The Caldwell Objects

105 & 108
tudes for these objects. At least that will give you
something to write about in your book!
These observations reveal one overarching
truth about deep-sky objects: we still have a lot
to learn!
Through binoculars NGC 4372's tiny puff of
light is overpowered by the neighboring 7thmagnitude
star, but see if you can't pick it out
with averted vision. The view through large binoculars
or a 60-mm refractor gives the best
impression of Dunlop's conical ray of light following
the star like a comet's flowing tail. I could
barely see NGC 4372 in the 4½-inch finder at 19x
through the moonlight, so weak is its glow. At
high power the cluster displayed loose ribbons of
starlight lying atop a wrinkled, semitransparent
sheet. I could also imagine a slightly denser sheet
punctuated by two roughly 12th-magnitude
stars. Seen through the 20-inch, each ribbon was
part of a wavy row of 13th- to 14th-magnitude
stars; the rows flowed more or less parallel to one
another in a northeast-southwest direction. Dark
troughs separated these waves, though they were
not completely black, as they were lightly
peppered with dim stars. These wavy features
and dark troughs reminded me of the "rippled
dunes" of starlight I saw in the open cluster M25
in Sagittarius through the 4-inch Genesis. If you
422
defocus the cluster slightiy and use averted
vision, can you locate some very thin "dark doodads"
marring the wrinkled sheet? My drawing
of NGC 4372 is a composite of low-power views
with the Auckland Observatory 4½-inch finderscope
and higher-power views with the 20-inch.
Over the years astronomers have characterized
the light curves of 19 variable stars in
NGC 4372. Eight of these variables belong to the
SX Phoenicis class (metal-poor analogs of the
pulsating Delta Scuti stars), and eight are contact
binaries. The variables range in brightness from
14th to 19th magnitude, so all are within the
reach of modest-sized telescopes when they are
at maximum light. CCD owners should be able to
monitor all of these stars throughout their
variability cycles. All the SX Phoenicis variables
are concentrated toward the center of the
globular and are blue stragglers. The contact
binaries are distributed throughout the cluster.
Two more variables, possibly binaries, have also
been detected. The cluster's total lack of RR Lyrae
variables confirms that it is extremely metal-poor,
like NGC 4833. Each of NGC 4372's stars has
about 1 / the 1 2 iron 0 content of our Sun. The
cluster is approaching us at a speed of 72 km per
second and has an estimated age of 15 billion
years, plus or minus 4 billion years.
Deep-Sky Companions: The Caldwell Objects