  The bright circles observed around stars are usually considered as produced
by shock waves; but this interpretation does not explain easily the bright
spots of the "pearl necklace" of NS 1987A supernova.
  Assuming that the central object of SN 1987A is a neutron star heated by the
accretion of a low density cloud, non-linear optics, in particular
superradiance and impulsive stimulated Raman scattering (ISRS), is needed to
take into account the high intensity of the radiated light.
  Where the temperature of the surrounding gas decreases enough to allow a
combination of protons and electrons into atomic hydrogen in despite of the low
density, a spherical shell absorbs in particular the Lyman alpha line, but does
not populate much the 2P state because a tangential superradiance appears until
the exciting line is almost absorbed; the increase of the 2P population
resulting from the disappearance of the superradiance produces a redshift, so
that almost all energy of a wide band is transferred to tangential modes making
an UV pearl necklace in a given direction of observation.
  In a column of UV light making a pearl, atomic lines are excited enough to
produce new, co-linear superradiances, in particular visible.
