& 0.502 & 0.0115 & 0.0048 & $3.9\times10^{-4}$

![The evolution in the H-R diagram of a $1\,M_\odot$ star with initial helium abundance $Y_i=0.23$ (solid line) and $Z=0$ from the ZAMS up to the helium shell-burning phase. The open circles labelled by A and B correspond to the onset of the first (major) and the second He-flash, respectively. The fast transition from A to a phase of slow shell burning is indicated by the dotted segment. Also shown, for comparison, is the track of the same model, but with an initial helium abundance of 0.24 (model S3), which does not experience a hydrogen flash. The filled circles indicate the beginning of the central He-burning phase.\[fig2\]](fig2.eps){width="\columnwidth"}

The major He core flash occurs when the surface luminosity is equal to $\log(L/L_\odot)=2.314$, i.e. a factor of $\approx 1.5$ lower than the value attained by the model of Iben and coworkers. At this stage, the mass of the He core is $M_{\rm cHe} = 0.482\,M_\odot$. The mass shell of maximum energy release by He burning, $M(\epsilon^{\rm He}_{\rm max})$, is located at $M_r=0.151\,M_\odot$. These values are in fair agreement with the ones obtained by @CC93. The evolution of $M_{\rm cHe}$ and $M(\epsilon^{\rm He}_{\rm max})$ is shown 