[Fe/H] versus stellar age. The stars have been divided in two groups: an old sequence shown with filled red circles and a younger sequence shown with filled blue circles. Two stars (HD 59711 and HD 183658) having intermediate ages are shown with filled black circles and the Sun with the ⊙ symbol. The components of the visual binary star, ζ Reticuli, are marked with black squares and the Na-rich stars, HD 13724 and HD 189625, with yellow squares.