Soon after the development of the atom bomb in 1945, scientists started developing a more powerful bomb. This resulted in the development of another highly destructive bomb called the ‘hydrogen bomb’. An American scientist Edward Teller and his team tested the first crude form of hydrogen bomb in 1952. It was a 10 Megaton bomb, about 700 times more powerful than the atom bomb dropped in Hiroshima. The hydrogen bomb makes use of ‘nuclear fusion’. In this process, four hydrogen nuclei combine at extremely high temperature to form one helium nucleus, producing tremendous amount of heat energy. Similar reactions take place in the Sun and other stars due to which, they have been continuously producing enormous amounts of heat and light energy. The shell of the hydrogen bomb is made of a strong alloy. Two isotopes of hydrogen (deuterium and tritium) are kept inside this cover along with the atom bomb. First the atom bomb is exploded and this initiates the fusion reaction of the hydrogen bomb. The atom bomb produces a very high temperature of millions of degrees, and at this temperature, deuterium and tritium combine to form a helium nucleus, producing huge quantity of heat. This entire reaction is completed in one-millionth of a second. In fusion, not only are helium nuclei formed, but neutrons are also produced. These neutrons carry out fission reaction in uranium by which heat is continuously produced to help further fusion reaction. It is for this reason that hydrogen bomb is more powerful. Till date, hydrogen bombs have been developed by U.S.A., Russia (former U.S.S.R.), U.K., France, China, India and possibly Pakistan.