THere are huge clouds of gas and dust in the Galaxies.They are called Nebula..Due to some reason mass gets accumulated at center and this mass attracts more gases..Then it become a p-roto star.When mass become very huge due to pressure and temperature at about 15 million degree K. at center nuclear fusion stars changing hydrogen into helium..4 hydrogen atoms fuse into 1 helium atoms and remaining 0.7% mass converted into energy. Picture credit science how the stuff works.com.

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