Generalities: the Sun is a star classified in the spectral class G2, corresponding to a surface temperature of 5,500-6,000 degrees.
Its esteemed age is of around 5 billion years, and  its life will last as long, until  it is transformed at first into a  red giant and then into a white dwarf , to get definitely cold, because of the exaustion of the nuclear fuel.

Structure: the internal  structure of the Sun shows different areas, each one of them is characterized by a  different type of activity.
- Nucleus: the nucleus is the area where the thermonuclear reactions take place  and it occupies the 20-25% of the solar ray. Here hydrogen, because of the high temperatures and pressures,  creates reactions  of nuclear fusion, which produce helium nuclei. The temperature in this area is of about 15 million degrees.
- Radiative area: getting further  toward the outside the so-called radiative area is found, the widest, which goes to about 3/4 of the solar ray. In this area, the great energy, emitted in the nucleus by the thermonuclear reactions,  is transmitted in the form of high energy electromagnetic radiations  (gamma rays, X rays and ultraviolet rays). Energy cannot propagate in the form of heat since the atoms, because of the  temperatures even higher than 2 million  degrees, are strongly ionized and therefore unable to transmit  heat for convection. High energy radiations are continually absorbed and emetted by ionized atoms again, progressively losing energy and transforming themselves into visible and infrared radiations. This process requires very long times, often superior to million years.
- Convective area: going away from the center of the Sun the so-called convective area is found . The progressive decrease in  temperature causes  temperatures to get  lower and lower and  the atoms to recover partially at least  their electrons, creating therefore  a decrease in  ionization. For this reason, the energy emerged from the deepest areas of the sun begins to be transmitted in the form of visible light and heat. This means that  in the convective area the transmission of heat takes place  toward the surface through enormous columns of gas that go up again, while analogous columns of gas, which have handed by now their heat over toward  the outside, go down again in depth. The dimension of the convection cells, or of the columns of ascending and descending gas, is greater in the deepest area in the convective area, while next to the solar surface, the circulation of the gases takes place in much smaller cells, which create a granulation of the solar surface, with small brighter areas and other less bright ones, said rice grains, which move continually.
- Photosphere: the solar surface is said photosphere and it corresponds to  the external limit of the solar disk observable  from the Earth. Its temperature is  5,500-6,000 Celsius degrees, therefore extremely inferior to that of the most internal areas of the sun. The photosphere shows the phenomenon of the sunspots, observable with suitable instrumentation, also from the Earth.
The sunspots are darker areas since  they have a temperature inferior to that of the rest of the photosphere (about 4,000 degrees). The sunspots surely originate from complex interactions among the gases ionized of the sun and the strong magnetic field introduced by our star. Through them it has been possible to notice the sun rotation  around itself, since even these formations  move in agreement with the star surface. The life of a sunspot can last from a few days to some months. The activity of the sunspots follows a cyclical course, which reaches a climax every  11 years, when the number of sunspots and the magnetic activity of the sun in general are at the most. The descent from the maximum  to the minimum takes seven years, while the ascent to the maximum activity  is more rapid (4 years).
The areas of the sunspots are often center of the solar twinklings, brief and sudden raisings of the brightness, tied up to the solar magnetic field.
- Cromosphere: beyond the sun surface  the cromosphere is found, so called for its bright pink color. Here the mattert is more rarefied, but extremely turbulent. From the cromosphere  in fact the protuberances develop, constituted by throws of incandescent gas  that  raise like  tongues of flame up to 300-400 thousand Km. from the surface, moving  at high speed. Each of these phenomena can last  short periods or  a few weeks . From the cromosphere the spicules are also produced, which are simply phenomena similar to the protuberances, but of notably inferior course.
- Solar crown: the last solar formation, the most peripheral is the crown, which shows very rarefied ionized gases, but at temperatures sharply overcoming those of the fotosphere. The temperature of the crown gases  goes up again to 2 million degrees and this, being not explainable through the laws according to which the heat is transmitted,  seems due once more to the effect of the powerful  solar magnetic field on the  gases present in this area. The crown shows a jagged form from which wider areas depart, which take the name of plumes. The solar crown is visible only from the Earth on the occasion of the total sun eclipses . From this area of the sun  the so-called solar wind expands toward the external space, that whole of  loaded particles uttered by our star and reaching also our planet, creating moreover the phenomenon of the polar auroras.

Motions of the Sun: it is possible to  distinguish essentially three of them: the rotation around its own axis the translation in the space and the revolution around the center of the Milky Way. The rotation around its own axis in about 25 days and 8 hours is completed  for the areas of the equator. Not being formed by rigid material, the sun rotates with different speed according to the latitude. The polar zones are slower, employing about 34 days to complete a turn. The translation is the motion that the sun, with all its planetary system carries out toward a point in the direction of the constellation of Hercules, at a speed of 19.4 Km/sec.
The revolution around the center of the galaxy (from which it is 25,000 light-years far)  is a motion that the sun carries out together with all the other stars. A complete turn requires a very long time: 240 million years.
 

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