Central (solid) and surface (dashed) abundances as a function of time of a 240 M_⊙ star for Z = 10⁻¹¹ (left), Z = 0.001 (center), and Z = 0.014(right). The abscissa is the time until the last model, and the ordinate is the mass fraction of each element. Light blue, light green, pink, dark blue, purple, dark green, and orange refer to H, ⁴He, ¹²C, ¹⁴N, ¹⁶O, ²⁰Ne + ²²Ne, and ²⁴Mg + ²⁵Mg + ²⁶Mg abundances, respectively. Colored bars at the top indicate the type (H, He, and C) and duration of core burning phases. The red square marks the time when X_H^S < 0.3 in mass fraction.