Formation of a bipolar H ii region, according to the simulation of Bodenheimer et al. (1979; their Fig. 4; the thickness of the parental plane is 1.3 pc, its density 300 H₂ cm^-3). The ionized material appears in red, the neutral material in blue; the density of the material is indicated by the saturation level. Schema a) shows the dense molecular plane surrounded by low density material, the initial Strömgren sphere around its central exciting star (T_∗ = 4 × 10⁴ K). In schema b), at 3 × 10⁴ yrs, the expanding H ii region reaches the border of the parental cloud, and the low density material is quickly ionized; a bipolar nebula forms. In schema c), at 1 × 10⁵ yrs, the high density ionized material flows away from the central region; high density molecular material accumulates at the waist of the bipolar nebula, forming a torus of compressed material.