The Sun,of a vital importance for the inhabitants of the planet Earth,is a star of medium dimension similar to other stars.
lts predominant role in the formation of the Earth and in the evolution of all the organisms is due to its neamess.
At the distance of 750 million kilometres, sunlight takes only 8.3 minutes to reach the Earth.
With light, the Earth is also reached by heat and other forms of solar radiations which nourishe life on earth and guide many
physical prolesses on Our planet.
About 4.6 billion years ago a nebula,a big cloud ofgas and dust,became sufficiently dense that its gravity began to push the hydrogen and other substances into a nebula even more compact which spun taster. The cloud callasped spinning on itself faster and faster until fragments starter to breakaway.
These fragments became planets and their moons.
The Sun contains enough hydrogen which has kept the production of energy relatively constant for the major part ofits 6 billion years of history,and it is very likely to continue for another 5 billion years to come. In the centre,the Sun has a temperature of 15.000.000 C,on its visibile surface the temperature is about 6.000 C.
In the end however all the nucleo hydrogen will be transformated into helium. When this happens,the Sun will start to die. The Suns existance will finish like a cold,black,tiny speck.
In the radiative zone there is no thermal convection; while the material grows cooler as altitude increase, this temperature gradient is less than the value of a diabatic lapse rate and hence cannot drive convection.
Heat is transferred by radiation-ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions.
The density drops a hundredfold from the bottom to the top of the radiative zone. In the outer layer, from its surface down to
approximately 200,000 km , the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward through radiation (in Other words it is opaque enough).
As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sum Once the materia/ cools off at the surface, it plunges downward to the base ofthe convection zone, to receive more heat from the top of the radiative zone.
The visible surface of the Sun the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing
amount of H— ions, which absorb visible light easily.
The parts ofthe Sun above the photosphere are referred to collectively as the solar atmosphere They can be viewed with telescopes.
Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. lt is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses ofthe Sun.
The temperature in the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. In the upper part of chromosphere helium becomes partially ionized.
The corona is the extended outer atmosphere of the Sun which is much larger in volume than the Sun itself. The corona continuously expands
into the space forming the solar Wind, which fills all the Solar System.
While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.