How stars shine? From what does they produce energy? The gas clouds which are formed when a supernova explosion occurs gets organised due to gravity and gradually they become closer and closer. As they become closer the temperature inside the cloude increases because the density increases. When they get the required temperature they start to fuse hydrogen(H) into helium(He) and radiate energy by the equation E= MC^2 where E is the energy, M is the mass and C is the speed of light. The outward pressure produced by this fusion reaction keeps equilibrium with the inward pressure of gravity. When the star exsausts all the hydrogen, they start to fuse Helium( He). Similarly the fusion reaction continues  upto iron(Fe) one after another when they exsausts one type of element. After that they can’t produce havier elements by fusion reaction. The bigger stars have to fuse hydrogen or helium more rapidly to keep equilibrium between gravity and the outward pressure due to fusion. Hence they exhaust all the fuels for fusion faster than a smaller star. So lifetime of a bigger star is less than a smaller star. The age of our Sun is about 5 billion years and it will stay another 5 billion years. After that it will become a red giant. The Sun radiates 46% of its energy in the form of infrared radiation, 49% in visible region and 5% in ultraviolet radiation. Temperature of the outer atmosphere of the sun is 6000 Kelvin.  It doesn’t have enough temperature to produce x-ray. 

Why does the stars in the night sky twinkle? The star light got refracted  in various gases when it enters earth’s atmosphere. So when we see the star we see its apparent position which changes time to time. In other words starlight follows a zig zig path to our eyes. That’s why we see a star twinkle. Stars don’t twinkle if we see it from outer space because there is no gas atmosphere.


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