The Caldwell Objects

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I did not notice f brightening, but instead recorded
star e looking fainter. In other words, the
apparent dimming of star e could have been
caused by the brightening of star d. Admittedly,
star f was rather outside my scope of interest (no
pun intended!) because it is not associated with
any nebulosity. So consider yourself warned
about the reality of these fluctuations. But here's
the clincher. While studying NGC 246 a little
more than four years later (on January 15,2001), I
was taken by an obvious diamond of light: stars
a, b, c, and f. When I compared my drawing to
those made in 1996,1 found that star f was clearly
brighter in 2001 than star d, while star e was still
its faint self. I encourage others to monitor these
stars to see if such fluctuations are real. (By the
way, star b is the nebula's central star.)
NGC 246 is at its finest at moderate power
(72x). Most prominent is an arcing swath of
nebulosity north of the central star; the two are
separated by a dark void. This nebulous section
ends abruptly on the northeastern side at a bright
knot that looks like a fuzzy 13th-magni-tude star.
The eastern side of the planetary's shell cannot be
seen in the 4-inch. I call NGC 246 the Pac-Man
Nebula because, in photographs, its dark eastern
side looks like the mouth of that voracious videogame
character, which is about to devour the dim
star e. The dark cavity north of the central star is
the creature's eye.
The western border of the roughly circular
nebula is well defined; it arcs gracefully through
the brightest star (a) in the "cluster." A wedge of
bright gas joins star a to the central star, and it
forms an interstice between two voids: one
(already described) to the north of the central
star, and a dimmer one to the north
226
of star c, the third-brightest star superimposed on
the nebula. It's a challenge to see these features;
averted vision and lots of time are required. I find
it best to relax between efforts. That being said, I
find NGC 246's "eyes" easier to detect than the
Owl's. But don't be fooled into looking for
something obvious. The nebula's dark eyes are
difficult to define because of the bright
intervening stars. They are mere suggestions. I
recommend spending several nights with this
object to average out any atmospheric vagaries
that may affect your observations. Use high
magnification to determine the brightnesses of
the stars associated with the nebula. Houston
said that objects like NGC 246 show up better in
long-focus instruments since they tend to scatter
less light into the field of view than do shortfocus
optics. He also suggested using a Barlow
lens to obtain higher magnification: " low-power A
eyepiece and Barlow seem to scatter less light and
produce a higher-contrast view than a highpower
eyepiece alone." See if you agree.
For those big-telescope users who like to
compete with the Hubble Space Telescope for
optical prowess, Howard Bond (Space Telescope
Science Institute) notes that NGC 246's central
star is also a resolvable binary. The 14th-magnitude
companion is a type G8-K0 V star and lies
3.8" from the primary. By fitting this star onto the
main sequence, Bond derived a distance to the
nebula (1,600 light-years) that, he says, "is one of
the more secure distances known for a galactic
planetary nebula, but it could be improved still
further with a better estimate of the metallicity of
the system." Assuming that distance is correct,
NGC 246's physical diameter is 1.8 light-years —
six times that of the Saturn Nebula (Caldwell 55).
Deep-Sky Companions: The Caldwell Objects