Strömgren Sphere

The Rosette Nebula , a Strömgren Sphere

The Strömgren sphere , named after Bengt Strömgren , is the H-II region around a star in theoretical astrophysics in which it can ionize a hydrogen nebula through its ultraviolet radiation . In the ideal case of homogeneous gas density , this nebula is spherical.

At the edge of the Strömgren Sphere, all ionizing radiation is consumed and the ionized area ends. Precise calculations show that this transition from mainly ionized to neutral gas takes place over a distance that is short compared to the size of the nebula; the Strömgren sphere is thus sharply delimited. The radius of a Strömgren sphere depends on the spectral energy distribution and the density of the hydrogen nebula. Normally only stars of the spectral classes  O or B generate radiation fields with enough ultraviolet radiation to have significant Strömgren spheres.

Mathematical description

The equilibrium between the number of recombinations of ionized hydrogen and free electrons to neutral hydrogen H ⁰ (left side) and the number of ionizations of neutral hydrogen (right side), each per second in the entire ionized nebula, can be described as

${\displaystyle {\frac {4}{3}}\pi \,r_{1}^{3}\,N_{\rm {H}}^{2}\,\alpha _{B}=Q({\rm {H^{0})}}}$

With

• the volume of the (assumed spherical) area of ​​ionized hydrogen ${\displaystyle {\frac {4}{3}}\pi \,r_{1}^{3}}$
  • the radius of the area of ​​ionized hydrogen, also known as the Strömgren radius . For a hydrogen nebula with a density of one atom or ion per cubic centimeter, this radius is about 100  pc for the hottest main sequence stars of the spectral type O5 and only about 10 pc for the spectral type B0.5; correspondingly smaller at higher density. ${\displaystyle r_{1}}$
• the particle number of hydrogen ${\displaystyle N_{\rm {H}}}$
• the recombination coefficient, a parameter of dimension 1 / (volume * time) determined by the atomic properties of hydrogen and the temperature in the fog ${\displaystyle \alpha _{B}}$
• the emission rate of ionizing photons (energy> 13.6  eV ) of the exciting star. ${\displaystyle Q({\rm {H^{0})}}}$

literature

• Arnold Hanslmeier : Introduction to Astronomy and Astrophysics , 2nd edition 2007, Spectrum Akademischer Verlag, ISBN 978-3-8274-1846-3 , pp. 391–392
• James B. Kaler: Stars and their spectra , Spektrum Akademischer Verlag 1994, ISBN 3-86025-089-2 , p. 244