As we are learning about the life span of a star, we need to understand the nuclear reactions.
A nuclear reaction is a reaction in which the nuclei take part and new nuclei are formed as a result. In these reactions the nucleus of the reacting elements got changed so that produced elements are different than the parent reacting elements. There are two types of nuclear reactions.
When two or more lighter elements fuse together to form a bigger element the reaction is known as nuclear fusion reaction. There is lot of energy produced during the nuclear fusion because of the mass of the new element is less than the total mass of the elements which got fused together. To trigger the nuclear fusion reaction a very high temperature is needed along with a certain ion density of particles.
Nuclear fusion is responsible for the huge energy emitted by The Sun and other stars in the universe.
When a larger nucleus breaks down into smaller nuclei the reaction is called the nuclear fission. Following is the example of fission of Uranium-235. When a neutron hit the U-235 isotope, U-235 breaks into two smaller nuclei of Krypton and Barium.
In the process huge energy is released along with 3 more neutrons. Nuclear fission is used in nuclear reactors to generate electricity. Also the atomic bombs are based on nuclear fission reactions
Please note that in both the nuclear reactions, huge energy is released. You know why? Because mass is being converted into energy, according to Einstein famous equation
