  Stromgren studied the action of an extremely hot source on a diluted pure
hydrogen cloud; a very ionized, spherical hydrogen plasma surrounded by neutral
atomic hydrogen is formed. A relatively thin intermediate, partially ionized,
hydrogen shell, is cooled by the radiation of the atoms.
  Stromgren was unaware of that this plasma, similar to the plasma of a gas
laser, can be superradiant at several eigen frequencies of atomic hydrogen; the
superradiant rays emitted tangentially with the sphere appear resulting from a
discontinuous ring because of the competition of optical modes. The
superradiance intensely depopulates the excited levels, including the continuum
of proton-electron collisions, by cascades of transitions combined into
resonant multiphotonic transitions so that the gas is cooled brutally beyond
the radius of the Stromgren sphere.
  The extreme brightness of the rays emitted by the source allows a
multiphotonic non-resonant absorption leading in stationary states or the
ionization continuum. This absorption combines with the superradiant emissions
in a multiphotonic diffusion induced by the superradiant rays. Although its
brightness remains higher than that of the superradiant rays, the source
becomes invisible if it is observed through a small solid angle.
  The lines emitted inside the sphere are all the more weak as they arrive of
an internal area, lower in atoms, and more reddened also by a parametric
transfer of energy towards the thermal radiation catalyzed by excited atomic
hydrogen present in the sphere only.
  The Stromgren sphere appears to help to simply explain the appearance and the
spectrum of supernova 1987A.
