Published online by Cambridge University Press: 30 March 2016
Giant (20 – 260 pc) and supergiant (600 – 1400 pc) filamentary ionized shells are found in several galaxies. Some giant shells are collisionally ionized supernova remnants but many surround ≃ 5 – 50 OB stars and, if approximately spherical, could be formed by the combined effects of successive, 1050 erg, supernova explosions, pressure of the Lyman photons and stellar winds. Other mechanisms are required for their supergiant counterparts for perhaps they are toroidal rings rather than radially expanding spherical shells.