The source of sunshine

Scientists are one step closer to understanding sunshine through an experiment conducted beneath the mountains of Italy.

The research provided Princeton physicists a deeper understanding of the sun’s centre and the mysterious class of subatomic particles born there.

Working as part of an international collaboration at the underground Gran Sasso National Laboratory near L'Aquila in Italy, the US scientists made the first real-time observation of low-energy solar neutrinos, which are fundamental particles created by nuclear reactions that stream in vast numbers from the sun's core.

"Our observations essentially confirm that we understand how the sun shines," said Frank Calaprice, a professor of physics and principal investigator of the Princeton team.
 
"Physicists have had theories regarding the nuclear reactions within the sun for years, but direct observations have remained elusive. Now we understand these reactions much better."

The scientists' precise measurements of the neutrinos' energy gave long-sought proof of the theory regarding how these neutrinos are produced.

In stars the size of the sun, most solar energy is produced by a complex chain of nuclear reactions that converts hydrogen into helium.

Beginning with protons from hydrogen's nucleus, the chain takes one of several routes that all end with the creation of a helium nucleus and the production of sunlight.

Steps along two of these routes require the presence of the element beryllium, and physicists have theorised that these steps are responsible for creating about 10 per cent of the sun's neutrinos.

However, these technological limitations had made the theory difficult to test until now.

Low energy neutrinos

The Gran Sasso lab's giant Borexino detector, located more than a kilometre below the Earth's surface, overcame these limitations, permitting the team to observe low-energy neutrinos, which interact extremely rarely with other forms of matter.

Scientists have wanted a way to detect them, because they emerge largely unchanged from their journey through the sun's interior to the Earth - offering a clear glimpse into the processes that forged them.

Most particles that emerge from the sun take so long to escape the interior that they change drastically before scientists can study them, so it has been difficult to prove how the sun creates energy.

Neutrinos provide a key because they escape before they have time to change.

"The findings show that science's understanding of the chain of nuclear processes that make the sun shine is essentially correct, as least as far as the part of the chain that involves beryllium is concerned," Calaprice said.

"The reaction does not generate a large percentage of the sun's energy, but confirming that we understand it makes us more certain that we know how the other processes that create sunlight work."

The Borexino experiment's entire research team, which includes more than 100 scientists from many institutions worldwide, will publish its findings in an upcoming edition of the scientific journal Physics Letters B.

The experiment is funded by the National Science Foundation.

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