triple-$\alpha$ reaction from the beginning of the main-sequence. For galactic stars contribution of this reaction is at the end of the main-sequence only. This is shown in Figure \[bar\_fig15\]. For mass losing stars, the nuclear energy production via the pp-chain, CNO-cycle and 3$\alpha$ reaction is higher than in the conservative case, and 3$\alpha$ reaction starts from the beginning of the main-sequence, as it is shown in Figure \[bar\_fig16\].

As a result of lower energy-generation rate in the core, the stars maintain high central temperatures to support the stellar mass against the gravitational collapse. This high temperatures in the core and reduced opacity in their envelopes make these stars hotter and more compact than their enriched counterparts, depending on the initial stellar mass and metallicity.

The lower is the metallicity, the more compact and hotter are the stars. Table 1 shows models with $100\,M_{\odot}$ when they settle down on the main-sequence. A galactic star with metallicity $Z=10^{-6}$ has a radius of $7.68\,R_{\odot}$, while a pregalactic star with $Z=10^{-9}$ has $4.49\,R_{\odot}$ or with $Z=10^{-10}$ it has $3.69\,R_{\odot}$. Concerning the effective temperature this is 69582, 91865 and 101800 K, respectively. On the ZAMS, the pregalactic lower metallicity stars are denser, hotter and slightly 