A Strömgren sphere is a sphere surrounding an OB star with the conditions to host an HII region. It is the sphere within a calculated radius at which ultraviolet from the star will keep hydrogen ionized, called the Stömgren radius. 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*/(4πn²β2))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 and a hydrogen temperature of a few thousand K.