The Caldwell Objects

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the companion would have to be located about
4.5 solar radii from the central star to account for
the misalignment of the inner ellipsoids.
A recent discovery also revealed a dim inner pair
of condensations that resembles the well-known
outer pair, with one knot on either side of the
central star. The condensation pairs may have
originated in separate shells of ejected matter that
have since been pushed outward by the central
star's ultraviolet radiation or particulate winds.
The International Ultraviolet Explorer satellite
found no evidence of light-scattering
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dust particles in NGC 7009.
In April 1996 the Hubble Space Telescope imaged
NGC 7009, giving astronomers an unprecedented
look at this complex planetary nebula. An
enhanced color version of this image (a blackand-white
version is shown below) shows a
central star trapped in the cavity of a pale blue
chrysalis of gas. Large hollow lobes of yellow gas
extend from this chrysalis, making the inner
nebula look like some sort of psychedelic sushi.
The green outer envelope (the star's former outer
layers) appears some-
what boxy and multidimensional,
like a time-lapse
exposure of an open beer
barrel rolling in a circle.
Thin green jets extend along
the nebula's major axis and
end in bright blood-red
knots, as if the jets were
recently cauterized veins. If
the nebula is 1,400 lightyears
distant, these knots lie
0.15 light-year from the
central star. Despite its
visual com
plexity, the HST image is giving astronomers a
better understanding of the mysterious processes
that transform a low-mass star like our Sun into a
white dwarf. A computer model based on the
HST data indicates that NGC 7009's progenitor
star first expelled one solar mass of gas, which
now constitutes the "barrel." This gas confines
stellar winds flowing from the central star. The
confined gases slide along the walls of the
barrel's cavity until they converge at its tips. As
Bruce Balick (University of Washington)
explains, "Each ansa [terminal knot] is joined to
the tips of the cavity by a long greenish jet of
material, much like the hot gases that
follow a bullet from the barrel of a rifle __ The
tips act somewhat like a nozzle that form gas
Deep-Sky Companions: The Caldwell Objects