How do solar neutrinos provide evidence about nuclear reactions in the Sun?

Solar neutrinos provide direct evidence of the nuclear reactions occurring in the Sun's core, where hydrogen is fused into helium via the proton-proton chain.

Because these fusion processes release neutrinos as by-products, detecting them allows scientists to observe the Sun's interior energy generation in real time. While photons generated in the core take approximately 170,000 years to reach the surface due to constant scattering, neutrinos escape the Sun almost immediately, providing a "snapshot" of current solar activity.123

Historically, experiments detected fewer electron-neutrinos than theoretical models predicted, a discrepancy known as the solar neutrino problem. This was eventually resolved by the discovery that neutrinos undergo "oscillations," changing from electron-neutrinos into other types (muon or tau neutrinos) during their journey to Earth. This phenomenon confirmed that neutrinos possess a small, non-zero mass, a finding that significantly advanced both nuclear theory and our understanding of fundamental particle physics.1425