A Strömgren sphere is a sphere surrounding an OB star with the conditions to host an HII region. The term is used for a star's (theoretical) sphere within a particular calculated radius, and also for an observed case where the expected phenomenon is seen to occur. The radius (Stömgren radius) is that at which ultraviolet from the star will keep hydrogen ionized. The calculation is based upon a balance between ionization due to the electromagnetic radiation and the expected 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.