News in Science

Scientists see the 'soul' of the Sun

Neutrino photo Scientists have confirmed how the Sun makes 99 per cent of its energy.

They have detected subatomic particles called pp neutrinos, they report in the journal Nature.

The discovery, by an international team of scientists led by Assistant Professor Andrea Pocar of the University of Massachusetts, confirms existing theories that most of the Sun's total energy output is produced by proton-proton fusion in its core.

"Our experiment has taken a neutrino photograph of the Sun," says Pocar.

"We have basically confirmed that our understanding of the Sun is correct, by directly measuring the most abundant neutrino source in the Sun, those produced through pp fusion."

Stars, like the Sun, shine by fusing together hydrogen atomic nuclei called protons into helium. The process also produces photons of energy and pp neutrinos.

Neutrinos are virtually mass-less particles which barely interact with other matter, making them hard to detect.

Hungry Sun

The Sun consumes over 600 million tonnes of hydrogen every second, reaching temperatures of over 15 million degrees at its core and radiating over 384.6 yottawatts of power (a yotta being ten to the power of 24).

Pressures and densities deep inside the Sun are so great, that it may take a photon 100,000 years to make its way from the Sun's core to the surface.

In contrast the weakly interactive nature of neutrinos allows them to travel virtually unimpeded, taking a little over eight minutes to cover the 150 million kilometres from the Sun's core to the Earth.

"As far as we know, neutrinos are the only way we have of looking into the Sun's interior," says Pocar.

"If the eyes are the mirror of the soul, with these neutrinos, we are looking not just at its face, but directly into its core, we have glimpsed the Sun's soul.

"This is important because it lets us see the Sun as it was eight-and-a-half minutes ago, not a a hundred thousand years ago as it was when the photons left the Sun's core."

By comparing the photon and neutrino data, Pocar and colleagues can infer that the Sun is in a state of thermodynamic equilibrium, and it has been so for at least 100,000 years.

Almost impossible to find

Neutrinos from other processes in the Sun have been detected previously, but those produced by proton-proton reactions are particularly hard to detect because their low energy places them in a range where natural radioactivity masks their interactions.

Pocar and colleagues used the Borexino neutrino detector at Italy's Gran Sasso National Laboratories to successfully find these elusive particles.

Borexino is the only detector on Earth capable of observing the entire spectrum of solar neutrinos simultaneously.

Pocar and colleagues plan to use the Borexino neutrino detector to try and study the internal chemical composition and structure of the Sun.

"The precision of this measurement wasn't enough to discriminate between two models of the Sun which differ in the amount of heavier elements like carbon and oxygen," says Pocar.

"If we could determine the metallicity [elements heavier than hydrogen and helium] in the Sun, it would tell us a lot about stellar physics, and give us a lot of information about heavier stars further away which fuse these heavier elements."