance the force of gravity. But in the larger, far more numerous stars, the puzzle remained until yesterday of how they were not victims of the force of gravity by collapsing, considering the known reaction insufficient to counter it. We tried to capture the neutrinos that are emitted by these reactions and launched by the Sun, which tell what happens inside the star, explains Bellini.
The challenge was difficult because it was necessary to find those with particular characteristics and with a lower energy, able to unmask unknown aspects. And the first obstacle was to build a machine capable of detecting these particles which by their nature are elusive because, not interacting with matter, they also pass through our body by the billions every second without us realizing it. Furthermore, as their energy is very low, the background disturbance generated by natural radioactivity had to be overcome. After having begun to study the problem and created a small prototype that demonstrated the feasibility of the research, in the 1990s Luciano Maiani, then president of Infn, approved a great experiment. Thus was born Borexino, a sort of dome covered with detectors designed to capture the elusive neutrinos and containing 2,400 tons of water needed to trap the ephemeral particle.
Bellini, who returned to Italy after ten years spent at the Fermilab in Chicago (before he had also worked in the Soviet Union), put together an international group of researchers dedicated to the new adventure. We have discovered that stars larger than the Sun, adds Bellini, are able to shine because another nuclear reaction between carbon, nitrogen and oxygen takes place, called CNO by their chemical symbols, which guarantees precisely that temperature and pressure that counteracts the force of gravity This cycle also exists in the Sun, but to a much lesser extent, around 1 percent, compared to the major stars. But it was enough to launch the neutrinos that interested us and that we captured.
The hypothesis of the new cycle existed theoretically, but so far the proof now conquered has never been found. Now we have the first fundamental experimental confirmation of how stars heavier than the Sun shine, concludes Professor Bellini, an international authority in the field of neutrinos, so much so that it has received the most illustrious recognition for scholars in the sector, namely the Pontecorvo Prize awarded by a commission formed by Russian, American, Japanese and Italian scientists. But also obtaining the Fermi Prize, the most important awarded by the Italian Physics Society.
November 25, 2020 (change November 25, 2020 | 23:46)