The Caldwell Objects

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most apparent under clear, dark skies. I estimated
the galaxy's apparent visual magnitude to
be 8.9 — fully 30 percent brighter than the Deep
Sky Field Guide's value, which, in turn, is 1.5
magnitudes (4 times) brighter than Burn-ham's
value. Christian Luginbuhl and Brian Skiff
confirm my estimate to within 10 percent; they
list magnitude 8.8. Together, NGC 247 and its
attendant 9th-magnitude star look like a silken
fiber attached to a milkweed seed adrift in the
cool winter wind. With alternating direct and
averted vision, the "fiber" glints, as if by the light
of the Moon.
NGC 247 is among the closest galaxies to our
Milky Way. It belongs to the Sculptor Group of
galaxies, the closest aggregation of island
universes to our Local Group. It is also a member
of the South Galactic Pole Group of galaxies,
which includes NGC 55, NGC 253, and NGC 300
(all Caldwell objects). NGC 247 lies 6½° from the
South Galactic Pole, so its light is hardly
absorbed at all by dust in our own Milky Way. In
photographs, the highly inclined galaxy displays
a bright central mass with a short bar (oriented
northeast-southwest) and ill-defined spiral arms
studded with knots. One long arm extends to the
north before looping east, then dipping south,
where it reconnects with the central mass.
Whether it's illusion or reality, the sight leaves
the impression that an enormous dark hollow
lurks in the disk. A second arm shoots due south
off the stubby bar and then curves just beyond
the 9th-magnitude star. This sudden jog makes
the arm look like a broken bone that hasn't
properly healed. Accepting R. Brent Tully's
distance estimate of 6.8 million light-years, the
galaxy measures 34,600 light-years across. It is
receding from us at 159 km per second.
The Rosat satellite picked up X-rays from
hot gas within NGC 247's disk and from what is
probably a cluster of young, low-mass stars
246
or brown dwarfs within the galaxy. Dwarf
galaxies like NGC 247 are believed to be dominated
by dark matter, a significant fraction of
which is thought to exist in their halos as small,
compact objects like brown dwarfs or giant
planets — the so-called MACHOs (for massive
compact halo objects). NGC 247 offers
astronomers the possibility to observe a halo of
MACHOs being formed. The material associated
with the X-ray emission weighs in at about 1
billion solar masses, or 10 percent of the galaxy's
total mass.
The view through the 4-inch Genesis does
not disappoint. With a glance at 23x NGC 247
immediately displays a noticeable core, with faint
extensions to the north and south. These
extensions appear to swell and contract as I
alternate between averted and direct vision, first
appearing as a thin thread of light, then as a luminous
tassel seemingly tacked to the sky by the
9th-magnitude star. The galaxy's patchy edges
are difficult to trace. Its southern extension is
more noticeable, and there's a suggestion of a
break in the northern extension, which makes it
easy to imagine that a dark void lurks there.
At 72x NGC 247 sits in a triangle of 9th- to
11th-magnitude stars. Little else seems to shine
Deep-Sky Companions: The Caldwell Objects