Therefore, it is less important for energy production of the Sun. The CNO main cycle dominates in stars of which the mass is larger than 1.5 × M ⊙ with a higher core temperature than the Sun. The cycle transforms 4 protons to 4He and the relative abundance of C, N, O does not change. Those cyclic processes could go on continuously. The 12C produced during the last reaction would be the input nucleus of the first reaction. If all of the solar luminosity did originate from the gravitation energy of the contraction, then the corresponding age of the Sun would beġ2 C + p → 13 N + γ + 1.95 MeV, 13 N → 13 C + e + + ν e + 2.22 MeV, 13 C + p → 14 N + γ + 7.54 MeV, 14 N + p → 15 O + γ + 7.35 MeV, 15 O → 15 N + e + + ν e + 2.75 MeV, 15 N + p → 12 C + 4 He + 4.96 MeV. It was called the Helmholtz–Thomson (Kelvin) contraction. The conversion of gravitational energy released during contraction can be thought to be a possible origin of the solar energy. 1) But the necessary number of meteorites to keep the Sun shining was not validated observationally, and the idea gradually diminished.Īnother idea came along with the concept that the Sun itself contracted slightly, resulting in the liberation of light and heat. Transformations of the energy of one form to another form were thought to be a possibility.Ī constant falling of meteors or asteroids into the Sun may produce heat by the conversion of kinetic energy to thermal energy. The discovery of energy conservation and the discovery of the fundamental laws of thermodynamics at around the same time stimulated consideration about the source of heat in the Sun. In the middle of the 19th century, the origin of solar energy was a big question. The corresponding solar energy created at 150 million km away from Earth is 3.83 × 10 26 J/s, which is called the solar luminosity. The total energy of 1.37 kW/m 2 reaching the top of Earth’s atmosphere, is called the solar constant. The Sun produces a vast amount of energy, and is the mother of all the living creatures on Earth. The present and the future solar neutrino studies are described in section 9. The early solar neutrino experiments and the solar neutrino problem before the start of Super-K are summarized in sections 5 and 6, The Super-K detector is described in section 7 and the struggle towards understanding solar neutrino oscillation can be found in section 8. Basic concepts and formula of neutrino oscillation are given in section 4. The neutrino interactions relevant for the solar neutrinos are discussed in section 3. We briefly summarize historical notes on the Sun and solar neutrinos in section 2. In this review, we explain the long and winding road to obtain convincing evidence of solar neutrino oscillations with an emphasis on the role of the Super-K experiment. The discovery of neutrino oscillations has opened up a new horizon. Neutrino studies will be of significant importance in the future of elementary particles and cosmology. Then there may have been a possibility to create baryon number in the early universe through the CP violating process in the lepton sector. Neutrino oscillation may violate CP invariance. It was something like to solve a jigsaw puzzle to draw a consistent view among the experiments. Nevertheless, all of the experiments indicated deficits of the observed solar neutrinos. The different neutrino experiments cover different energy regions, so the effect of neutrino oscillation seen in the individual experiment is different. Many experiments to detect solar neutrinos were conducted using different target materials: chlorine, gallium, water, heavy water and scintillator, and with different technologies. However, the missing solar neutrino problem, first indicated in the early '70s, by a historical chlorine experiment, took more than a 30-year struggle to settle the issue. It took about 10 years to discover atmospheric neutrino oscillations after the initial indication of the atmospheric neutrino anomaly. Today about 20 years after the oscillation discovery, neutrino oscillation is still the only compelling and convincing evidence for physics beyond the standard model. Amazingly, this discovery was achieved 14 years before finding the Higgs boson, the last piece of the standard model. This is evidence of physics beyond the standard model of elementary particle physics, since neutrinos are massless in the standard model. Neutrino oscillation indicates that neutrinos have both masses and mixings. Three years later it was also shown that solar neutrinos oscillate, by comparing two independent experimental results from Super-K in Japan and SNO in Canada. The discovery of neutrino oscillation was announced in 1998 based on an observation obtained by studying atmospheric neutrinos in Super-Kamiokande (Super-K).
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