The brightest star near to earth is called sun. The emission of electromagnetic radiations in radio as well as visible regions is done by sun. Its other name is ‘Yellow Dwarf Star’. Some of the main properties of sun are discussed below:
1. Solar Interior: Hydrogen is the major component present in sun. 70% of sun’s atmosphere consists of hydrogen gas. Helium is only 28% and the remaining 2% consists of some other elements from lithium to uranium. So, in other words we can say that the whole sun is made up of gases. All the three things i.e. temperature, pressure and density goes on increasing as we move from surface towards the interior of sun. The internal temperature of the sun is near about 14 X 106K. The temperature present on the surface of sun is 6000K. The density at the centre is also higher than that of the surface. The density at its centre is 104Kgm-3. The density on the surface of sun is nearly 10-4Kgm-3.
2. Structure of the sun: Sun consists of a layer known as ‘Photosphere’. It is the brightest layer of the sun and is present at its centre. All the light which we are receiving on earth is due to its Photosphere. Chromosphere is the hottest layer of sun. It consists mainly of hot gases. This layer surrounds the Photosphere.
3. Source of Energy of Sun: The radiations coming from the sun are continuous from billions of years. The energy rate of sun’s radiations is 4 x 1026Js-1. This is a very huge amount of energy. It can’t be produced from any small chemical process like burning of petrol, burning of coal, etc. So, not even a simple fission reaction can be the source of energy of sun.
Mr. Bethe conducted an experiment in 1939. He suggested that the source of sun’s energy is due to the nuclear fusion reactions. In fusion reaction, two lighter nuclei fused together to form a bigger nucleus. In case of sun, two lighter hydrogen nuclei fused together to form bigger hydrogen nucleus. During this fusion reaction, some amount of mass is reduced automatically and some amount of energy is released. This mass and energy are related to each other by Einstein’s Energy relation:
E= mc2
In the above equation c is the velocity of light. In the sun and the stars the nuclear fusion reactions taking place can happen in two forms. Either they can form carbon nitrogen-cycle or they can form proton-proton cycle. In sun four hydrogen nuclei fused together to form a bigger helium nucleus. During this reaction formation of two positrons also occurs and nearly 26.7MeV energy is released. The equation for this reaction is shown below:
4 1H1 
2He4 + 2 1eo + 26.7MeV
As we know that sun contains a large amount of hydrogen. That’s why the brightness of sun will remain same forever.
4. Surface Temperature of the sun: Solar luminosity and solar constant are the two factors on which the value of the temperature depends.
Now we will discuss the solar constant. The amount of radiant energy received per second by a small unit area from the surface of a body which is perfectly black, held at right angles to the direction same as that of the direction of the sun at an average distance of earth from the sun. Different values of solar constant in different units are shown below: 2 cal.cm-2 per minute or 1388watt.m-2.
