After studying the mechanism that powers the Sun, we addressed the one operating in stars. In stars of equal or smaller size than the Sun, the mechanism remains the pp chain. However, in massive stars—with a mass at least 30% greater than that of the Sun—the mechanism is different and is known as the CNO cycle. The opportunity to study this cycle arises from the fact that the same mechanism also occurs in the Sun, although it contributes only about 1%. Reconstructing the energy of neutrinos from the CNO cycle is challenging due to the reduced flux—just 1% of the solar neutrino flux—and the overlapping presence, within the same energy window, of neutrinos from the CNO cycle and radiation from the contaminant Bismuth-210. These energy distributions also have a very similar shape. To address this, the flux of Bismuth-210 was measured using Polonium-210, present in the scintillator and in secular equilibrium with it. Additionally, it was necessary to stabilize the temperature of the detector to prevent convective movements in the scintillator. Finally, we succeeded in measuring the flux of CNO neutrinos, confirming their existence in a discovery awaited for 90 years.