The Caldwell Objects

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International Ultraviolet Explorer satellite found
that many planetaries' central stars in fact have
fast winds. He since has been able to invoke these
winds in computer models that account for the
detailed shapes of many planetaries. Of course,
his models are a bit more complicated than
described here, for they include asymmetric
cocoons of matter, donuts of molecular gas and
dust, and the effects of companion stars.
NGC 2392's singular appearance earned it
the honor of being the first object imaged by the
Hubble Space Telescope after the December 1999
repair mission, which replaced the orbiting
observatory's six gyroscopes. Hubble resolved
the nebula's inner portion into a weblike network
of filamentary structures on the surfaces of two
overlapping bubbles. The light-year-wide outer
shell (the Eskimo's fur-lined parka) displays a
halo of cometlike nodules whose tails radiate
outward, away from the windy central star.
The appearance of a planetary nebula
depends on its orientation with respect to our
sightline. In NGC 2392's case we are peering
more or less straight down the major axis of an
inverted funnel of gas. The complex inner nebula
(about 13" in diameter) looks like two rings
superimposed on one another, though slightly
offset. And indeed that is what we are seeing.
Planetary-nebula observer Jay McNeil of
Houston, Texas, likens the inner nebula to "an old
wooden barrel" (but a really big one, about two
trillion km across) with both ends blown out. We
see the translucent barrel's waist as well as the
smaller rim at either end. The barrel has a majoraxis
to minor-axis ratio of about 2 to 1, and its
long axis is inclined about 10° to our sightline.
Support for this model comes from the expansion
velocity of the Eskimo's equatorial region, which
measures 60 km per second while wind escapes
from the
barrel's hollow ends at roughly twice that speed.
Since the Eskimo is some 4,000 light-years distant,
we are seeing it as it appeared when the nomadic
Hyksos ruled Egypt in the Thirteenth Dynasty. If
you were to somehow magically break the lightspeed
barrier and see the Eskimo as it really is
today, it would appear measurably larger than 46".
To find this exotic object, start with magnitude-3.5
Delta (δ) Geminorum, also known as
Wasat. Meaning "the middle," this name refers to
the star's position in Gemini, though it may also
refer to the star's proximity to the ecliptic. This pale
white F0 IV star lies 59 light-years distant and has a
fine magnitude-8.2 K 6 dwarf companion about 7" to
the northwest; the noted 19th-century observer
Adm. William Henry Smyth saw this companion
shining with a purple light. (Of course no two
observers see exactly the same colors in stars, and
their tints may be functions of atmospheric clarity,
the telescope used, and, to some degree, the
observer's imagination. That's one of the things
that make our hobby so fascinating.) Delta
Geminorum holds a special place in the hearts of
those who knew the late Clyde Tombaugh. In 1930
Tombaugh discovered Pluto a mere ½° east of that
star after comparing a pair of photographic plates
he had taken during his yearlong search for
Percival Lowell's Planet X. Delta Geminorum's
"face" soon appeared alongside Pluto's in
newspapers around the world. And Delta
Geminorum, shining 44,000 times more brilliantly
than dim 15th-magnitude Pluto, was every
astronomer's guiding light to that distant world.
Now, more than 70 years after its discovery, Pluto
has left Delta Geminorum well behind. But the
star's role in history will still shine in the memories
of countless astronomy enthusiasts.
Today we can use Delta Geminorum to guide
us to NGC 2392. It is the northernmost
156 Deep-Sky Companions: The Caldwell Objects