The Caldwell Objects

the triple-alpha process, wherein the star fuses
helium to make carbon nuclei. The ultraviolet
spectrum of NGC 246's central star displays
remarkable Ρ Cygni profiles. (P Cygni is a
"novalike" variable that erratically changes
brightness, and its pairs of slightly offset emission
and absorption lines attest to an expanding
outer atmosphere.) Spectroscopic observations of
the nebula itself reveal that its outer shell is
expanding more rapidly than its inner region; the
acceleration is most probably caused by radiation
pressure from the central star. NGC 246's central
star may be near the point of
exhausting its nuclear fuel. That means it soon
will contract from its present radius (5 to 8
percent of our Sun's) to become a white dwarf
barely larger than Earth.
To locate NGC 246 first pick up 2nd-magnitude
Beta (β) Ceti. About 7½° due north you'll
find 4.8-magnitude Phi 1 (φ 1 ) Ceti and, 1½° farther
east, 5.2-magnitude Phi2 (φ 2 ) Ceti. Confirm that
you've located these stars with your binoculars,
then point your telescope toward their midpoint.
NGC 246, 1⅓° to the south of their midpoint,
makes a nearly equilateral triangle with these
two stars. Be careful, though, because the 12thmagnitude
galaxy NGC 255 is just ½° northnortheast
of your tar-
56
get. NGC 246 lies at a high southerly galactic
latitude (-74°), so we see it through a virtually
transparent part of the Milky Way.
At low power in the 4-inch, NGC 246 lies in a
field of stars resembling a crooked Taurus. At
first I passed over the nebula while sweeping for
it, noticing only a small clustering of stars as I
moved by it. Of course, the "cluster" is the tiny
grouping of stars superimposed on the nebulosity.
At 23x it's hard not to imagine that the nebulosity
encompasses all the bright stars in the area,
but a careful look reveals the planetary has two
connected crescents of nebulosity that arc
through an obvious triangle of bright stars. The
southwestern rim of one arc curves between the
stars labeled c and d on the photograph. This arc
then tapers and fades to the east toward a void
surrounding star e.
Now for yet another mystery. Looking back
through my notes on NGC 246,1 noticed that in
my initial sketch of the region (dated December 3,
1996), the star labeled f on the photograph was by
far the dimmest of the six I plotted. On the
following evening, however, my sketch shows f
just slightly fainter than star d. Curiously,
The Caldwell Objects 225