Nuclear Fusion Inside Stars / Fission (Nuclear bombs)

Stars are dense, highly pressurized and extremely hot objects. In these extreme environments, plasma is formed (a state of matter after 'gas') where all the particles get 'unbound' in a way and merge into a singular chargeless fluid.

In this chargeless fluid, as all the particles are unbound and zip around at extremely high speeds, hence many nuclei collide and combine to make a new heavier element. This process releases an extenuatingly large amount of energy. The process was used to make the H-Bomb (many times more powerful than the A-bomb released on Hiroshima and Nagasaki). Essentially nuclei in a plasma state combine and create new nuclei and hence new elements and this process releases energy.

Hence in stars, due to the large amounts of pressure and heat at the cores, nuclear fusion takes place which creates new elements. Our sun in fact constantly fuses hydrogen into helium. However what's really fascinating is the fact that fusion stops at iron. There is a 'binding energy' that is needed to fuse nuclei and iron actually has the greatest binding energy despite not having the greatest molar mass. Hence fusing beyond iron is impossible.

Now moving onto nuclear fission. As some of you may know, isotopes decay through alpha, beta, and gamma radiation and do this based on their half-life. Technitium-99 a common medical isotope decays and has a half-life of 6 hours (extremely short). Alpha and beta radiation both cause a change in the element while gamma does not affect the original element. (Beta decay releases an electron and alpha decay releases a helium nucleus). Hence some scientists started thinking about atoms splitting into constituent elements aka nuclear fission. 

What basically happens is that uranium 235 or plutonium 239 are hit with a neutron beam or with a neutron. The new isotope now undergoes the process of fission and splits into two separate atoms. At this point, three more neutrons are released. If the uranium was in a pile then the new neutrons would irrigate the other uranium and a chain reaction would begin. Each 'fission' of uranium would release 2.5MJ of energy which is well.... a lot. A ton of this uranium would release that much more energy. Hence this chain reaction is what is used in the A-bomb as the large amount of energy/radiation causes the explosion and after-effects.