Strömgren sphere

The Strömgren sphere, named after Bengt Strömgren is, in theoretical astrophysics, the H II region around a star in which this can ionize by its ultraviolet radiation a hydrogen fog. Ideally, a homogeneous gas density of fog is spherical. On the edge of the Strömgren sphere all ionizing radiation is used, and the ionized region ends. Precise calculations that the transition from primarily ionized to neutral gas over a short distance is presented in comparison to the size of the nebula, the Strömgren sphere is so sharply defined. The radius of the Strömgren sphere of a star depends on the spectral energy distribution and the density of the hydrogen fog. Normally, only stars of spectral types O or B produce radiation fields with sufficient ultraviolet radiation to have significant Strömgrensphären.

Relationship between Strömgren radius, gas density, and properties of the ionizing star

The balance between the number of recombinations of ionized hydrogen and free electrons to neutral hydrogen per second and the number of ionizations neutral hydrogen per second throughout the ionized nebula can be described as

In which

Radius of the sphere ionized hydrogen or Strömgren radius. For a hydrogen fog with a density of one atom or ion per cubic centimeter, this radius is about 100 pc for the hottest main sequence stars of spectral type O5 and only about 10 pc in the spectral type B0.5, correspondingly smaller at higher density.

Density of the hydrogen (cm -3).

Recombination coefficient, one of the atomic properties of hydrogen and the temperature in the fog of certain parameters ( cm 3 s -1).

Emission of ionizing photons ( energy > 13.6 eV) of the exciting star (s- 1).