The evolution of a 5 AJo population I model (Xn = 0.708, Z = 0.02) is described and compared with the evolution of the 3 AJO model presented in Paper II of this series. The paths in the HertzsprungRussell diagram, the relationship between interior processes and changes in external characteristics, and the relative time scales associated with different evolutionary phases are qualitatively quite similar for the two stars. Quantitative differences (in addition to a considerable difference in absolute time scales) are of importance. In 5 Alo evolution there are two major phases of core helium burning separated in the H-R diagram by a short phase of rapid transit; the path in the H-R diagram during core helium burning extends to higher surface temperatures in the S i'fo case, crossing the region of classical Cepheids; at the red-giant tip, the convective envelope is shallower in the 5 Alo case, leading to a less pronounced increase in the surface ratio of N14 to C11, although the reduction in the surface abundance of Li is about the same as in the 3 Alo case. The effect of the uncertainty in the Cu(n, y)OlI reaction rate is investigated. It is found that, by reducing the C1I(n, )O16 cross-section, the maximum surface temperature reached during the core helium-burmag phase is decreased. The total time spent in core helium burning is reduced and the final core concentration of C12 is considerably enhanced relative to the final core concentration of 016. It is shmvi that the primary maximum found in the number versus magnitude distribution of the classical Cepheids (in any galaxy with a large component of young population I stars) is to he expected as a result of passages through a postulated instability strip during core helium burning.
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Jr. Iben Icko (1966) studied this question.