First detection of neutrinos that can be traced to carbon-nitrogen-oxygen fusion, known as the CNO cycle, inside the sun


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May 6, 2010
Boston, MA
This is HUGE: “This is the first evidence that the CNO cycle is at work in the sun and the stars"

In research published Wednesday in the journal Nature, scientists reported that they’ve made the first detection of almost-ethereal particles called neutrinos that can be traced to carbon-nitrogen-oxygen fusion, known as the CNO cycle, inside the sun.

It’s a landmark finding that confirms theoretical predictions from the 1930s, and it’s being hailed as one of the greatest discoveries in physics of the new millenium.

“It’s really a breakthrough for solar and stellar physics,” said Gioacchino Ranucci of the Italian National Institute for Nuclear Physics (INFN), one of the researchers on the project since it began in 1990.

The scientists used the ultrasensitive Borexino detector at the INFN’s Gran Sasso particle physics laboratory in central Italy – the largest underground research center in the world, deep beneath the Apennine Mountains, about 65 miles northeast of Rome.

The detection caps off decades of study of the sun’s neutrinos by the Borexino project, and reveals for the first time the main nuclear reaction that most stars use to fuse hydrogen into helium.

Almost all stars, including our sun, give off huge amounts of energy by fusing hydrogen into helium – effectively a way of “burning” hydrogen, the simplest and most abundant element and the main fuel source in the universe.

In the case of the sun, 99 percent of its energy comes from proton-proton fusion, which can create beryllium, lithium and boron before breaking them down into helium.

But most stars in the universe are much larger than our sun: the red-giant Betelgeuse, for instance, is about 20 times more massive and about 700 times as wide.

Trillions of neutrinos from the sun pass through the Borexino detector every second, but it detects only dozens of them each day by looking for faint flashes of light as they decay in its dark 300-ton water tank.

Ranucci said the Borexino detector has spent decades measuring neutrinos from the sun's main proton-proton chain reaction, but detecting its CNO neutrinos has been very difficult – only about seven neutrinos with the tell-tale energy of the CNO cycle are spotted in a day.

The discovery required making the detector ever more sensitive over the last five years, he said, by shielding it from outside sources of radioactivity so that the inner chamber of the detector is the most radiation-free place on Earth.

The result is the only direct sign of CNO fusion ever seen anywhere: “This is the first evidence that the CNO cycle is at work in the sun and the stars,” Ranucci said.

Gabriel Orebi Gann, a particle physicist at the University of California, Berkeley, called the discovery "a major milestone."

“This discovery takes us a step closer to understanding the composition of the core of our sun, and the formation of heavy stars," she said.

Orebi Gann is the author of a scientific article in Nature about the new study, but she was not involved in the research.



May 25, 2010
SF Bay Area
Missed this article back in November, thanks so much. The key to the finding is that the CNO cycle occurs at a higher temperature than the pp reaction, and the mix of the two gives a much better insight into the structure of the core of the sun. It has been long predicted that there should be some CNO cycle going on in sun (we studied this in grad school in the '60's), but neutrino detection is notoriously difficult. The energies of the neutrinos created in the pp cycle are different from those created in the CNO cycle, and you can "see" the difference.

The energy (photons) created in the sun's core takes a very long time (thousands to millions of years) to get out to the surface of the sun because the sun is so dense and so big, and then only eight minutes for the light to get from the surface of the sun to the earth. So if the core of the sun were to suddenly drop in the temperature and turn off its fusion reactions, we wouldn't know it for a very long time. Neutrinos are different - very hard to stop, so they fly right out of the core of the sun, making it to the earth in eight plus minutes (an added half million miles to go the radius of the sun plus the 93 million miles from the surface of the sun to the earth). Because neutrinos can pass through matter so easily, they are also extremely hard to detect. They just fly through everything (your body has neutrinos entering and leaving your body all the time). So these neutrino telescopes are the oddest telescopes in the world, buried deep underground to prevent contamination from radiation from other particles and the neutrinos are "seen" only when they collide with the material in the huge tanks (300 tons of water in this case) and telltale radiation is emitted. The first neutrino telescope was built in an abandoned mine in South Dakota (IIRC) where the receiving tank was filled with dry cleaning fluid.

Your astronomer friend, Larry
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