The sun: Nearest of Stars

The sun lies in the Orion arm of the Galaxy, which is marked by the dark and bright nebulae from which new stars are constantly born. More than twenty star subsystems lie within 13 light-years of the Sun.Only four of these are visible to the naked eye.The nearest subsystem, Alpha Centuari is triple. Several others are binary stars.
The Sun is the most dominant member of the Solar System because of its gigantic mass.Its diameter is 1,392,000 kilometers more than 109 times the equatorial diameter of the Earth and its volume is 1,303,600 times that of our planet Earth. The distance between the Earth and the sun is called 'One Astronomical Unit' and is used to measure the distances in the Solar System.
When the Sun was studied through telescopes, using special filters, it was found that it is not a solid body as it seems to the naked eye, but it is completely gaseous. The surface layer of the Sun as we see it is the 'Photosphere'. This photosphere is about 500 km thick, and we do not see anything inside.It is from this region that the Solar radiation escapes. To the eye it appears smooth but under telescope magnification its mottled structure can be seen.
The mottled structure of the Photosphere or Granulation is caused by columns of hotter gases rising from below the Photosphere.The darker intergranular regions are caused by the cooler gases sinking back.Individual granules have a life of several minutes only.

Solar Atmosphere:

Overlying the Photosphere there is a thick layer known as the Chromosphere.It has a thickness of about 10,000 km. The outermost zone that lies above the Chromosphere is known as the Corona. In some parts of the Corona, temperatures may rise as high as 1,000,000 degree K.However, its density is very low. During total Solar Eclipse the Corona becomes visible to the naked eye as a beautiful pearly halo surrounding the Sun. The Corona is referred to as the Sun's atmosphere. Deep inside the Sun at its innermost region, the temperature is about 14,000,000 degree K.This is the Core of the Sun.The energy liberated from the Core is transported to the Photosphere through the two zones called the Inner Radiative Zone and the Outer Convective Zone.The diameter of the Sun as defined by the Photosphere is about 1,392,000 km.The volume of the Sun is about 1,300,000 times that of the Earth.Its mass is about 750 times the combined mass ot the all the other bodies in the Solar System.Deep in its interior, the density of the sun increases and the hot gases under severe compression become 10 to 20 times denser than iron Earth.

Sun Spots