Sun

The Sun (or Sol) is the closest star to Earth and the largest element of the Solar System. The stars are the only bodies in the universe that emit light. The Sun is also our main source of energy, which is manifested mainly in the form of light and heat.Untitled Prezi - Prezi
The Sun contains more than 99% of all matter in the Solar System. It exerts a strong gravitational pull on the planets and spins around.
The Sun was formed 4,650 million years ago and has fuel for 5,000 million. Then begin to get bigger and bigger, until it becomes a red giant. Finally, it will sink under their own weight and become a white dwarf, which may take a trillion years to cool.
==Structure==
The structure of the Sun contains the following layers:
* Core – the innermost 20-25% of the Sun's radius, where temperature (energies) and pressure are sufficient for nuclear fusion to occur. Hydrogen fuses into helium (which cannot currently be fused at this point in the Sun's life). The fusion process releases energy, and the helium gradually accumulates to form an inner core of helium within the core itself.
* Radiative zone – Convection cannot occur until much nearer the surface of the Sun. Therefore, between about 20-25% of the radius, and 70% of the radius, there is a "radiative zone" in which energy transfer occurs by means of radiation (photons) rather than by convection.
* Tachocline – the boundary region between the radiative and convective zones.
* Convective zone – Between about 70% of the Sun's radius and a point close to the visible surface, the Sun is cool and diffuse enough for convection to occur, and this becomes the primary means of outward heat transfer, similar to weather cells which form in the earth's atmosphere.
* Photosphere – the deepest part of the Sun which we can directly observe with visible light. Because the Sun is a gaseous object, it does not have a clearly-defined surface; its visible parts are usually divided into a 'photosphere' and 'atmosphere'.
* Atmosphere – a gaseous 'halo' surrounding the Sun, comprising the chromosphere, solar transition region, corona and heliosphere. These can be seen when the main part of the Sun is hidden, for example, during a solar eclipse.
==Sunspots==
Sunspots have a dark central part called umbra, surrounded by a lighter region called the penumbra. Sunspots are dark because they are cooler than the surrounding photosphere.
The spots are the place of strong magnetic fields. The reason why cold sunspots are not yet understood, but one possibility is that the magnetic field in the spots allows convection beneath them.
Sunspots generally grow and last from several days to several months. The sunspot observations revealed first that the Sun rotates in a period of 27 days (as seen from Earth).
The number of sunspots on the Sun is not constant, and changes over a period of 11 years known as the solar cycle. Solar activity is directly related to this cycle.
==Solar prominences==
The solar flares are huge jets of hot gas ejected from the sun's surface, extending to many thousands of miles. The biggest flares can last several months.
The Sun's magnetic field deflects some bumps that form a giant bow well. They occur in the chromosphere which is about 100,000 degrees.
The bumps are spectacular phenomena. They appear in the limb of the Sun as flaming clouds in the upper atmosphere and lower crown and consist of clouds of material at lower temperature and higher density than its surroundings.
Temperatures in the central portion are approximately one hundredth of the temperature of the crown, while its density is about 100 times that of the corona environment. Therefore, gas pressure inside the protrusion is approximately equal to its surroundings.
==The solar wind==
The solar wind is a stream of charged particles, mostly protons and electrons that escape the Sun's outer atmosphere at high speeds and penetrate the Solar System.
Some of these charged particles are trapped by the terrestrial magnetic field spiraling along the lines of force from one to another magnetic pole. The northern and southern lights are the result of interactions of these particles with air molecules.
The solar wind speed is about 400 kilometers per second in the vicinity of Earth's orbit. The point where the solar wind is coming from other stars is called the heliopause, and is the theoretical limit of the solar system. Is at about 100 AU from the Sun The space within the boundary of the heliopause, containing the Sun and the solar system, is called the heliosphere.
==References==
Category:Big bodies