The Caldwell Objects

ulars; challenge yourself and your friends to
hunt it down this way at your next star party.
NGC 7331 belongs to the Pegasus Spur of
galaxies, a small gathering of 35 systems that
includes one member of the famous Stephan's
Quintet (described below). NGC 733l's spiral disk
is tilted 22° from edge on. It whisks away from
the solar system at a generous 820 km per
second. In detailed photographs the galaxy looks
like an oblique aerial view of a hurricane at
night; the glowing eye and spiral arms of the
"storm" seem poised to snuff out the lesser stellar
metropolises surrounding it.
The galaxy's modest apparent size and
brightness belie its true enormity. In fact, NGC
7331 is among the largest galaxies known. If we
accept a distance of 47 million light-years (a
value that has been verified by the Hubble Space
Telescope), NGC 733l's linear diameter becomes
130,000 light-years. NGC 7331 appears to have a
total mass of 300 billion Suns. Thus it equals in
size and mass the great Andromeda Galaxy and
outranks our Milky Way.
As early as 1961, astronomers recognized that
NGC 733l's structure is less like that of M31 and
more like that of M63 in Canes Venatici or NGC
2841 in Ursa Major (though NGC 733l's spiral
arms appear thicker and are more easily seen
than those of the latter two galaxies). NGC 7331
is now classified as an Sbc galaxy, meaning it is a
spiral with somewhat tightly coiled arms, a
relatively small nucleus, and no bar.
Spectroscopic data indicate that there has been
large-scale star formation in NGC 733l's nucleus.
Spectroscopy and photography also have helped
astronomers determine which side of the giant
spiral is closer to us. The data suggest that the
northern end of the spindle is approaching us
and that its western side — the one marked by
the especially prominent dust lane — is closer;
the spiral arms are trailing. The stars in the
central 3"
The Caldwell Objects
30
(650 light-years) of the galaxy's disk, while relatively
young (about 2 billion years old), are very
rich in "metals" (the chemical elements, heavier
than helium, that stars produce via nuclear
fusion). This region might represent a
circumnuclear ring formed during a recent
starburst episode. NGC 733l's starlike nucleus
also probably experienced a secondary starformation
burst because, at an age of about 5
billion years, it is two to three times younger than
the surrounding bulge. Rosat made the first deep
X-ray observations of NGC 7331, and in the
process discovered a nuclear X-ray source — a
finding that bolsters the notion of a massive black
hole at the galaxy's heart. In turn, that black hole
may somehow be related to NGC 733l's modest
low-ionization nuclear emission-line region, or
LINER.
It is curious that William Herschel believed
that he had resolved NGC 7331. But as Larry
Mitchell points out in Appendix C, Herschel
suspected that all nebulae would be resolved into
clusters of stars, if only they could be viewed
with a sufficiently large telescope. Mitchell says
this belief came as a result of Herschel's
examination of the Messier objects, which he
found either to be made of nothing
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