Data from Wikipedia and NASA (C)


Star centers of our solar system; round to it rotate the planets, comets, asteroids and minor bodies likee meteoroida and powder.


Since the antiquity has been attempted explanations on the structure and composition of the sun. Greek philosopher Anassagora thought that it was a great sphere of inflamed metal, while Eratostene was the first one to calculate with precision the distance of the Earth from the Sun, in III century a.C., approximately 149 million kilometers, value much neighbor to that one modern. Copernico was the first one to put it to the center of the solar system in 16° the century, while Galileo with the first observations with the telescope discovering the solar spots. Isaac Netwon discovered its spectra but it studied Fraunhofer later on, while William Herschel discovered the infrared radiation. Lord Kelvin proposed that the sun was liquid and in cooling, and that it emitted its last inner heat. An other wrong hypothesis was the meteoric hypothesis on its formation. Only Rutheford in the 1904 began to propose a mechanism of radioactive decay like source of inner energy of heat. After the discovery of the relation between mass and energy of Einstein, Arthur Eddington proposed the idea of fusion reaction nuclear to its inside. The idea was studied from the physicists Chandrasekar and Bethe.

The reactions in the sun

The sun has a inner temperature of 15 million °C and a density 1,50 g/cm³ maintained by means of the fusion reactions proton-proton of the hydrogen nuclei); every second 600.000.000 tons of hydrogen are transformed in 595.500.000 tons of helium. The lacking 4.500.000 tons hydrogen transform directly in energy second the equation of Einstein E=mc². The energy generated every second is equal to 405 x 1024 Joule, than expressed in watt-hour TWh (is equivalent to 112.500.000.000 for comparison the world-wide production of electric power in 2005 has been of 17.907 TWh).


The Sun is a star of main sequence of spectral type G2 (a yellow dwarf). A star of G2 type, before exausting all the its fuel completely, has a life of approximately 10 billions of years, and currently the sun is to half of its vital cycle. The energy generated is released initially through gamma rays and X, but during the emission towards the outside it is continuously absorbed and released from surrounding atoms. The result is that its frequency is lowered, and becomes therefore a electromagnetic radiation more lowland. The passage of the energy from the nucleus to the surface of the sun employs also 10 million years. To the end by means of its emission the sun appears to us like a body to approximately 5500 °C. Part of the energy is emitted under shape of neutrini, kinetic and thermal energy of the solar wind and a part goes to form its magnetic field. The inner zones of the Sun (nucleus and radiative zone) have highest values of temperature and pressure and the matter is found in a degenerate system said plasma. In the more external zones (convettive zone, fotosfera, cromosfera and solar corona) the matter is found instead under gas shape. It is understood that the Sun is not a solid body and is subject to a differentiates rotations : as an example, the Equator rotates more fastly of the poles. The  differentiates rotations provokes them a distortion of the lines of magnetic field that form the solar spots then. The solar spots have a cycle of approximately 11 years. During this period there is the maximum and minimal peak of visible solar spots. The maximum peak corresponds on the earth to a warmer period, caused from an intense solar activity, while the minimal peak corresponds to a colder period. The solar magnetic field is extremely complex and is subject to periodic cycles of activity and passing, with frequent reversals of the magnetic poles. The solar surface is extremely turbulent, comes called fotosfera and, beyond to the spots, it has phenomena like the solar eruptions. The solar corona, endured over the solar surface, is a lot rarefied, with a temperature superior to the million degrees.


The nucleus is the inner zone of the Sun, where happen the thermonuclear reactions of fusion of hydrogen. The temperature is of the order of 15 million kelvin, the density is of approximately 160 g/cm³ and the pressure around to 500 billions of atmospheres. It is hidden to the direct observation. The information on its state come from the eliosismologia, that it takes advantage of the vibrations of the Sun, and from the emitted analysis of the neutrini from the fusion reactions. Other data as the production of energy total and the chemical composition of the surface allow to define the theoretical models. To these temperatures the hydrogen atoms of the nucleus separate in protons and electrons. The thermal energy is therefore high that when protons met met accidentally, gains the electrical repulsion electrical between same charges and they join to form a helium nucleus. Every second 594 million tons of hydrogen are converted. Therefore the sun is lightened every second of 4 million tons. Its total mass is enough large because, also after 10 billions of years of active life, its mass is reduced only imperceptibly. The energy freed from the fusion nuclear is introduced initially under photon shape. But they do not make much road interacting with other atoms. Once caught up the surface of the Sun they travel in the interplanetary space. The neutrini, other by-product of the fusion nuclear reactions, passes instead through the matter, and exits from the Sun online straight. A smallest part is intercepted from the little detectors of neutrini in activity on the planet. To the conditions enforced in the center of the Sun the medium proton must wait for 13 billions of years before melting with others three and forming a helium nucleus. As the time passes to the probability of the reactions it increases. The solar brightness increases slowly therefore, that it has induced some theorists to assume that between 500 million or a billion years the Sun will be too much warm in order to concur the life on the Earth. This increase is independent from the stellar evolution to which the Sun will go encounter, and that it will carry it between approximately 5 billions of years transforming itself in a giant red. The solar nucleus will become still more concentrated warmth than today: the fusion of helium, typical of the red giants, demands hundred of million degrees.

Radiative zone

Situated to the outside of the nucleus, it absorbs the produced energy and it transmits for radiation to the superior layer. Pressure and temperature are still enough elevated  to maintain the matter in state of plasma.

Convective zone

Situated to the outside of the radiativa zone, transports the energy towards the outside by means of convettivi motions. The gas that composes this zone moves, schematically, like the water of a pot in boiling. It has a thickness of approximately 450000 km, in this zone the gases forgiveness and absorb energy, but for minor the temperature it does not give place to nuclear reactions.


The fotosfera is the superficial layer of the Sun, that is the zone of emission of the visible light, is thick approximately 300 km. In it the temperatures are a few inferior to the 6000 kelvin, and are center of phenomena like the solar spots and flare. To the direct observation the fotosfera has a granular surface, since are present the convettivi grains, that is the emissions of heat that emerges from the inner layers through the convettivi motions. The solar fotosfera is composed from cells of convection calls grains; every grain is a wide fire storm from 500 to 1000 km, to the center of which warm gas it go up from the inside of the star, cooling off itself and falling back to the edges for convettivo motion. A single grain has a average life of single 8 minuten, but they form new continuously, giving to the fotosfera an aspect similar to a slow boiling. Between normal grains they are found large supergrains until 30.000 kilometers, able resisting until to a day.


The cromosfera is the superior layer respect the fotosfera. It is transparent, visible only with special filters or during the totals eclipsess of Sun. This layer is interested from various emissivi phenomena like spicule and the solar swells. It is thick 10,000 kilometers and its red color can be observed during an eclipse total or in filtered light, like the H-alpha. The more common visible formations on the solar fotosfera are the spicule, long luminous gas jets that exit from the fotosfera. The spicule go up until the top of the cromosfera and then they fall back more low, approximately in 10 minuten. An other cromosferica formation is the fibrils, horizontal gas layers similar like dimensions to the spicule, but with a double average life. The most spectacular formations, and also rarest, are the solar swells, gigantic gas eruptions that catch up heights of 150.000 kilometers. Only the solar glitters can exceed them in energy.

Corona - Crown

 the solar corona is the more external part of the solar atmosphere, it does not have defined limits and it extends for tens million km. It is constituted from particles of ionized gases (hydrogen) and vapors from the below layers of the solar atmosphere. The temperature is much elevating (more than a million kelvin), superior to the fotosfera and the gas is a lot rarefied being in the state of plasma. Moreover the solar corona can be the solar wind for the entire solar system . It produces millionth of the visible light. The crown is separated from the fotosfera from the cromosfera. The exact mechanism of heating is a topic of scientific debate, but the main hypotheses include the induction of the solar magnetic field and the waves of sonic pressure (the last possibility are less probable now that has discovered the presence of crowns in primordiali stars). The crown is dispersed from the solar wind in its extremities. The crown is not uniform distributed around the surface: during the periods of quiet approximately it is confined in the equatorial regions, with "holes" in the polar regions, while during the periods of solar activity it is distributed around the Equator and to the poles and is mainly present in the areas of activity of the solar spots. Transienti the "transienti" of the crown are enormous amounts of material of the crown that travel more than million than km/h, and containing approximately 10 times the energy of flare that it provokes them. Some greater expulsions can emit hundred of million tons of matter in the space: when they catch up the Earth can damage the satellites and disturb the telecommunicationses.

Solar spots

A solar spot is a region of the fotosfera characterized from a smaller temperature of the surrounding atmosphere and from strong magnetic activity. Also if the solar spots are extremely luminous, because they have a temperature of approximately 5000 kelvin, the contrast with the surrounding regions, still more luminous thanks to a temperature of 6000 kelvin them renders clearly visible like dark spots. If they were observable isolatedly they would be more luminous than one filament lamp. The number of solar spots is correlated with the intensity of the solar radiation. During the Minimum of Maunder they nearly disappeared, and the Earth in the same period was cooled off in consisting way. The correlation between the two events is object of arguments in the scientific community. Observation of the solar spots : the first probable references to the solar spots are those of the Chinese astronomers of the first millenium d.C., than probably they could see the groups of larger spots when the splendor of the sun was diminished from the powder raised from several deserts of Asia centers. They were observed telescopically for before the time in the 1610 from frisiani astronomers Johannes and David Fabricius, that they published a description in the june of 1611. In this date Galileo it was already showing the solar spots to the astronomers to Rome. The solar spots had a some importance in the debate on the nature of the solar system. They showed that the Sun rotates on itself, and the fact that appeared and disappeared demonstrated that the Sun endured of the changes, against to the instructions of Aristotle. The details of their appearing motion could not be explain unless in the eliocentrico system of Copernico. The searches on the solar spots continued in the XVII and the beginning of XVIII century, because of the Minimum of Maunder nearly no solar spot was visible for many years. But after the resumption of the solar activity, Heinrich Schwabe could bring back in 1843 a periodic change in the number of the solar spots, that then it would have been called the undecennal cycle of the solar activity.

Magnetic field

The plasma and the electric particles that form the Sun generate a powerful magnetic field, connected to many solar phenomena like the spots and the solar eruptions. The layers of eliosferiche currents extend until the external edges of the solar system, and derive from the interaction of the solar magnetic field with interplanetary plasma. The Sun rotates to the Equator (approximately 25 days) regarding the greater latitudes (approximately 35 days close to the poles). This differentiates rotations them to the various latitudes of the Sun provokes to the twisting of the lines of the magnetic field and the consequent formation of solar spots and solar swells.

Solar wind

The solar wind is a particle current emitted from the solar atmosphere. The solar wind is one particle current (more than other protons with high energy, ~500 keV) emitted from the solar atmosphere. Also the other stars show the same phenomenon, and it is spoken therefore about stellar wind, or also of loss of mass. In 50 years ' a German scientist of name Ludwig Biermann studied comets, and the fact that their tail always headed in opposite direction to the Sun. Biermann postulated that this happened because the Sun emitted a constant particle flow that pushed far away the tail of the comet. Eugene Parker understood that the flow of heat from the sun in the model of Chapman and the tail of the comet tip via from the sun in the hypothesis of Biermann had to be the result of the same phenomenon. Since the gravity force is weakened with the distance from the sun, the external solar corona escapes in the interstellar space. In the solar system, the composition of this wind is identical to the crown of the Sun: 73% hydrogen and 25% helium, with the rest formed from traces, and heavy are ionized. Close to the Earth, the speed of the varied solar wind varied from 200 km to the second to 900 km/sec. The Sun loses approximately 800 kg material to the second ejecting it under shape of solar wind. The solar wind is a plasma and bring with himself the magnetic field of the Sun in all the interplanetary space until to a distance of approximately 160 astronomical units. It moves in radial direction regarding the Sun, and because of the rotation of this the lines of the magnetic field are bent so as to form a spiral. Particularly violent emissions of solar wind, call solar flare, and other phenomena of "solar meteorology" call "solar storms" emit great amounts of radiations that can damage the probes space and the satellites. The particles of the solar wind caught from the earth magnetic field stretch to group themselves in the bands of Van Allen, and cause the boreal dawn and that austral one when they close hit the earth atmosphere to the poles. Other planets with magnetic fields similar to those of the Earth have. The zone of the solar system in which the force of the solar wind he is not more sufficient to push behind interstellar means is known like eliopausa, and often is considered like the external border of the solar system. The distance of the eliopausa is not known with precision. Probably it is much smallest on the side of the solar system that is found "" regarding the orbital motion of the solar system in the galaxy. It could also vary to second of the speed of the solar wind at the moment, and to second of the local density of interstellar means. It is knows that it is very beyond the Pluto orbit. The probes space them Voyager 1 and Voyager 2, after to have finished their planetary exploration, they are heading towards the outside of the system and it hopes that they will arrive until the eliopausa.

Missions space

The first satellites plan to observe the Sun were Pioneer 5, 6, 7, 8 and 9 of NASA, launch between 1959 and 1968. These probes carried out the first measures detailed of the solar wind and the magnetic field. In years ' 70 operated the probes Helios 1 and Skylab. They came observed the first emissions of mass and the holes of the solar corona. NASA launch in 1980 the Solar Maximum Mission, constituted from a probe planned in order to observe the ultraviolet radiations, gamma rays and X rays coming from the solar flare during a period of high activity. The Japanese satellite Yohkoh came launch in 1991 and observed solar flare to the wavelengths of i X rays. The collected data allowed to identify various types of flare and demonstrated that the solar corona, also in the various periods from those of maximum activity, is more active and dynamics than how much was not supposed. The Ulysses probe came instead planned in order to study the polar regions; it came launch and directed so to go away from the ecliptical plan. Make observations of the solar wind and the magnetic field strength. The Genesis mission executed test on the solar wind and was planned for having a direct measure of the composition of the solar material