(a) Nuclear fusion
Nuclear fusion is the process by which two nuclei of light elements combine to form a heavy nucleus.
When deuterium atoms are heated to an extremely high temperature under extremely high pressure, two deuterium nuclei combine to form a heavy nucleus of helium, and a neutron is emitted. A tremendous amount of energy is liberated in this fusion reaction.
(b) When two positively charged nuclei are brought together, they repel each other. Therefore, a very high temperature is required to force the lighter nuclei to fuse together to form a bigger nucleus.