more luminous than galactic ones.

With effective temperatures of $T_{\rm eff}\sim 10^{5}$ K these stars are very efficient at producing photons capable of ionizing the hydrogen and helium [@Bromm2001].

Lower metallicity stars are shifted to the blue upper part of the HR-diagram. When stars are gradually enriched with heavy elements they evolve redwards. This is shown in Figure \[bar\_fig17\] for a $100\,M_{\odot}$ model without a mass loss. Mass losing stars evolve reducing their mass, luminosity and effective temperature, as it is shown in Figure \[bar\_fig18\].

In our models, evolutionary tracks describe different paths on the HR-diagram depending on stellar mass and mass loss rates. Galactic and pregalactic Population III mass losing stars move from the left to the right in the upper part of this diagram. With higher mass loss rates, the luminosity and the effective temperature decreases more than in the conservative case.

![Nuclear energy generation via pp-chain, CNO-cycle and $3\alpha$-reaction for galactic and pregalactic Population III stars with initial metallicity $Z=10^{-6}$ to $Z=10^{-10}$, without mass loss during the hydrogen burning.[]{data-label="bar_fig15"}](figure15.eps){width="82mm"}

![Nuclear energy generation via pp-chain, CNO cycle and $3\alpha$-reaction for galactic and pregalactic Population III stars with initial metallicity $Z=10^{-6}$ and $Z=10^{-9}$, with mass loss during the hydrogen 