Passage: Standard solar models have historically predicted a specific flux of neutrinos produced by nuclear fusion in the core of the Sun. However, for decades, detectors on Earth consistently measured only about one-third of the expected neutrino count, a discrepancy known as the solar neutrino problem. This led to a crisis in astrophysics: either the solar models were fundamentally flawed, or the understanding of neutrino physics was incomplete. The resolution emerged with the discovery of neutrino oscillation. According to this theory, neutrinos possess a non-zero mass and can spontaneously change flavor—transforming from electron neutrinos into muon or tau neutrinos—during their flight from the Sun to Earth. Since early detectors were sensitive only to electron neutrinos, the other two flavors remained undetected, creating the illusion of a deficit. Experiments at the Sudbury Neutrino Observatory eventually confirmed that the total flux of all three neutrino flavors matched the predictions of solar models perfectly. This confirmation not only resolved the mystery but also necessitated a revision of the Standard Model of particle physics, which had previously assumed that neutrinos were massless.