During the past 75 years, the picture of how the sun produces the enormous amount of energy it radiates has slowly come into focus. But one key observation has been missing – until now.
For the first time, scientists at an observatory deep beneath Italy's Apennine Mountains have detected elusive subatomic byproducts of a nuclear reaction responsible for producing 99 percent of the sun's energy.
These byproducts are known as neutrinos, formed under intense heat and pressure at the center of stars. Scientists have seen some versions before, such as the ones produced when helium fuses with other helium ions. But scientists had never seen neutrinos from a more fundamental solar reaction – protons fusing with protons.
Now, an international team of physicists has produced the first direct evidence for this more-basic process.
On one level, the results, published in Thursday's issue of the journal Nature, represent "a capstone" for efforts to use neutrinos to verify generally accepted ideas of the physics driving the sun's nuclear furnace, says Andrea Pocar, a physicist at the University of Massachusetts at Amherst and a member of the team reporting the results.
But the work also opens the possibility for using solar neutrinos as probes of the composition of the cloud of dust and gas that collapsed to form the sun and planets some 4.6 billion years ago, adds Wick Haxton, a nuclear astrophysicist at the University of California at Berkeley, who did not take part in the study.
In particular, researchers are interested in how rich the nebula was in elements heavier than hydrogen or helium, also known as metals. Encouraged by the results in the new study, the researchers would like to improve their neutrino detector to take on this new challenge. The techniques they used mark an important step in that direction.
The neutrinos involved in such detective work come from a particular type of fusion that is much less common in the sun than the proton-proton version chronicled in Thursday's study. Their signatures also are harder to separate from those of terrestrial processes that can trigger false detections.
This kind of fusion converts hydrogen to helium and is facilitated by carbon, nitrogen, and oxygen in the core, researchers say. Because the reaction uses carbon, nitrogen, and oxygen as catalysts, rather than as fuel, these elements are not converted to something else. Thus, in principle, neutrinos from this reaction provide a direct probe of the metal content of the nebula that gave rise to the solar system.
"If you have to look at any place in the solar system where you're pretty sure you're looking at at the original gas, it would be there at the center of the sun," Dr. Haxton says. "That was the first stuff to condense out of the gas cloud."
In stars more massive than the sun, this type of fusion reaction dominates, rather than the proton-proton version. The sun's core, by contrast, "is on the ragged edge of being hot enough to burn" using this reaction, he says.
Neutrinos are subatomic particles with miniscule masses. They rarely interact with other matter. Traveling at nearly the speed of light, they flit through Earth and everything on it only 8 minutes after they were formed at the sun's core. Radiation generated at the core, by contrast, takes roughly 100,000 years to leave the core because it interacts with intervening matter on its way out.
The collaboration that detected the proton-proton neutrinos used an underground neutrino telescope known as Borexino located some 1,400 feet below the surface. It consists of a spherical tank whose core is filled with 300 tons of a highly purified liquid similar to benzene. On the rare occasion when a neutrino interacts with the benzene, it triggers in a faint flash of light picked up by photomultiplier tubes that surround the container.
Earth is bathed in a steady stream of solar neutrinos, which zip through largely unnoticed at a pace of some 420 billion per square inch per second, based on the team's measurements. By some estimates, proton-proton neutrinos comprise 86 percent of all the neutrinos the sun generates.
Now, the team is now directing its efforts to try to detect solar neutrinos formed using carbon, nitrogen, and oxygen as catalysts. That means further purifying the detector's liquid, which "already is by far the cleanest mass of liquid that we know of," Dr. Pocar says. "It's a really challenging task."
The target neutrinos travel with enough energy for Borexino to spot them. But over the range of energies they carry, there is no sharp peak or valley in the energy spectrum for an high-confidence Eureka moment. And the range of energies this type of neutrino carries closely matches that of confounding sources from the telescope's natural surroundings.
Given the challenge, it may be that the best the team can do is set an upper limit on the number of CNO neutrinos they pick up. Even that, however, could allow researchers to begin testing models of CNO processes.
"There's no certainly of success," he says. But "it's exciting basic science."