The Caldwell Objects

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across, that is tilted at a
45° angle to our line of
sight. The Einstein and
Rosat spacecraft
revealed that X-rays
emanate not from the
nebula's cavity but from
its visually brightest
parts. The X-ray emissions
originate in
filamentary structures
close to the nebula's
inner border, suggesting
that they are generated
as material cools after
being shocked by the
fast wind from HD
192163. The nebula's
outer edges, by contrast,
show the first point of
contact between the
advancing Wolf-Rayet
wind and the
atmosphere of the
former red supergiant.
The revolutionary
Infrared Astronomical
Satellite has shown that
a vastly larger (1.8° x
1.5°) shell surrounds the
Crescent and is also
centered on the Wolf-Rayet star. This extended
structure may be the infrared signature of a
100,000-year-old supernova remnant. Since HD
192163 is believed to be a binary star, it's possible
that the outer infrared shell was generated by the
explosion of the primary star, while a very recent
outflow from the secondary star formed the
Crescent Nebula. Indeed, Wolf-Rayet stars are
believed to be the progenitors of some supernova
remnants, Cassiopeia A among them. But
Argentinean astronomer
110
Cristina E. Cappa (Argentine Institute of Radio
Astronomy) and her colleagues have questioned
the notion that the HD 192163 system has hosted
a supernova explosion. In a 1996 Astronomical
Journal paper they argue that HD 192163 could
have ejected the extended outer shell during the
star's life on the main sequence; material ejected
by the star later in life then encountered the
innermost wall of the outer shell and created
NGC 6888. However, David A. Green (Mullard
Radio Astronomy
Deep-Sky Companions: The Caldwell Objects