in physics nuclear fusion is also a importent topic

                           it  is the process by which multiple nuclei join together to form a heavier nucleus.  It is accompanied  by  the   release  or  absorption of energy depending on the masses of the nuclei involved. Iron and nickel nuclei have the largest binding energies per nucleon of all nuclei and therefore are the most stable.
                           The fusion of two nuclei lighter than iron or nickel generally releases energy while the fusion of nuclei heavier than iron or nickel absorbs energy; vice-versa for the reverse process, nuclear fission. Nuclear fusion of light elements releases the energy that causes stars to shine and hydrogen bombs to explode. Nuclear fusion of heavy elements (absorbing energy) occurs in the extremely high-energy conditions of supernova explosions. Nuclear fusion in stars and supernovae is the primary process by which new natural elements are created.    It is this reaction that is  harnessed in  fusion power.  It takes considerable energy to force nuclei to fuse, even those of the lightest element.

A large atomic nucleus splits up into two nuclei is fission. Two light atomic nuclei combine into one heavy nucleus is nuclear fusion. Fusion may release a nucleon.both the Fission and fusion generally release energy. Hydrogen fusion produces million times more energy than burning hydrogen with oxygen. Fusion reactions power stars such as the Sun. In terms of energy supply, fusion has many advantages:Fuel supply widely available.Fuel supply sufficient for millions to billions of yearsNo CO2 or other emissions to cause acid rain, global warming or ozone destructionMuch lower radioactivity hazardMuch lower land use and environmental impact than many forms of renewable energyTo achieve nuclear fusion, two light atoms must come within a distance of 10-15 m. The strong force becomes effective at this distance, and the two nuclei unite into one nucleus.Since atomic nuclei have positive charges, they must overcome the Coulomb potential in order to approach each other within 10-15 m. Thus, the light nuclei must be moving at high speed in their collision. Thus, nuclei are either accelerated or heated to a high temperature