what is meant by its binary form?
If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe.
Star
 If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. 
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. in which unit is it measured? Joule. can you mention any one of its characteristic? Luminosity, size, evolution, lifespan, and its eventual fate.
wiki:25611513
Star [SEP] Binary stars
The post–main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe, the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other. When the Roche lobe is violated, a variety of phenomena can result, including contact binaries, common-envelope binaries, cataclysmic variables, and type Ia supernovae.