a new branch of observational astronomy that deals with the search for and study of neutrino fluxes from extraterrestrial sources. The neutrino is the only form of radiation that reaches a terrestrial observer from the interior of the sun or a star; the neutrino thus carries information about the internal structure of the sun or star and the processes occurring in it. Present methods of detecting neutrinos permit the observation of neutrino radiation only from the sun and supernovae in our galaxy.

Solar neutrino astronomy. The existence of an intense stream of neutrinos from the sun follows from the modern concept of the sun’s origin and structure; according to this concept, the sun’s luminosity is due entirely to energy from the thermonuclear transformation of hydrogen into helium in the core of the sun. As solar-model computations show (seeSTELLAR MODELS), the principal contribution to energy generation comes from the hydrogen cycle (proton-proton chain), while the contribution provided by the carbon-nitrogen cycle is less than 1 percent. The synthesis of each ⁴He atom is accompanied by the emission of two electronic neutrinos v_e and the total neutrino flux as determined from the sun’s luminosity is about 6.5×10¹⁰ neutrinos/(cm².sec) at the earth’s surface, with the neutrinos carrying away about 3 percent of the energy of the thermonuclear fusion.

The observation of solar neutrinos would be a convincing confirmation of basic ideas about the sun’s thermonuclear evolution. The measurement of neutrino fluxes from the various reactions, using an appropriate set of detectors, constitutes a complete program for studying the internal structure of the sun. Since the solar-neutrino flux undergoes seasonal variations with an amplitude of about 7 percent (which is connected with the eccentricity of the earth’s orbit), observation of these variations would serve as proof that the detected neutrinos were indeed solar neutrinos. Another method of determining the incident direction of the neutrinos consists in measuring the angular distribution of the electrons formed during the capture of the neutrinos in a detector (see below): because of the nonconservation of parity in beta decay, these electrons should be emitted primarily in the direction of the sun.

The first experiments on the observation of solar neutrinos were performed in 1967–68 by the American scientist R. Davis and co-workers with a radiochemical neutrino detector containing 610 tons of liquid perchloroethylene (C₂Cl₄). The detector was set up underground at a depth of 1,480 m in order to block out the cosmic-ray background. The detection of the neutrinos was based on a method proposed in 1946 by B. M. Pontecorvo. Solar neutrinos with energies greater than 0.814 megaelectron volt (MeV) from radioactive ³⁷ Ar with a half-life of 35 days in the reaction ³⁷C1 + v_e → e^- + ³⁷ Ar. According to calculations, the principal contribution (76 percent) to the effect must be due to neutrinos of the highest energy (up to 14 MeV) from the decay ⁸B → ⁸Be + e⁺ + v_e in the rarest branch of the hydrogen cycle. The flux of these neutrinos varies with the temperature T asT²⁰; thus the chlorine neutrino detector is a unique “thermometer” for measuring the temperature T_c in the sun’s core. Theory has predicted a value T_c ≈ 15×10⁶°K.

In Davis’ experiments, the ³⁷Ar was accumulated in the detector for 100 days, then extracted by bubbling helium through the liquid, adsorbed on activated charcoal at a temperature of 77°K, and placed in a proportional counter, which recorded the number of disintegrations of ³⁷Ar atoms. Measurements taken in 1972, as well as the first measurements in 1967–68, showed that the neutrino effect observed was several times smaller than that predicted by theory and did not exceed the background count of the detector—in the detector, not more than eight atoms of ³⁷Ar were accumulated per experiment instead of the expected 45.

Although solar neutrinos were not detected with certainty, the results of these experiments are an important achievement of neutrino astronomy, since they show that contemporary ideas about solar neutrinos are somehow incorrect. The solution of the solar-neutrino mystery can be sought in three ways. First, it is possible that T_c is less than the theoretical values predicted by standard models of the sun and is about 13×10⁶ °K; that is, the value lies below the sensitivity threshold of the “neutrino thermometer.” This means that the structure of the sun is different from what had been assumed up to now. Second, it may turn out that incorrect values were used for nuclear reaction rates in the calculations of the models; this would imply that the scale of the “neutrino thermometer” was incorrectly calibrated. Third, the “neutrino thermometer” may prove to be entirely “faulty” if something happens to the neutrinos on their way to the earth; for example, the neutrinos may decay if they turn out to be unstable particles, or they may oscillate and be transformed into a state in which they do not interact with chlorine. In order to solve the problem conclusively, it is necessary to increase the sensitivity of the chlorine neutrino detector and conduct additional experiments with detectors sensitive to lower-energy neutrinos, for example, ⁷Li, ⁷¹Ga, ⁸⁷Rb, and ⁵⁵Mn.

Another important problem of neutrino astronomy is the observation of solar neutrinos from the reaction²H + p + e^- → ²H + v_e (using ³⁷C1 and ⁷Li detectors), which necessarily accompanies the hydrogen cycle. The detection of these neutrinos would prove the occurrence of the hydrogen cycle on the sun and would lead to the rejection of hypotheses about anomalous properties of neutrinos, thereby confirming the correctness of the conclusion that the carbon-nitrogen cycle does not make a significant contribution to the sun’s energy generation. (If the carbon-nitrogen cycle made the principal contribution, about 300 ³⁷Ar atoms would be formed in Davis’ detector.)

Neutrino bursts. Neutrino fluxes from other “quiet” stars, even those that are very near, are extremely small and cannot be detected by contemporary methods. At the same time, the observation of neutrino bursts from a star at the moment of the gravitational collapse of the star is completely feasible. The most likely objects for observation are supernovae in our own galaxy, in which the core collapses just before the burst occurs. A neutrino burst can be detected even when the supernova cannot be observed optically. The duration of such a burst is about 0.01 sec, and the neutrino flux at the earth is 10¹⁰-10¹² neutrinos/cm² for each burst. By measuring the delay times of the onset of the burst at detectors at different places on the globe, we can establish the incident direction of the neutrino radiation. The bursts can be detected as a characteristic series of pulses by using several hundred tons of a hydrogen-containing scintillator. Such experiments are planned in the USSR and the USA.

Neutrino astrophysics. The need for studying astrophysical phenomena in which neutrinos participate has created a new branch of astrophysics—neutrino astrophysics. According to current thinking, neutrino emission, which increases rapidly with temperature, has a decisive influence on the final stages of the evolution of stars, when the temperature of the star’s interior reaches 10⁹ °K and higher. This situation is associated with the emission of neutrinos from the hottest innermost regions of a star (since the free length of neutrinos in matter is much greater than a stellar diameter); therefore, the neutrino emission determines the rate of energy loss in these stars. An example is the effect of the hypothetical electron-neutrino interaction predicted by the universal theory of weak interactions (see) on the evolution of the cores of planetary nebulae; by taking this effect into account, we can bring the observed data on the time of evolution into closer agreement with theoretical computations; the improved agreement, in turn, is an argument in favor of this interaction.

When the temperature at the center of a star reaches a value of about 10¹¹ °K, the free length of an electronic neutrino becomes comparable with a stellar diameter; with a further increase in temperature, the star becomes opaque to neutrinos. However, since the neutrino free lengths remain much greater than the photon free lengths, energy in the star is transported by the neutrino gas (neutrino heat conduction), and the loss of energy continues to be determined by the neutrino emission. At temperatures greater than about 2×10¹¹ °K, stars also become opaque to muonic neutrinos v_μ. These stages in the life of a star are most puzzling and interesting. It is assumed that neutrino emission plays a decisive role in the mechanism of supernova outbursts.

The development of neutrino astronomy and neutrino astrophysics promises to provide valuable information not only about the structure of celestial bodies but also about the nature of the neutrino itself and the properties of weak interactions.