A Strömgren sphere is a spherical region surrounding an OB star with the conditions to host an HII region (ionized hydrogen). The term is used for the theoretical spherical volume with a particular calculated radius, and is also used for an observed case where the expected phenomenon is observed to occur. The radius (Stömgren radius) is the maximum distance at which ultraviolet from the star will keep hydrogen ionized. The calculation is based upon a balance between the photoionization and the random recombination of ion and electron.
RS = (3S*/(4π n²β2))1/3
(Recombinations down to the ground state are balanced by the ionizing radiation they produce.) This formula gives rough conditions for the presence of such a cloud, given a hydrogen density of a few thousand per cm3 and a hydrogen temperature of a few thousand K.