Does the sun have any heavy elements

For the first time, the sun's second fusion process has been detected

The sun is basically a gigantic fusion reactor in which hydrogen is continuously converted into helium. This process, also known as hydrogen burning, takes place in two ways: Around 99 percent of the energy comes from a process of fusions and decays that begins with two hydrogen nuclei and ends with a helium nucleus via the deuterium intermediate stage, the so-called proton-proton or pp chain. In the second reaction chain, however, the heavier elements carbon (C), nitrogen (N) and oxygen (O) are involved as catalysts and intermediates. It is therefore called the CNO or Bethe-Weizsäcker cycle. While the pp reaction dominates in light stars like the sun, the CNO cycle is the main process for generating energy in heavier and hotter stars.

This second cycle was postulated in the 1930s by the physicists Hans Bethe and Carl Friedrich von Weizsäcker independently of one another as the energy supplier of the sun, but has not yet been experimentally confirmed. But now the physicists of the Borexino experiment, which is located in the Italian Gran Sasso underground laboratory near L’Aquila in Abruzzo, have succeeded in proving this for the first time. The basis for the detection are special neutrinos that arise during the CNO cycle.

Elusive ghost particles

Neutrinos are considered "ghosts" among the elementary particles - and these particles actually have a whole range of ghostly properties: The second most common particles after the photons are electrically neutral, almost massless and therefore zoom through space at almost the speed of light. Practically nothing can stop them: They effortlessly pass planets and stars alike. The Austrian Nobel laureate in physics, Wolfgang Pauli, predicted its existence as early as 1930. But because they penetrate conventional matter more or less unimpressed and interact with it solely through the weak interaction, neutrinos were only credibly detected in 1956.

In order to be able to detect the neutrinos that are created in the interior of the sun during the fusion processes, huge systems are required. They reach the earth billions of times and normally penetrate it unhindered. "With the huge detector of the Borexino experiment 1,400 meters below ground, however, we can track down these neutrinos," says Michael Wurm, neutrino physicist at Johannes Gutenberg University Mainz (JGU) and member of the Borexino collaboration. "Then they allow an unobstructed view of the processes inside the sun."

Sources of interference eliminated

While the Borexino collaboration has been able to detect neutrinos from several reactions along the pp chain in recent years, the neutrinos of the CNO cycle were difficult to distinguish from those produced by the radioactive decay of tiny traces of other elements due to their energy distribution. In particular, bismuth-210 from trace impurities on the surface of the detector wall hid the signals of the CNO cycle. Due to convection movements, these contaminants got into the detector liquid. In order to eliminate the disturbance, the convection inside the Borexino detector had to be brought to a standstill, which was technically extremely complex.

"For a long time I thought it was impossible that this measurement would be successful," says Stefan Schönert, professor for experimental astroparticle physics at the Technical University of Munich. "After six years of effort, we have now succeeded, so that we have now been able to detect the CNO neutrino signal for the first time." Even more: the research team was also able to estimate the total flow of CNO neutrinos arriving on earth. With around 700 million of them flying through a square centimeter per second, their share is about a hundredth of the total number of solar neutrinos, the physicists report in the journal "Nature". "That fits perfectly with the theoretical expectations that the CNO cycle in the sun is responsible for around one percent of the energy gained," says Daniele Guffanti, postdoc in Michael Wurm's group and also a member of the Borexino collaboration.

New evidence of the sun's metallicity

The physicists rate the new results as an important milestone towards a complete understanding of the fusion processes that drive our sun, but also heavy stars, and make them shine in the universe. They also pave the way for a better understanding of the elemental composition of the solar nucleus, particularly with regard to how often heavier elements such as carbon, nitrogen and oxygen can be found in the solar plasma in addition to hydrogen and helium - researchers speak of "metallicity" here. Here too, neutrinos are the only direct messengers. (red, 11/26/2020)