The Caldwell Objects

highly suspect by some. However, in 1991 Jean-
Rene Roy (Laval University, Canada) and his
colleagues provided further evidence for a bar in
the galaxy.
In exposures taken through large telescopes NGC
4631 looks like the rotting corpse of a galaxy.
Warped and withered extensions seem to slump
lifelessly on either side of its bulging core, while
reams of dust seem to bore into the bundles of
starlight that cover the galaxy's quilted surface.
These features are all battle scars, for NGC 4631
is an outstanding example of an interacting
galaxy undergoing intensive star formation. Its
highly energetic core complex may have been
triggered into activity by the tidal influences of
its two nearest neighbors: NGC 4627, a dwarf
elliptical or spheroidal galaxy merely 3' to the
north, and NGC 4656, a very irregular spiral
about 30' to the southeast. (Actually, two NGC
numbers — 4656 and 4657 — describe different
parts of this very peculiar object, which some
amateurs know as the Hockey Stick. You can see
NGC 4627 and the Hockey Stick in the
photograph on page 126.) Radio telescopes have
discovered five spectacular tidal spurs of
hydrogen gas extending as far as 130,000 lightyears
from the disk of NGC 4631, while visiblelight
images have revealed a faint bridge of light
connecting NGC 4631 to NGC 4627. Radio
astronomers also have recently discovered two
expanding super-shells of gas in NGC 4631. The
larger one, in the galaxy's eastern limb, spans
10,000 light-years in diameter, making it one of
the largest known in a galactic disk. If supernova
explosions created that expanding cavity of
hydrogen gas, they must have numbered some
10,000 to 35,000. A more feasible explanation for
the superbubble may be the impact of a fastmoving
hydrogen cloud onto the galaxy's disk.
To find NGC 4631 look about halfway down,
and a little southeast of, an imaginary
The Caldwell Objects
32
line from Alpha 1,2 (α 1,2 ) Canum Venaticorum (Cor
Caroli, a superb double star) to Gamma (γ)
Comae Berenices. The galaxy shines at magnitude
9.2, and with averted vision I have seen it
(albeit with some difficulty) through 7x35
binoculars under dark skies. At 23x the galaxy is
one of two equally bright elongated hazes ½°
apart. (NGC 4631 is the more northerly one; the
other is the Hockey Stick). Long and cigarshaped,
its pale light reminds me of a glowing
meteor train. Its central region appears brightest,
while its tapered tips gradually fade into
darkness on either end. Increasing the magnification
to 72x shows the galaxy in all its splendor.
The disk is a beautifully detailed cloud of light
and dark vapors entwined in a delicate embrace.
Star clumps pepper the disk like snowballs
tossed onto the side of a house. To fully
appreciate this very complex sight, take the time
to capture moments of good seeing, because a
maze of visual treats awaits you. (The galaxy's
central region itself warrants hours of attention.)
Look for a triangle of bright patches just
southwest of a 12th-magnitude star punctuating
the northern rim of the galaxy; a dark but delicate
grid of dust separates these patches from the
other blossoms of light populating that region.
Because of its dust complexes, Barbara Wilson
considers NGC 4631 one of the finest galaxies in
the sky. "This
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