Nuclear fusion is a nuclear reaction in which atomic nuclei join to form heavier atomic nuclei. In order for fusion to occur there needs to be high temperature and or pressure in order for there to be sufficient kinetic energy for the nuclei to be able to overcome the electrostatic repulsion between the positively charged nuclei and get close to one another; when this occurs the very short range Strong Nuclear Force attracts the two nuclei and they forge together to form a new heavier nuclei. For light mass nuclei mass is not conserved in this reaction but converted into both kinetic energy of the products of the redaction and electromagnetic radiation. The equation explaining this conversion is Einstein’s energy-mass equivalence formula (E=mc^2; see entry under Albert Einstein). An example of this reaction is the fusion of hydrogen nuclei (deuterium and tritium isotopes) into helium nuclei within the sun.
The Sun is effectively a self-balancing system between the kinetic energy released from the exothermic fusion nuclear reaction within the core and gravity.
