The Caldwell Objects

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NGC 4631
Type: Barred Spiral Galaxy
(SBd)
Con: Canes Venatici
RA: 12h 42.1m
Dec: +32° 32'
Mag: 9.2
Dim: 14.7' x 3.5'
SB: 13.3
Dist: 22 million light-years
Disc: William Herschel,
1787
W. H ERSCHEL:
[Observed 20 March
1787] Very bright. Much
extended from south
preceding to north fol-
lowing [from southwest to northeast]
but near the parallel. Much brighter in
the middle. 16' long. (H V-42)
GC: Remarkable, very bright, very large, extremely extended
toward approximate position angle 70°, bright in the middle to a
nucleus, bright star nearby.
TO THE NAKED EYE CANES VENATICI LOOKS
anemic. Its brightest star, Cor Caroli, shines at
3rd magnitude, and its other stars are fainter than
4th magnitude. But the constellation hosts a
dynasty of telescopic splendors: the "colorful"
globular cluster M3; the dramatic Whirlpool
Galaxy, M51; the delicate Sunflower Galaxy,
M63; and the mysterious, black-hole-bearing
Seyfert galaxy, M106. And there's more. In the
obscure southwestern quadrant of the constellation
lurks one of the most distinguished —
yet often overlooked — deep-sky objects of its
class: the edge-on galaxy NGC 4631. Shining at
magnitude 9.2 and spanning nearly ¼°, NGC
4631 is, in fact, among the brightest and largest
126
NGC: Remarkable, very bright, very large, extremely
extended toward approximate position angle 70°,
bright in the middle to a nucleus, magnitude 12 star
attached at north.
edge-on galaxies in the night sky, and a vivid
reminder that Messier's catalog is not an
exhaustive list of celestial greats.
At its distance of 22 million light-years, NGC
4631 is quite large, having a true linear diameter
of 94,000 light-years and a total mass of some 50
billion Suns. We see the system inclined only 5°
from edge on. But if we could tilt NGC 4631 over
and view it face on, we would probably see a
late-type barred spiral with very clumpy and
loosely wound arms.
The late Gerard de Vaucouleurs was the
first (in 1963) to suggest that NGC 4631 was a
barred spiral. The radial-velocity data upon
which his conclusion rested were considered
Deep-Sky Companions: The Caldwell Objec