Strömgren sphere
In theoretical astrophysics, there can be a sphere of ionized hydrogen (H II) around a young star of the spectral classes O or B.
Very hot stars of the spectral class O or B emit very energetic radiation, especially ultraviolet radiation, which is able to ionize the neutral hydrogen (H I) of the surrounding interstellar medium, so that hydrogen atoms lose their single electrons.
The photons lose energy as they travel outward from the star's surface, and are not energetic enough to again contribute to ionization.
A Strömgren sphere is the theoretical construct which describes the ionized regions.
In its first and simplest form, derived by the Danish astrophysicist Bengt Strömgren in 1939, the model examines the effects of the electromagnetic radiation of a single star (or a tight cluster of similar stars) of a given surface temperature and luminosity on the surrounding interstellar medium of a given density.
To simplify calculations, the interstellar medium is taken to be homogeneous and consisting entirely of hydrogen.
The formula derived by Strömgren describes the relationship between the luminosity and temperature of the exciting star on the one hand, and the density of the surrounding hydrogen gas on the other.
Using it, the size of the idealized ionized region can be calculated as the Strömgren radius.
This is caused by the fact that the transition region between gas that is highly ionized and neutral hydrogen is very narrow, compared to the overall size of the Strömgren sphere.
A very small amount of hydrogen atoms appear at a density that increases nearly exponentially toward the surface.
Thus a Strömgren system appears as a bright star surrounded by a less-emitting and difficult to observe globe.
The density of excited hydrogen is low, but the paths may be long, so that the hypothesis of a super-radiance and other effects observed using lasers must be tested.
A supposed super-radiant Strömgren's shell emits space-coherent, time-incoherent beams in the direction for which the path in excited hydrogen is maximal, that is, tangential to the sphere.
In Strömgren's explanations, the shell absorbs only the resonant lines of hydrogen, so that the available energy is low.
Assuming that the star is a supernova, the radiance of the light it emits corresponds (by Planck's law) to a temperature of several hundreds of kelvins, so that several frequencies may combine to produce the resonance frequencies of hydrogen atoms.
In supernova remnant 1987A, the Strömgren shell is strangulated into an hourglass whose limbs are like three pearl necklaces.
[2] In 1938 the American astronomers Otto Struve and Chris T. Elvey published their observations of emission nebulae in the constellations Cygnus and Cepheus, most of which are not concentrated toward individual bright stars (in contrast to planetary nebulae).
They suggested the UV radiation of the O- and B-stars to be the required energy source.
[3] In 1939 Bengt Strömgren took up the problem of the ionization and excitation of the interstellar hydrogen.
The resulting images more closely resemble many actual H II-regions than the original model.
This is important, as the electric dipole mechanism always makes the ionization up from the ground level, so we exclude n=1 to add these ionizing field effects.
is the recombination coefficient of the nth energy level in a unitary volume at a temperature
as the number of nucleons (in this case, protons), we can introduce the degree of ionization
