To peer into the heart of the sun, a 13.7-meter-wide stainless steel shell lined with over 2,200 light-gathering sensors hides deep under a mountain in central Italy. Known as the Borexino experiment, it watches for flashes of light from neutrinos, ghostly subatomic particles shot out of the sun’s core.
After seven years of searching, Borexino scientists report in the Aug. 28 Nature that the detector has for the first time caught a glimpse of the neutrinos cast out of the sun’s main nuclear reaction.
The sun supports itself by transforming hydrogen into helium. Neutrinos are one by-product of this alchemy. These particles have so little mass, they barely exist at all; roughly 10 billion trillion pass through the Earth every second without touching a single atom.
To spot such elusive prey, Borexino’s sensors surround a vat filled with 300 tons of a liquid hydrocarbon. Occasionally, a neutrino slams into an electron within the liquid and generates a flash of light. By recording the rate of detections and how much energy is in each burst, researchers can identify the source of the neutrinos.
Borexino has detected solar neutrinos before, but these are the first from the simple fusion of two protons that leads to 99 percent of the sun’s energy. By watching neutrinos arrive from the sun, researchers can test ideas about what powers both our star and billions of others.