A Strömgren sphere is the idealized ionized region around a steady source embedded in uniform hydrogen gas, bounded where integrated recombinations consume the source's ionizing-photon supply.
For ionizing-photon rate , uniform pure-hydrogen density , and Case B recombination coefficient , the Strömgren radius is
Before hydrodynamic expansion matters, photon counting in a uniform initially neutral medium givesThe ionization front approaches the Strömgren radius on the recombination time.
A dusty Strömgren sphere includes ionizing photons absorbed by dust before they can ionize hydrogen. Dust therefore decreases the ionized radius below the dust-free Strömgren radius at fixed source luminosity and gas density.
