There are many sources of neutrinos in the universe. For example, neutrinos are produced inside the sun, the earth, and the entire atmosphere. Neutrinos are also produced during the birth, collision, and death of stars. Particularly huge flux of neutrinos is emitted during supernovae explosions. Most of the neutrinos that pass through the earth come from the sun and are produced in the nuclear fusion reactions going on inside the sun's core. In addition, (anti)neutrinos are produced in nuclear power plants. The beams of high energy protons striking a target material produce intense flux of pions and kaons, which then decay to produce neutrinos. There are neutrinos around us which were born almost 13.8 billion years ago,
soon after the birth of the universe; they constitute cosmic neutrinos (relic of the Big Bang). Neutrinos produced inside the core of the earth are called geoneutrinos.
Figure 17.1 represents the different sources of neutrinos. Besides these sources, neutrinos are also produced by our body; they are also present in vegetables and fruits, etc. In fact, we are living in a sea of neutrinos having a wide range of energies. Simply speaking, through an area of 1 cm2 of our body, almost a billion neutrinos pass every second. The theoretically obtained energy ranges of these neutrinos are quite broad, ranging from micro electron volts for the neutrinos left over from the Big Bang, right up to peta electron volts for the neutrinos produced in the violent gamma-ray bursts in the universe. Figure 17.2 shows the predicted neutrino flux as a function of neutrino energy from a variety of neutrino sources. Some of these neutrinos have been studied by various neutrino experiments. In this chapter, we will discuss some of the important neutrino sources and their detection techniques in various energy regions.
The sun is the main source of energy for the earth. It was first suggested by Eddington  that nuclear fusion is the source of solar energies as well as the source of energies of other stars. However, it was Bethe  in 1939, who explained the mechanism of thermonuclear fusion reactions, stating that hydrogen burning takes place inside the core of the sun, in which neutrinos are being produced, along with helium and positrons.