Neutrinos ( little neutron in Italian, coined by Enrico Fermi) reveal themselves when one strikes an atom and generates a flare of light, but the event is rare.

Although it takes many hundreds of thousands of years for the energy generated in the sun’s core to fight its way to the surface and then fly to Earth, the neutrinos come straight out. So the surface brightness tells us about the ferocity of the nuclear reactions that were taking place hundreds of thousands of years ago. The neutrinos tell us about the reactions taking place today – and there was a clear difference in the two rates.