Tiny phenomena, big discovery

By catching glimpses of some of the smallest things in the universe, a team of researchers in Italy says it has finally proven what makes many stars burn.

Nearly 100 scientists at the Borexino neutrino observatory announced they spotted neutrinos produced by a type of nuclear fusion reaction responsible for about 1 percent of the sun’s energy. The discovery, published in a Nov. 25 report in Nature, provides the first tangible evidence that the carbon-nitrogen-oxygen fusion cycle (CNO) operates within the sun.

The sun creates light by turning hydrogen into helium in a nuclear fusion process known as the proton-proton chain reaction. But since the late 1930s, scientists have speculated that larger, hotter stars use a more complicated six-step fusion reaction. The study's authors said observing special neutrinos produced only by this CNO reaction and tracing their paths to the center of our solar system demonstrates that our sun uses such processes, though at much lower levels than larger stars.

“It’s really a breakthrough for solar and stellar physics,” Gioacchino Ranucci, an Italian scientist who has worked on the project since 1990, told NBC.

Neutrinos are hard to see because they are “both almost massless and almost noninteracting,” Ohio State University professor John Beacom said. Scientists call neutrinos an elementary particle because they have no constituent parts and cannot be divided. In that way, neutrinos are like electrons, only smaller. But neutrinos don’t respond to the electromagnetic forces that bind the protons, neutrons, and electrons of an atom together. Because of that, neutrinos are typically able to pass through solid objects by shooting the gaps between other particles that make up the physical world. Scientists at the IceCube South Pole Neutrino Observatory theorize that nearly 100 trillion neutrinos pass through a human’s body every second.

Proving the CNO cycle in stars required scientists to observe a special kind of neutrino bearing the signature of that particular reaction. Using a dark 300-ton water tank lined with sensors, the Borexino observatory routinely spots neutrinos. But, according to Ranucci, only seven neutrinos created by a star’s CNO cycle flash through the observatory in a typical day.

Like the wind, scientists study neutrinos not by direct observation, but by the rare effects they can have on the physical world. At big neutrino observatories like IceCube and Borexino, they occasionally produce a faint blue light when passing through water or ice.