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3. The Sun         

3.1. Introduction and History            

           The Sun is the central body of the Solar system. It represents a high-temperature plasma full-sphere consisting mainly of the atomic hydrogen. The Sun is shown in Fig.3.1.



Fig.3.1 The Sun 
Beside hydrogen the Sun also comprises about 25 % of helium. The number of atoms of all other elements in total is 1000 times less than the number of atoms of hydrogen. The weight of the Sun is 332958 times more than the weight of the Earth. 99,866 % of the weight of the Solar system is concentrated in the Sun. The diameter of the Sun is 1,392109 m , which is 109 times more than that of the Earth. At the same time the Sun is an ordinary star of our Galaxy. Therefore, such subjects as the sources of energy of the Sun, its structure, the formation of the spectrum are common for the physics of the Sun and stars.
The Sun as a star is a typical yellow dwarf located in a mean part of main sequence of stars in Herzshprung-Ressel's diagram. The velocity of the motion of the Sun relative the set of the nearest stars is 19.7 km/s. The Sun is located inside one of spiral branches of our Galaxy in about 10 Kpc apart from its center. The period of its revolution around the center of the Galaxy is about 220 million years. The power of the total radiation of the Sun make up 3,831026 W, about 21017 W. of which comes to the Earth.
It is considered that the power source refilling the energy lost by the Sun and supporting the high temperature of about 6000K are nuclear reactions of the transition of hydrogen into helium, which takes place inside the Sun. However, actually the internal structure of the Sun and sources of its energy follows only from the models used for calculation. No direct experiments have been made till now, because the Sun is not transparent for electromagnetic waves and for any particles except for the neutrino. Exploring the flows of neutrino it is possible to define the sources and the structure of the Sun. But, long-term researches directed to searching of the neutrino radiation concomitant to the thermonuclear synthesis have not resulted in success. This determines searching for new sources of energy of the Sun that are not connected with thermonuclear synthesis.

 
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The atmosphere of the Sun is formed by the external layers accessible to observation. Almost all radiation of the Sun is emanated from the lower part of its atmosphere called a photosphere that has the temperature about 6000K. In the sub-photosphere zone of the Sun there is visible a graininess, which is the so-called granulation surface texture of the Sun. Unlike other formations on a surface of the Sun, the granulation is identical along all surface and does not depend on the solar activity.
Sun-spots and plumes are frequently observed in the photosphere. Sun-spots are dark formations having a lower temperature – up to 4500K. Generally, the spots form groups. Groups of spots are always surrounded by plumes and protuberances. The mean altitude of protuberances above the surface of the Sun makes up 30-50 thousand km. The temperature of protuberances does not exceed 5000-10000 K. The nature of these phenomena has not been found till now. The layer of the Sun's atmosphere named the chromosphere is located above the atmosphere. The temperature of this layer at first drops with the increase of the altitude from 6000K up to 4000K, and then sharply increase up to 1,510K at the altitude 2000 km and also passes over into the solar corona. The solar corona is the most external and rarefied part of the Sun extended to more than 10 solar radiuses. In the external parts of the corona the majority of energy is taken away by the solar wind. The solar wind is a flow of the coronal gas, having the velocity that is sufficient for overcoming the Sun's gravitation.
Recently, there were found new phenomena in the surface of the Sun in the form of stationary oscillations of its surface – the sunquakes. Their nature is also an enigma.
The history of the research of the Sun stared with the telescopic observations made by Galileo who discovered sun-spots in 1611 and defined the time of the cycle of the Sun about the axis. In 1843 the German astronomer G. Schvabe found out a repetition of the solar activity. In 1814 J. Fraunhofer discovered dark lines in an absorption spectrum of the Sun that started studying its chemistry. Since 1836 regular observations of the Sun's eclipses have been conducted. This resulted in discovery the Sun's corona and the chromosphere and also the solar protuberances. In XX century the radio emission of the Sun was studied. There was determined the Sun's temperature and the oscillations of the surface. The description of the power sources of the Sun is still a conventional problem.

 
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