  Stroemgren defined a model made up of an extremely hot source plunged in a
constant density, huge, static cloud of low pressure hydrogen. The present
studies of this model apply qualitatively with relatively small and cold
sources, but without these assumptions, we must take into account that: - the
source has, at all frequencies, the spectral radiance of a laser at a single
frequency, so that the multi-photon absorption of few lines involves the whole
continuous spectrum; - a spherical shell containing large column densities of
excited atoms emits strongly super-radiant beams selected by competition of
modes; - the long paths in excited atoms allow observations which require in
the labs the use of ultrashort laser pulses. A necessary, qualitative update
watch that: - the whole continuous spectrum of the source pumps the atoms to
excited states, so that almost all energy emitted by the source is transferred
to a line spectrum; - the main fraction of this line spectrum is emitted by a
strong super-radiance in a spherical shell where the temperature becomes cold
enough for a notable de-ionization of the atoms; by competition of modes, a
dotted ring appears; - inside the ring, a less bright region emits
spontaneously lines made extremely broad by parametric transfers of energy
between the light beams and thermal radiation; the intensities of these lines
decrease down to zero, from increased laboratory frequencies to decreased
frequencies. Supernova remnant 1987A shows these properties.
