in which unit is it measured?
Joule.
Energy
 The SI unit of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton.
where does the sun's luminosity come from Nuclear fusion reactions in its core. can you explain this term you mentioned just now? It is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles. do this process is experienced by any other planetary body? Stars.
wiki:7769250
Energy [SEP] Introduction
In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton. Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature.