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Physicists wrap neutrino detector in cosy blanket to shed light on the Sun's secondary fusion cycle

Direct observation published for first time

As we near the northern winter solstice, the Sun continues to produce a steady power output of 384.6 yottawatts resulting from the fusion of hydrogen into helium in two distinct nuclear reactions. Direct observation of the secondary cycle was published in the journal Nature for the first time yesterday.

A team of physicists and engineers going by the name of The Borexino Collaboration have been working on their neutrino detector for 30 years, shedding light on the Sun's nuclear processes at the same time. Through the gradual finessing of the apparatus, including stabilising temperature with layers of thermal insulation, they have been able to reveal the actions of these elusive subatomic particles emitted by the Sun's secondary carbon–nitrogen–oxygen (CNO) cycle.

The primary source of nuclear energy in our solar system's star comes from the proton-proton (p-p) chain, which is responsible for about 99 per cent of its energy output. In the secondary CNO cycle, helium is produced from hydrogen fusion using carbon, nitrogen and oxygen as intermediary catalysts in a process also emitting wayward neutrinos.

As it turns out, neutrinos are particularly useful in revealing the Sun's internal workings. While we feel the heat and see the light from the Sun's photons, they take tens of thousands of years to escape its massive body.

Because they scarcely interact with matter, neutrinos "can escape the Sun and reach Earth in just eight minutes," wrote Gabriel Orebi Gann, associate professor, University of California Berkeley, in an article accompanying the research paper.

"This gives us a unique window into the core of this blazing star," she said.

The weakness of their interactions with other matter makes neutrinos frustratingly difficult to detect and measure. Although proposed by Wolfgang Pauli in 1930, the first experimental evidence for them did not emerge until 1955. The particles barely interact with ordinary matter - to the point that they are hardly deterred from their path even when they have to pass through the entire body of the Earth.

The Italian Borexino experiment, led by Gianpaolo Bellini, professor at the University of Milan, detects light produced when solar neutrinos scatter off electrons in a large vat of liquid scintillator – a medium that produces light in response to the passage of charged particles.

Recent refinements to the detector include thermal insulation to control temperature variations in the detector, helping the team produce precise measurements needed to detect solar neutrinos produced by the Sun's secondary solar-fusion cycle.

The evidence from the experiment at the Gran Sasso National Laboratories of the Italian National Institute for Nuclear Physics represents a breakthrough in our understanding of the Sun's physical processes and how they will develop over time.

"This result is a huge leap forward, offering the chance to resolve the mystery of the elemental composition of the Sun's core. In astrophysics, any element heavier than helium is termed a metal. The exact metal content (the metallicity) of a star's core affects the rate of the CNO cycle. This, in turn, influences the temperature and density profile – and thus the evolution – of the star, as well as the opacity of its outer layers," Orebi Gann said.

It is also a result that can help our understanding of stars like the Sun but that are more than twice as heavy (the Sun + 1.3 solar masses). In these stars, the CNO cycle is the predominant fusion process and the Borexino experiment may well provide evidence of the main process for burning hydrogen in the universe.

"Now, we finally have the first ground-breaking, experimental confirmation of how the stars heavier than the Sun shine," said the University of Milan's Bellini. ®

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