The Caldwell Objects

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2867 as a "star," but a fair bit of magnification and
a stable atmosphere are required to identify it as
a planetary nebula. (That is, unless you slip an
Oxygen III filter in front of your eyepiece, then
out; this will briefly pop the nebula in and out of
view.)
Several times I looked at NGC 2867 when it
was low to the horizon from Hawaii. Under such
circumstances the field stars were so bloated,
they all looked like planetary nebulae, even at 23x
in the 4-inch, and higher powers only magnified
their swollen masses. But when the object was
higher in the sky, and the stars appeared as tacksharp
points of light, the nebula revealed itself as
a slightly swollen disk, especially with averted
vision. At 72x its tight inner disk was surrounded
by a faint, thin ring of diffuse light. I have seen
this feature several times. It shows best with
medium magnification; high power tends to
magnify the dim light beyond recognition. The
inner disk is so condensed, however, that you can
crank up the power to ridiculously high levels
while viewing it. But don't be disappointed if you
still cannot see much more than what I've
described here. Even when seen through a
356
12½-inch telescope, the object has been reported
as "small." Andrew Murrell (Ilford, Australia)
used a 20-inch f/5 Dobsonian at 200x on NGC
2867 and saw a "small bright planetary with a
diameter of 20"." He also said it had an even
surface brightness, and he saw no central star.
"The planetary appeared round," he continues,
"even with high magnification. At low power, the
planetary had a very pale aqua colour which was
lost when observing above 200x." Ernst Hartung
also saw a "pale blue" color. Using 72x with the 4inch
on May 24, 2000, I had momentary
impressions of a pale glacial blue cast, like that of
the daytime sky reflected in ice. I know of no
positive sightings of the 15th-magnitude central
star, which may vary in brightness.
Fortunately we have instruments like the
Hubble Space Telescope, which can turn the
mundane into the magnificent. HST showed
NGC 2867 to be a multishelled planetary with a
prominent ring (like M57's) that appears
"dented" on its eastern edge, as if a car had
accidentally backed into it. Close inspection
shows that ring to be clotted and irregular. That
bright annulus also is surrounded by a turbulent
collar of glowing green gas. Inside the ring is a
faint ellipsoidal cloud, with the hot central star
"pushed" against its western edge. It's as if we
are looking down a barrel at the central star from
a slight angle. In essence, we are doing just that;
the inner oval is what we see when we look
down the funnel of a bipolar flow of gas, while
the fainter outer halo is the remnant of the slow
wind that the central star cast off when it was
still a red giant. Recent studies place NGC 2867
about 5,500 light-years distant, making the shell's
true physical extent 0.3 light-year. Some
estimates, however, place the nebula 1,600 lightyears
farther out, which would increase the
planetary's true diameter by about 30 percent.
Deep-Sky Companions: The Caldwell Objects