have initial radii $R=7.67$, $9.54$ and $11.15\,R_{\odot}$, respectively, and for pregalactic stars with $Z=10^{-9}$, their radii are $R=4.48$, $5.55$ and $6.46\,R_{\odot}$, respectively.

With decreasing effective temperature, the radius increases. For stars evolving with high mass loss parameters, their radii at the end of the hydrogen burning increase even more.

The changes of the radius and effective temperature can be understood in terms of changes in the convective core size. To satisfy the boundary condition

$$L=\pi acR^{2}T_{\rm eff}^{4}$$

the effective temperature decreases with increasing radius. An expanding convective core implies a decreasing radius and increasing effective temperature.

### b) Convective core size {#b-convective-core-size .unnumbered}

During the hydrogen burning the convective core evolves in different ways depending on whether the stars are mass losing or not. If a star evolves with mass loss its convective core decreases continuously during the hydrogen burning phase. In the conservative case, a helium core is formed only at the end of hydrogen burning when the star is contracting during its transition to helium burning.

At the end of hydrogen burning, the convective core size, defined as the ratio $q_{\rm cc}=M_{\rm cc}/M$ of the convective core mass $M_{\rm cc}$ to the total stellar mass $M$, is larger 