  Assuming a spherical symmetry, the extreme UV emitted by a very hot source
ionizes low pressure molecular hydrogen making a transparent bubble of H II
(Protons and electrons). For an increase of radius, intensity of extreme UV and
temperature decrease, so that the plasma contains more and more atoms. A
spherical shell, mainly of neutral atoms (H I) appears. If this shell is
optically thick at Lyman frequencies of H I, it is superradiant and a
competition of modes selects modes tangent to a sphere for which many atoms are
excited. Thus, a shell of plasma emits, into a given direction, tangential rays
showing a ring in which selected modes are brighter. While at Lyman
frequencies, absorption of rays emitted by the source excites the atoms able to
amplify the superradiance, a more powerful amplification of superradiance
results from an induced scattering of the radial beams, which extends to feet
of lines and progressively to the whole spectrum. Thermodynamics says that the
brightness of radial and tangential beams tends to be equal; if the solid angle
of observation is much larger for the ring than for the source, almost the
whole light emitted by the source is transferred to the rings, and the source
becomes invisible. Paradoxically, a glow due to incoherent scattering and
impurities around the source remains visible. As the scattering decreases with
the decrease of the radial intensity, the brightness of the ring decreases with
radius. These characteristics are found in supernova remnant 1987A.
