Why are the temperatures fairly constant in the surface zone and the deep zone

heat released by collision processes during accretion, and heat from decay of radioactive elements

explanation:

radiative zone

explanation    

radiative zone is the layer of the sun which energy is transferred away from the core by radiation. although, the core is the only part of the sun that produces an appreciable amount of heat through fusion. in fact, 99% of the energy produced by the sun takes place within 24% of the sun's radius.