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Scientists who are members of the Borexino Collaboration have provided the first experimental evidence of the occurrence of the so-called CNO cycle in the Sun: they managed to directly detect the distinctive neutrinos generated during this fusion process.
This is an important step on the way to better understanding the fusion processes occurring in the Sun. At the same time, although the CNO cycle plays a minor role in our Sun, it is most likely the predominant way of producing energy in other more massive stars and warmer. The results of the Borexino Collaboration were published in the latest issue of the journal Nature.
How does the Sun generate energy? Like a giant fusion reactor, it continuously converts hydrogen to helium, a process also known as “hydrogen combustion”. Essentially, this involves two types of processes. On the one hand, there is the proton-proton reaction (pp reaction). This begins with the direct fusion of two hydrogen nuclei to create the intermediate deuterium isotope of hydrogen from which helium is subsequently formed.
On the other hand, the heavier elements carbon (C), nitrogen (N) and oxygen (O) are involved in the second type of reaction chain, known as the CNO cycle or the Bethe-Weizsacker cycle. While the pp reaction is predominant in smaller stars like our Sun, the CNO cycle is the main process for generating energy in more massive and hotter stars.
As is the case with all fusion processes that occur within the Sun, countless neutrinos are produced in addition to helium and the enormous amounts of energy that make the Sun and its sister stars shine. Neutrinos reach the Earth by the billions and normally pass through it without obstacles.
“However, we are able to detect these neutrinos using the huge detector from the Borexino experiment located 1400 meters underground,” points out Prof. Michael Wurm, a neutrino physicist at the PRISMA + Cluster of Excellence at Johannes Gutenberg University. Mainz (JGU) and member of the Borexino collaboration. “They give us a clear view of the processes in the Sun’s core.”
While the Borexino Collaboration has been able to detect neutrinos originating from several reactions along the pp chain in recent years, their current result has been to explicitly identify neutrinos released in the CNO cycle, which are significantly less abundant in comparison.
“Although based on the model calculations we expected the CNO cycle to occur in the Sun as well, direct proof of this has never been obtained before. Only a characteristic neutrino signal can provide conclusive proof that this actually occurs – now we have that conclusive proof without a shadow of a doubt. “
Furthermore, the research team was also able to estimate the total flux of CNO neutrinos reaching Earth. About 700 million of them fly across one square centimeter of our planet every second, but this represents only one hundredth of the total number of solar neutrinos.
“This is consistent with theoretical expectations that the CNO cycle in the Sun is responsible for about one percent of the energy it produces,” adds Dr. Daniele Guffanti, a postdoc in Michael Wurm’s team and also a member of the Borexino Collaboration.
The two neutrino physicists from Mainz see the new findings as an important milestone along the way to gaining a complete understanding of the fusion processes that not only drive our Sun but also massive stars, and illuminate our night sky.
It also paves the way for a better understanding of the elements that make up the solar core, particularly in regards to how often heavier elements such as carbon, nitrogen and oxygen can be found in solar plasma in addition to hydrogen and helium – i researchers call it metallicity. Neutrinos may once again be our only guides to help us find this out.
Research paper
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Johannes Gutenberg Universitaet Mainz
Stellar chemistry, the universe and everything in it
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