The Caldwell Objects

1003, NGC 1023, and NGC 1058 (a nice line of
systems in Perseus, about 4° southeast of NGC
891). NGC 891 is receding from us at a speed of
529 km per second, which is about 100 km per
second slower than NGC 1003 and NGC 1023 but
about the same as NGC 1058.
If the distance of 31 million light-years is
accepted, then NGC 891's true diameter is quite
large, measuring 110,000 light-years, and its mass
totals some 150 billion Suns. If we could
somehow tilt the galaxy or look down onto its
hub, we'd probably discover a structure similar
to that of the textbook spiral M81 in Ursa Major
(though recent infrared observations suggest that
NGC 891 may be a barred spiral). In size and
mass NGC 891 is comparable to our own Milky
Way. Its rotational velocities — the orbital speeds
of stars, gas, and dust around the galaxy's core —
are also very similar to those in our galaxy. Radio
observations from the early 1990s show a steep
rise in rotation velocity as one goes outward
radially from NGC 891's nucleus while
remaining within the disk. A maximum speed of
more than 250 km per second is then followed by
a dip roughly 6,500 light-years from the hub. The
rotation curve becomes almost flat at about
10,000 light-years and remains so out to the edge
of the galaxy's visible disk. As with many spiral
galaxies, including our own, the rotation speeds
seem far greater than one would expect if NGC
891 contained no other material besides its visible
stars, gas, and dust. The implication is that NGC
891, like those other spirals, is embedded within
a halo of mysterious "dark matter" that exerts
gravitational forces but emits no measurable
electromagnetic radiation.
In 1961 Edwin Hubble determined that NGC
891's nuclear bulge is smaller than that in NGC
4565 (Caldwell 38), a similar and arguably more
famous edge-on system in Coma
The Caldwell Objects
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Berenices. Hubble also noted that NGC 891's dust
lane, like that of NGC 4565, has an intricate
pattern running perpendicular to the galaxy's
midplane. Thin fingers of darkness are seen in
silhouette against the galaxy's brightest parts;
they extend at least 30" (4,500 light-years) above
and below the galaxy's plane. Energy must be
pumped into the dust to keep it this far from the
midplane despite gravity's tendency to pull it
back. One energizing mechanism might be
supernova explosions. These violent ends to
massive stars' lives can blow holes through a
galaxy's disk and send streamers of dust and gas
out into its halo. But it seems unlikely that
supernovae alone could generate enough energy
to explain NGC 891's many dark, dusty fingers.
Other phenomena, such as gravitational and
magnetic interactions between interstellar clouds,
must also
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