2.  Fusion is the nuclear reaction in which the light atoms align themselves with heaver. At the same time energy released.  Fusion is in the sun and stars the main of the source of energy.  The heavier nuclei production spent more energy than lighter.  In a fission reaction the heavier nuclei released energy when it dicomposing to the lighter nuclei.

3.  The nuclei of atoms product from protons with a positive electric charge, and uncharged neutrons.  The positively charged nuclei between the affected electrical repulsio which prevents the grouping of nuclei in normal conditions.  When the substance is very hot, the nuclei moves so fast and the kinetic energy of their is so high that the electrical repulsion isn`t enough for prevents colisions of their.  The substance also should be sufficiently dense so that the small nuclei can hit each others.

4.  Sun, like the other stars, produce energy from the fusion of hydrogen to helium.  Change the hydrogen into helium can happen in several routes.  The sun fuses 620 million tons of hydrogen each second.  The energy released only if a result of fusion is iron or lighter element nuclei.