The Caldwell Objects

23
play a role. Eventually, though, gravity will in
time pull the dust in NGC 891's "geysers" back
into the galaxy's plane, just as volcanic ash falls
back to earth after an explosive eruption.
As stated above, NGC 891 seems to contain
copious quantities of mystery matter. Then again,
maybe not. Using the European Space Agency's
Infrared Space Observatory, two Dutch
astronomers, Edwin A. Valentijn and Paul P. van
der Werf, recently detected huge amounts of
relatively warm molecular hydrogen gas in NGC
891. In the September 1, 1999, Astrophysical
Journal Letters the scientists claim that the amount
of gas they found — 5 to 15 times as much
molecular hydrogen as atomic hydrogen —
"matches well the mass required to resolve the
problem of the missing matter." The molecular
gas they detected resides in the galaxy's flat disk,
not in a puffy halo, as most dark-matter theories
postulate. But the scientists think this is a
surmountable problem. If so, a prosaic substance
— mere hydrogen — may be keeping the
spinning spi-
98
ral, and others like it, from flying apart.
NGC 891's visual magic is fully revealed only
in photographs and CCD images. The
magnitude-9.9 galaxy is a challenge for eyeball
observers with small instruments because its
dark equatorial clouds hide so much of its light.
Interestingly, in 1940 Carl Seyfert demonstrated
that NGC 891 's dark lane is somewhat of an illusion.
The lane's surface brightness is a modest 0.6
to 0.9 magnitude fainter than that of the galaxy's
surrounding parts, so it only appears dark, just as
hot sunspots appear dark against the even hotter
face of the Sun. Regardless, NGC 891's 12'-long
glow has such a middling overall surface
brightness that dark skies are required to see it
well. The field is easily located 3½° east of the
fine double star Gamma1,2 (γ1,2) Androm-edae, the
constellation's easternmost bright star. The
galaxy is oriented north-northeast to southsouthwest,
its elongated form almost paralleling
a chain of five stars (ranging from magnitude 6 to
8) about ½° to the southeast.
Christian Luginbuhl and Brian Skiff report
seeing NGC 891 with difficulty in a 21 /2-inch
telescope, saying it is "visible as a tiny streak of
low surface brightness." With peripheral vision at
23x in the 4-inch, the galaxy is a ghostly sliver of
light; it all but vanishes with a direct look. Once
you locate NGC 891, try slowly rocking your
telescope back and forth across the field and see
if you don't catch glimpses of faint light
extending from what appears to be a sharp
central bulge. An increase in magnification (72x in
my case) should then show that the sharp bulge
is actually a magnitude-12.5 star just north of the
true hub, itself a dim mottled patch of light. (The
hub looks much brighter than that star in most
photographs because the region is overexposed).
I perceived the galaxy in its entirety more clearly
at 72x than at 23x. Its dim, tapered body is
peppered with several faint suns (stars belonging
to our own Milky
Deep-Sky Companions: The Caldwell Objects