Gaia DR2 colour absolute magnitude diagram showing the blue horizontal-branch star region. Top panel: Gaia DR2 objects that satisfy the modified (Lindegren et al. 2018) criteria (gray points) with those with the stringent halo criteria applied (small blue dots). Xue et al. (2008) objects (red circles) and Behr (2003) objects (yellow triangles) are superimposed. The green trapezium denotes the Gaia DR2 colour-magnitude coordinate space (final CMD criteria) considered as the blue horizontal-branch region. The main-sequence turn-off can be seen at around (G_BP − G_RP) = 0.6 and G_abs = 3. Blue stragglers are found extending the main-sequence beyond that. Our selection was designed to minimise the blue-straggler contamination and focus on the blue horizontal-branch stars. Bottom panel: for comparison, the Lindegren et al. (2018) ‘clean subset selection C’ comprised of disc stars within 100 pc which are primarily main-sequence stars (black) with red trapezium denoting the Gaia DR2 colour-magnitude coordinate space considered as the blue horizontal-branch region.