Constructed HB evolutionary tracks. The metallicity is Z = 0.001. The HB evolution is terminated when the helium abundance drops below 0.001 at the star center. The ZAHB is shown as the dotted line at the bottom of each evolutionary track. The time interval between two adjacent  +  symbols in each track is 10⁷ years. The region between the two vertical dashed red lines denotes the RR Lyrae instability strip.