<P> Helioseismology observations made it possible to measure the interior temperatures of the Sun; these agreed with the well established standard solar model . Furthermore, detailed observations of the neutrino spectrum from the more advanced neutrino observatories produced results which no adjustment of the solar model could accommodate: While the overall lower neutrino flux (which the Homestake experiment results found) required a reduction in the solar core temperature, details in the energy spectrum of the neutrinos required a higher core temperature . This happens because different nuclear reactions, whose rates have different dependence upon the temperature, produce neutrinos with different amounts of energy . All adjustments that could be made to the solar model worsened some aspect of the discrepancies . </P> <P> The solar neutrino problem was resolved with an improved understanding of the properties of neutrinos . According to the Standard Model of particle physics, there are three flavors of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos . Electron neutrinos are the ones produced in the Sun and the ones detected by the above - mentioned experiments, in particular the chlorine - detector Homestake Mine experiment . </P> <P> Through the 1970s, it was widely believed that neutrinos were massless and their flavors were invariant . However, in 1968 Pontecorvo proposed that if neutrinos had mass, then they could change from one flavor to another . Thus, the "missing" solar neutrinos could be electron neutrinos which changed into other flavors along the way to Earth, rendering them invisible to the detectors in the Homestake Mine and contemporary neutrino observatories . </P> <P> The supernova 1987A indicated that neutrinos might have mass because of the difference in time of arrival of the neutrinos detected at Kamiokande and IMB . However, because very few neutrino events were detected, it was difficult to draw any conclusions with certainty . If Kamiokande and IMB had high - precision timers to measure the travel time of the neutrino burst through the Earth, they could have more definitively established whether or not neutrinos had mass . If neutrinos were massless, they would travel at the speed of light; if they had mass, they would travel at velocities slightly less than that of light . Since the detectors were not intended for supernova neutrino detection, this was not done . </P>

The solution of the missing neutrino problem from the sun's core was resolved by the discovery of