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Sun, the star closest to the Earth, for which, directly or indirectly, constitutes a unique and essential source of energy and thus of life. By analogy, the name is only used by astronomers as a synonym for Star in general, especially as this is considered as the center of a planetary system. The distance that separates the Earth from the sun is variable and passes, in the course of the year, from a minimum of 147.1 million kilometers to a maximum of 152.1 million km, with an average value of 149.598 million kilometers (astronomical unit, AU ). Consequently, the solar light takes about 8 minutes to reach the Earth. The distance variations are variations of the angular diameter of the Sun. The Sun has R⊙ radius = 695970 kilometers (109 times that of Earth). It was not until today revealed a possible crushing of the disk: however, the polar diameter differs from the equatorial to less than 1 / 100,000. The Sun is classified as a star of the population The disc, which belongs to the luminosity class V (formed by dwarfs or main sequence) and the spectral type G2. ≃4,6 has an age of billions of years. It is located in the Orion arm of the Milky Way; rotates around this point with a speed of 220-250 km / s, making a complete revolution in about 250 million years. Than the surrounding stars, it has a peculiar speed of 19.7 km / s directed toward a point on the celestial sphere, called solar maximum, located near the constellation of Hercules. Starting from the center, there are three key areas: a) the core, site of the nuclear fusion reactions that supply the energy astro; b) the radiative region, the intermediate layer, where the energy, produced in the core, is transmitted primarily by radiation; c) the convective region, the outer layer, where the energy is transmitted mainly by convection. Above the solar surface extends the atmosphere. The electromagnetic emission of the Sun, essentially of photospheric origin, consists of a continuous spectrum, to which are superimposed numerous dark absorption lines, which constitute the Fraunhofer spectrum. The rotation was discovered in 1610 by Galileo, who noted how appear stains from an edge of the solar disk, and then gradually they moved from off the opposite edge after 13-14 days. Solar energy is produced by nuclear fusion of hydrogen into helium.