It is pointed out that the magnetic field of a star is originated from dynamo action associated with the stellar evolution. The magnetic field of a star is related with how much nuclear energy is generated in its phase of evolution. From this we can explain why some stars possess a magnetic field high than that of the Sun. In our case the magnetic field of the star is a by-product of the stars evolution and it has no influence on the internal structure of the star but it does have influence on the flare, chromosphere and coronal activities of the star. Again it is stressed that to confirm the activities of the star, the details of evolution of stars should be calculated according to the photon-neutrino coupling theory.