CMD of Tucana superimposed with a selected sample of evolutionary stellar models for the labeled assumptions about the initial stellar mass and chemical compositions. Note that, to avoid confusion, the tracks are plotted until the tip of the RGB, and that the filled symbols mark the helium ignition. We have adopted the values of distance modulus and reddening, from Bernard et al. (2008), as summarized in Table 1. Left: the 1.5 and 0.8  M _☉ bracket the entire sequence of candidate BSSs. The two black lines show the ZAMS for metallicity Z = 0.0001 (solid) and Z = 0.001 (dashed), which outline the blue edge of the sequence. Central: we show the ranking with metallicity of stars of 1.0 (green) and 1.2 (red)  M _☉. The shaded area encloses the central hydrogen burning phase, until the exhaustion marked by the asterisks. Right: comparison with models with fixed metallicity ( Z = 0.001) and three different masses (1.0, 1.2, and 1.5  M _☉).