Fire does not exist in the absence of air but how is it possible for the sun to have immense heat and chemical reactions in the absence of air?
Fire is a chemical reaction that needs a fuel and oxygen present in air to exist. It is called a chemical because it involves the electrons present in both the fuel and oxygen. These electrons interact with each other resulting in the formation of new compounds that either escape in the form of smoke or settle down as ash. Mass is conserved in this reaction i.e. total mass of the reactants is equal to the total mass of products at the end of the reaction. This reaction is exothermic, releasing energy as heat and light.
An atom consists of a dense positively charged nucleus and negatively charged electrons going around the nucleus. Reactions involving only electrons are called chemical reactions, while those involving the nucleus are called nuclear reactions. The latter type is the one occurring in the sun.
The sun mainly consists of two gaseous elements: hydrogen and helium. In the nuclear reaction occurring in the sun, called a fusion reaction, four hydrogen nuclei combine to form one helium nucleus and also release a large amount of energy. However, a tiny amount of mass from the four hydrogen nuclei is converted to a large amount of energy, given by Einstein’s equation E=mc2. “c” stands for the velocity of light, and this is a large quantity, so a small mass (m) can lead to large amount of energy (E).
In conclusion the reactions occurring in the sun and that of fire are very different from each other. Fire needs fuel, oxygen and a little initial heat to start a reaction; remove any of them and fire ceases to exist. The sun doesn't need any oxygen and can continue to produce heat by the continuous conversion of hydrogen to helium.
M.S. Ramaiah Institute of Technology