The sun gets its energy from the fusion of hydrogen to helium. However, since 1938 theories have suggested that in addition to a direct fusion of hydrogen nuclei, there must also exist a second fusion path catalyzed by heavier elements – the so-called CNO cycle. Now, for the first time, researchers have succeeded in directly demonstrating this second solar fusion cycle. With the help of a neutrino detector under the Italian Alps, they detected the particles that are released as a by-product of the CNO cycle. This is a critical breakthrough for solar research and astrophysics.
Our sun is a gigantic fusion furnace: the immense pressure of solar gravity and a heat of more than 15 million degrees cause hydrogen nuclei to fuse inside. Around 600 million tons of hydrogen are converted into helium every second. In the sun, the majority of these fusion reactions occur through direct fusion of protons. This so-called proton-proton fusion makes up around 99 percent of solar fusion. But as early as 1938, the physicists Hans Bethe and Carl Friedrich von Weizäcker postulated independently of one another that there had to be a second fusion path in stars. Heavy elements like carbon, oxygen and nitrogen act one after the other as catalysts for the fusion of hydrogen to helium. According to the models, this reaction path, called the CNO cycle after the element abbreviations, should account for around one percent of the nuclear fusion in stars with less mass, such as the sun, but represent the dominant form of fusion in more massive stars.
Neutrinos as messengers from within the sun
But how can these processes inside the sun be demonstrated? This is where neutrinos come into play: virtually massless particles that are released as a by-product of the fusion reactions. Hundreds of billions of such solar neutrinos flow through our body every second without our noticing. Because these particles hardly interact with other matter. They can therefore only be detected in detectors that contain enormous amounts of water, ice or other detector material. If a neutrino collides with an atom, it causes a tiny flash of light that is captured by photosensors. From the energy and the spectrum of these light signals, scientists can then draw conclusions about the properties of the triggering neutrinos and their origin. However, the detection of solar neutrinos is particularly difficult because, due to their low energy, they can easily be confused with the neutrinos released during radioactive decay reactions.
One system with which the detection of solar neutrinos is still possible is the underground Borexino detector at the Gran Sasso Laboratory. It is shielded in several ways against interference radiation by decay neutrinos. In addition to the rock cover and a shell of water, the 278 tons of organic detector liquid are surrounded by two further zones of shielding liquids. Thanks to this setup, the researchers of the Borexino collaboration have already succeeded in detecting solar neutrinos in the proton-proton collision in recent years. However, in order to detect the much rarer neutrinos from the CNO cycle, they had to develop additional purification steps and statistical filter methods. “The greatest challenge was to identify the small excess – it only corresponds to a handful of events per day and 100 tons of detector liquid,” explain the scientists.
720 million CNO neutrinos per second and square centimeter
But it did succeed: after evaluating the detector data from July 2016 to February 2020, the physicists have now for the first time clearly identified neutrinos that were released during the sun’s CNO fusion cycle. Their neutrino detector captured an average of 7.2 such CNO neutrinos per day and 100 tons of liquid. “That can be converted into 720 million CNO neutrinos that flow into the earth every second and square centimeter,” explain the Borexino scientists. Your values confirm that a fusion takes place in the sun according to the CNO cycle and also fit the models according to which this fusion path accounts for around one percent of the total solar fusion reactions. “With this we finally have the first, groundbreaking experimental proof of how stars produce their glow heavier than the sun,” says co-author Gianpaolo Bellini from the University of Milan.
The physicist Gabriel Orebi Gann from the University of California at Berkeley, who was not involved in the study, sees it similarly. He writes in an accompanying comment: “The Borexino collaboration presents results that represent a milestone in neutrino physics. This tremendous achievement brings us closer to a full understanding of our sun and the formation of massive stars. “
Source: The Borexino Collaboration (INFN Gran Sasso Laboratories), Nature, doi: 10.1038 / s41586-020-2934-0