The Caldwell Objects

5 6
Pac-Man Nebula
NGC246
Type: Planetary
Nebula
Con: Cetus
RA: 00 h 47 m 03.3 s
Dec:-11° 52' 19"
Mag: 10.9
Dim: 4.6' χ 4. 1’
Dist: 1,600 light-years
Disc: William Herschel, 1785
W. H E R S C H E L: [Observed27November 1785]
Four or five pretty [bright] stars forming a
trapezium of about 5' in diameter. The enclosed
space is filled up with faintly terminated milky
nebulosity. The stars seem to have no
[connection] with the nebulosity. (H V-25)
NGC 246 is ONE OF THE CALDWELL CATALOG'S most
visually intriguing planetary nebulae. Look at the
photograph above. Here we see a broken shell of
gas centered on a 12th-magni-tude star. Several
other conspicuous stars are superimposed on the
gas shell as well, and still others lie just beyond
the nebula's confines. Now read William
Herschel's discovery description. Does it cause
you to raise an eyebrow? Where is the 5'-wide
trapezium of stars? And isn't it strange that he
would write "four or five pretty [bright] stars"
[emphasis mine]? Note, too, that Herschel does
not say that the nebulosity is centered on a 12thmagnitude
star. Instead he says that the nebula
fills the space within the trapezium. The New
General Catalogue is more satisfying because it
seems to better describe what is seen, at least in
long-exposure photographs — namely four
bright stars associated with a diffuse nebula
(though these four stars do not form a trapezoid).
Here's another mystery, which could be
56
GC / NGC: Very faint, large, 4 stars in a diffused nebula.
related to the first. In the December 1984 issue of
Sky & Telescope, Walter Scott Houston reported
that, with an 8-inch f/5 reflector, skilled deep-sky
observer Ronald Morales of Arizona easily saw
NGC 246 "as a round, diffuse glow behind three
stars of similar brightness. A fourth was
glimpsed, but the central star was not seen."
Really? Would a skilled observer like Morales
miss seeing a 12th-mag-nitude star in an 8-inch
telescope? You wouldn't think so. Yet Morales
apparently is not alone. In August 1986 veteran
observer Barbara Wilson didn't see the central
star either, and she used a 13.1-inch f/4.5 reflector
at 222x. A drawing in Volume 2 of the Webb
Society Deep-Sky Observers Handbook does show
the central star in an annulus whose periphery is
dappled with the light of three obvious stars; the
drawing is based on a view through an 8-inch
telescope at 75x, and the stars in the drawing
match those seen in photographs. Ernst Hartung
also saw the central star. In his
The Caldwell Objects 223