Shining light on the Sun’s source of power

Scientists working with $AU60 million instrument in Italy have found a new class of neutrinos coming from the Sun that confirms, as has long been theorised, that the Sun burns its nuclear fuel via two different thermonuclear pathways. 

“We have completed a chapter of physics which started more than 80 years ago,” says nuclear physicist Gioacchino Ranucci, a spokesperson for the Borexino Collaboration at the Italian National Institute for Nuclear Physics Gran Sasso Laboratories, where the detection occurred.

Most of the Sun’s energy, Ranucci says, comes from the proton-proton (pp) process, in which colliding hydrogen nuclei (each composed of a single proton) fuse, via a series of steps, into helium. 

But as far back as 1938 it was proposed that the fusion of hydrogen into helium could also be catalysed by carbon, nitrogen and oxygen, in a series of reactions called the CNO cycle. 

Confirming this from Earth, however, requires looking at the neutrinos each produces as a byproduct and distinguishing those from the CNO cycle from those from the pp process – a decades-long challenge.

Neutrinos are subatomic particles with no mass and no charge, moving at nearly the speed of light. They easily pass through ordinary matter, with vast numbers of them sleeting all the way through the Earth every moment.

Occasionally, however, one hits an electron and knocks it free from its parent atom. To spot these rare encounters, neutrino detectors, such as the one in Gran Sasso, have large tanks of fluid lined with sensors that can detect the faint flashes of light that occur when these electrons interact with their surroundings. 

To prevent interference from other types of radiation, such as cosmic rays, these liquid-scintillation detectors are put in shielded vaults and buried underground. 

Even then it is difficult to keep out stray radiation. “The [new discovery] is the crowning of a relentless years-long effort that has led us to push the liquid-scintillator technology beyond any previously reached limit, and to make Borexino’s core the least radioactive place in the world,” says Marco Pallavicini, of the University of Genoa. 

The pp process and the CNO chain produce the same type of neutrinos. But their neutrinos carry different mixes of energies, and it is by carefully unscrambling these that the Borexino scientists were able to make their find. 

“This is the first-ever direct demonstration that the CNO cycle is actually occurring in the core of the Sun,” Ranucci says. “We complete a picture of how a star works.”

Better yet, his team found, the number of these neutrinos they detected, when compared to those from the pp process, indicates that the CNO cycle contributes about 1% of the Sun’s total energy – just as theory had long predicted.

That is important not just for understanding the Sun, but for understanding other stars, because astrophysical theory suggests that for those even as little as 30% more massive than the Sun, the CNO cycle should be their dominant energy source. 

Furthermore, the scientists suggest in their paper in the journal Nature, it may even be possible to refine the neutrino measurements enough to be able to calculate the amount of carbon, nitrogen and oxygen in our Sun’s core – a direct experimental measurement of what astrophysicists call its metallicity (its content of elements heavier than hydrogen and helium). 

That’s important because it refines not only our understanding of how the Sun produces energy, but also how that energy escapes from its interior to eventually warm our planet – a process affected by the amount of heavier elements present in its core.

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