This is the paper identified with the concept of the Strömgren sphere.

The hotter and more luminous the exciting star, the larger the Strömgren sphere.

The denser the surrounding hydrogen gas, the smaller the Strömgren sphere.

A Strömgren sphere is the theoretical construct which describes the ionized regions.

Here, is the radius of the Strömgren sphere and are the number densities of protons and electrons, respectively.

H19 is a compact cluster containing a Strömgren sphere which is ionized by a hot B0 V type star.

Strömgren's model also shows that there is a very sharp cut-off of the degree of ionization at the edge of the Strömgren sphere.

This radiation rapidly ionizes the surrounding interstellar gas of the giant molecular cloud, forming an H II region or Strömgren sphere.

A common explanation: The very hot stars are in a cloud of hydrogen which, heated, generate a Strömgren sphere made up of fully ionized hydrogen (protons+electrons).

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.

For a pure hydrogen nebula, the ionization equilibrium states that the number per unit time of ionizing photons from the central star has to be balanced by the rate of recombinations of protons and electrons to neutral hydrogen inside the Strömgren sphere of the nebula.

In theoretical astrophysics, there can be a sphere of ionized hydrogen (H II) around a young star of the spectral classes O or B. The theory was derived by Bengt Strömgren in 1937 and later named Strömgren sphere after him.