The locations of the massive stars of the Orion constellation are marked with green stars (shoulders and knees; the belt is indicated by a single star; M42 is at the tip of the sword). The two giant molecular clouds A and B are shown in blue, and the prominent H ii regions are indicated by the green area, which includes M42 and the Trapezium cluster. Barnard’s loop, which is very prominent in Hα, is indicated by the red line. The bubble surrounding λ Ori (grey) is also indicated (red, ionized gas; blue, swept-up molecular shell), as are the boundaries of the superbubble (yellow dashed and dotted lines). Diffuse ionized gas is indicated in grey. The approximate locations of the Orion OB sub-associations—Ia, Ib, Ic and Id—are marked in green. The dotted line labelled b = 0 indicates the Galactic plane.