Hertzsprung-Russel diagram of the evolutionary sequences with initial metallicity z = 0.001 from the zero-age main sequence (ZAMS) to the zero-age horizontal branch (ZAHB). See Table 1 for details about the different evolutionary sequences. The red stars mark the location of the peak of the He flash for each sequence. The red circles mark the maximum energy liberation of CNO burning in those sequences that experience a PIE.