We investigate the effects of the CNO cycles on the formation of the blue loop. By use of two ways to treat the CNO cycles, we find that models adopting the CN and NO bi-cycles develop extensive blue loops while those only considering the CN cycle do not. Taking advantage of this unique opportunity, we compare the properties of the CN and CNO models, in order to explore the triggering of the blue loop. We separately investigate the physical conditions in both stellar core and stellar envelope, and find that the abundance discontinuity on the chemical profile is the essential factor to cause the stellar luminosity increasing. As soon as the upper edge of the H-burning shell touches the abundance discontinuity, new H-abundant material adds into the shell source and the energy production rate begins to increase. On the other hand, we find that the star expands when the effective temperature decreases, or the effective temperature increases when the star contracts, because the stellar envelope basically observes the virial theorem. in a convection-dominated envelope, extra heating from its base will leads to more developed convective motion and the decrease of the effective temperature, and the star evolves up along the RGB. However in a radiation-dominated envelope, the increase of the luminosity requires the temperature increasing the enlarge the heat conductivity, and the star develops a blue loop. We define a parameter η to measure the fraction of convection zone in the total envelope mass, and find the critical value to determine whether a star develops a blue loop is between 0.3 and 0.45. According to the new theory, overshooting from the convective core hinders the formation of the blue loop by displacing the abundance discontinuity farther from the H-burning shell, while the opacity enhancement of OPAL over LAOL leads to stronger development of convection in the stellar envelope and makes the blue loop more difficult to occur.
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