molecular weight increases and the density and temperature increase. During hydrogen burning, both the central density and temperature increase slowly until hydrogen is exhausted in the core.

The most massive stars, and lower metallicity stars, have higher central temperature. According to [@Bromm2001] the central temperature is related to the mass and radius as

$$T_{\rm c}\sim\frac{M^{1/2}}{R}\; .$$

For stars with $M>100\,M_{\odot}$ the central temperature is insensitive to both mass and radius and depends upon the composition only. In the absence of metals, $T_{\rm c}$ is higher and the star self-generates the small amount of CNO elements needed for energy generation to support the star [@Castellani2000].

### b) Effective temperature and radius {#b-effective-temperature-and-radius .unnumbered}

At the beginning of the main sequence, galactic stars with metallicity $Z=10^{-6}$, and mass loss parameter $N=50$, have an effective temperature $T_{\rm eff}=69582$, $73416$, and $75586$ K for a $100$, $150$ and $200\,M_{\odot}$, respectively. For the same stellar masses, pregalactic stars with $Z=10^{-9}$ have $T_{\rm
eff}=91865$, $97109$, and $100090$ K, respectively. Due to mass loss, the effective temperatures decrease during core hydrogen burning. At the end, these temperatures are lower than for the conservative case.

On the main sequence, $100$, $150$, and $200\,M_{\odot}$ galactic stars with metallicity $Z=10^{-6}$, 