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A Strömgren sphere is a spherical region surrounding an OB star with the conditions to host an HII region (a region of ionized hydrogen). The term is used for the theoretical spherical volume with a particular calculated radius, and is also used for an observed case adhering to the model. The theoretical sphere's radius (Stömgren radius) is a calculated of the maximum distance at which ultraviolet from the star keeps hydrogen ionized. The calculation is based upon a balance between the photoionization and the random recombination of ion and electron. The calculation:
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.