The Caldwell Objects

sense, evinced by any known cluster. If you
remove the effect of the Sun's motion through the
galaxy, what remains is an excess velocity of
about 240 km per second. This isn't quite fast
enough for the globular to escape the bond of our
galaxy's gravity. But it does move the cluster
through space in a direction that opposes the
general rotation of the galaxy's disk. Yet, just as a
comet's retrograde orbit doesn't change its
physical characteristics, NGC 3201 remains a
bona fide globular cluster. Guillermo Gonzalez
(University of Washington) reported the following
in the Astrophysical Journal in 1998:
"Although NGC 3201 has an unusual Galactic
orbit, we find no new evidence . . . that it is a
captured cluster." In an earlier study Gonzalez
also noted that "the composition of the stars in
NGC 3201 is similar to that of typical halo
globulars despite its unusual Galactic orbit."
Typically, a globular star cluster spends most
of its time moving slowly through the halo, at the
outer extreme of its orbit, like a comet orbiting
the Sun; only briefly does it whip in and buzz the
galaxy's core. It's possible that at some time in its
history NGC 3201 was sent into a new orbit by a
close encounter with another globular. It's also
possible that it may have been part of an
ingested dwarf galaxy; some modern theories
suggest that the galaxy's entire halo was built up
of such remains.
NGC 320l's discovery is usually attributed to
Sir John Herschel, but James Dunlop listed it as
the 445th object in his 1827 catalog, which was
published six years before Herschel arrived at
the Cape. Of it Dunlop wrote: " pretty A large
pretty bright round nebula, 4' or 5' diameter,
very gradually condensed towards the centre,
easily resolved into stars; the figure is rather
irregular, and the stars are considerably scattered
on the south preceding [southwestern] side: the
stars are also of slightly mixed magnitudes."
79
The cluster is easy to find. First locate the
enormous naked-eye nebula Eta (η) Carinae
halfway between the Southern Cross and the
False Cross. Some 10° due north of Eta Carinae is
the magnitude-2.7 star Mu (μ) Velorum. Mu
Velorum marks the southeastern apex of a 10°wide
naked-eye acute triangle, with 5th-magnitude
q Velorum marking the northwestern
apex and the equally bright stars m and u
Velorum occupying the west-northwestern
corner. NGC 3201 is near the heart of that triangle,
about 4½° due east of the midpoint
between m and u Velorum. From Hawaii I find
its magnitude-6.7 glow just at the limit of nakedeye
visibility. Look for a "double star" in the
globular's position, because about 30' due south
of the cluster is a star of essentially equal
brightness (magnitude 6.6). The trick is to separate
the two objects with averted vision. It's not
difficult; the separation is 10 times greater than
that of the naked-eye double star Epsilon (ε)
Lyrae. Seeing the magnitude-6.6 star should be
no problem under a dark sky. The difficulty will
lie with seeing the globular, whose light is spread
across a much larger area (though in practice
you'll be looking for the light from its innermost
region, just a few arcminutes across). Plan on
spending some time looking. You must be
comfortable and relaxed; neck strain will
The Caldwell Objects 315