Gaia Hertzsprung-Russell diagram of candidate blue stars at high Galactic latitudes (gray cross symbols). The observed targets are shown as large circles. Solid curves represent the zero-age and terminal-age MS, while the evolutionary tracks for representative initial masses are plotted as dashed curves (Choi et al. 2016). The light-blue strip represents the horizontal branch (Dorman 1992). The reddening vector and typical error bars of the observed targets are displayed at the top edge of the figure.