Strömgren sphere

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

• R_S - Strömgren radius.
• S_* - photons/second of ionizing photons (energy 13.6 eV or higher).
• n - number density of hydrogen nuclei (or of electrons: should be the same).
• β₂ - recombination-rate per unit volume for the temperature of the hydrogen down to energy state level 2 or higher.

(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.