Spatial distribution of the O-type (black filled circles), early B-type (red filled squares), and mid-to-late B-type (blue filled triangles) stars in W3. The background is ¹²CO J = 2–1 emission in the velocity range −60 to −25 km s⁻¹ (Bieging & Peters 2011), which highlights the dense gas of the high-density layer (HDL) between the W3 GMC and the W4 H II region. Prominent features and specific stars of interest are labelled. Stars with Hα emission are marked with open circles, and stars with WISE IR excess are marked with open squares. The field of view of our 90Prime imaging is shown by the dashed lines.