Nuclear burning rates and population II stellar models

We discuss theoretical predictions concerning the evolution of globular cluster Pop.II stars vis-a-vis current estimates of standard errors in the determination of nuclear burning rates. Numerical evaluations are given for the dependence of TO and HB luminosity from the rate of the relevant nuclear reactions. We conclude that evolutionary predictions appear rather solid in this respect, with a maximum 3$\sigma$ error of about 1 Gyr in the evaluation of cluster ages derived from the calibration of the difference in luminosity between HB and TO. However, current evaluation of the original He content, as given on the basis of the R parameter, should wait for a much better determination of the C12($\alpha$,$\gamma$)O16 reaction before reaching a satisfactory accuracy.