Physics: The “ghost particles” emitted by the SUN shed light on how bright massive stars are

For the first time, elusive “ghost particles” produced in the depths of the Sun have been detected, helping to shed light on reactions that make huge stars shine.

The researchers were able to capture evidence of the particles as they passed through a special detector buried under a mountain near the city of L’Aquila, Italy.

The rare emissions – which traveled 90 million miles to reach us – are produced in some nuclear reactions that account for less than one percent of solar energy.

However, these reactions are believed to be more dominant in larger stars and could help explain their formation and evolution.

“Now we finally have the first, revolutionary experimental confirmation of how stars heavier than the sun shine,” said the article’s author and astroparticle physicist Gianpaolo Bellini of the University of Milan.

Stars are powered by the fusion of hydrogen into helium, which can occur through two different processes: the first is the so-called proton-proton chain, which involves only isotopes of hydrogen and helium. This is dominant in stars such as the sun.

In larger stars, however, the so-called carbon-nitrogen-oxygen (CNO) cycle – in which these three elements help catalyze nuclear reactions – becomes a more significant source of energy. It also releases spectral particles called neutrinos.

These are nearly massless and are able to pass through ordinary matter without giving up any indication of their presence.

Physicists wanted to study these emissions from the Sun, however, as a better understanding of how the CNO cycle works in our star will offer insight into how larger stars – where this process is dominant – burn their nuclear fuel.

To detect the CNO neutrino emissions of the sun, physicists used the so-called ‘Borexino detector’ – a 55-foot-tall, layered onion-shaped machine that contains at its heart a spherical tank called a ‘scintillator’ that is filled with 278 tons of a special liquid.

When neutrinos pass through this liquid, they can interact with its electrons, releasing tiny flashes whose brightness is indicative of the neutrino’s energy, with those produced by the CNO cycle being at the most intense end.

These are detected by camera-like sensors and analyzed by powerful hardware.

To ensure that the detector only picks up the rare neutrino signals – and is not overwhelmed by cosmic radiation – the Borexino experiment is both buried underground and further shielded by being wrapped in a water tank.

‘This is the culmination of a thirty-year effort that began in 1990 and more than ten years of Borexino’s discoveries in the physics of the Sun, neutrinos and eventually stars,’ said Professor Bellini.

According to the physicist Gioacchino Ranucci, also from Milan, the success of the experiment must be attributed to the “unprecedented purity” of the solution.

The detection of CNO neutrinos revealed how much of the sun is made up of the elements carbon, nitrogen and oxygen.

“Despite the outstanding successes previously achieved and an already ultra pure detector, we had to work hard to further improve the suppression and understanding of very low residual backgrounds,” added Dr. Ranucci.

This, he continued, allowed them to “identify the neutrinos of the CNO cycle”.

The discovery finally confirms that some of the sun’s energy is actually produced by the reactions of the CNO cycle, a notion that was first proposed in 1938.

“It is the culmination of a relentless and years-long effort that has led us to push technology beyond any previously reached limits,” said Borexino Experiment spokesman Marco Pallavicini, physicist at the University of Genoa.

This, he added, made “Borexino’s core the least radioactive place in the world”.

The full results of the study were published in the journal Nature.