Left: color image of W4 (West et al. 2007) combining HI, Hα and infrared data. For comparison with our model, due to the faint structures and low contrast, the overlay is shown in a separate figure (right panel). The height of the star cluster above the plane is z₀ = 35  pc, marked by the black dot. Right: Kompaneets bubble at overlaid on the same image. The upper part of the contour fits quite well (solid line), while the part of the shell below the cluster is not seen in the observations (dotted line).