  Wrong Monte-Carlo computations are used to study the propagation of light in
low pressure gas of nebulae.
  We recall that the incoherent interactions required for Monte Carlo
calculations and hindering coherent interactions are due to collisions that
disappear at low pressure. Incoherent interactions blur the images while
coherent do not. We introduce coherent optical effects or substitute them for
Monte Carlo calculations in published papers, improving the results and
avoiding the introduction of "new physics".
  The spectral radiance of novae has the magnitude of the radiance of lasers,
and large column densities are available in the nebulae. Several types of
coherent interactions (superradiance, multiphoton effects, etc..), well studied
using lasers, work in nebulae as in laboratories.
  The relatively thin shell of plasma containing atoms around a Str\^omgren
sphere is superradiant, so that the limb of the sphere is seen as a circle
which may be dotted into an even number of "pearls". The superradiant beams
induce a multiphotonic scattering of the light rays emitted by the star,
improving the brightness of the limb and darkening the star. Impulsive
Stimulated Raman Scatterings (ISRS) in excited atomic hydrogen shift the
frequencies of electromagnetic waves: UV-X lines of the Sun are red- or
blue-shifted, the microwaves exchanged with the Pioneer 10 and 11 probes are
blueshifted (no anomalous acceleration needed), the far stars are redshifted.
  Without any "new physics", coherent spectroscopy works as a magic stick to
explain many observations.
