Answer: fusion
Step-by-step explanation:
Nuclear fusion is a process which involves the conversion of two small nuclei to form a heavy nuclei along with release of energy.
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Nuclear fission is a process which involves the conversion of a heavier nuclei into two or more small and stable nuclei along with the release of energy.
Red shift is a phenomenon where electromagnetic radiation from an object undergoes an increase in wavelength.
Nuclear decay is a type of process in which a less stable nuclei decomposes to a stable nuclei by releasing some radiations or particles like alpha, beta particles or gamma-radiations.