Strömgren Sphere

Spherical region of ionized hydrogen (HII) around a single or tight cluser of young stars of spectral classes O or B. THe region is ionized by the strong UV radiation produced by such stars. The size of the sphere is controlled by two main factors: (1) radiation output of the central star(s), with hotter more radiant star(s) producing larger spheres, and (2) gas density of the surrounding cloud, with denser gas yielding smaller spheres.

Strömgren sphere of the Rosette Nebula.