Figure 5.17: The positions of 32 sub dwarfs in the HR diagram. The sub dwarfs are
shown as circles against the background of the general HR diagram for stars with
parallax accuracies better than 5 per cent. The colour of the circles indicates their
metallicity index, from -2 (dark blue) to -0.5 (red). The lower sequence consists
almost entirely of dwarfs with metallicity index less than -1.