mass loss rates. Nuclear lifetimes also are different depending on the initial chemical composition of the stars. For $M\geq 100\,M_{\odot}$ galactic and pregalactic Population III stars the nuclear lifetimes during the hydrogen burning are $\tau_{\rm H}\leq 3\times 10^{6}$ years. The helium burning lifetimes are shorter than the hydrogen burning lifetimes.

![Evolutionary tracks in the HR-diagram for a $100 M_{\odot}$ stars with different low-metallicity at constant mass. The lower is the metallicity the bluer are the loci of the stars.[]{data-label="bar_fig17"}](figure17.eps){width="82mm"}

![Idem dito. Evolutionary tracks in the HR-diagram for a $100
M_{\odot}$ stars with different low-metallicity for mass losing stars by using a mass loss parameter $N=50$.[]{data-label="bar_fig18"}](figure18.eps){width="82mm"}

As discussed in Section \[massloss\], it is still an open question if the lower metallicity stars could have lost a substantial part of their mass and which could be the mechanisms. We argue that a mass loss is necessary to have a good mixing and transport of the produced elements to the surface of the star. In the conservative case, without any mass loss, the mixing is very deficient and the CNO elements are not transported to the surface. However, in a scenario with mass loss, the mixing is very efficient. The main result is 