The Caldwell Objects

pie refractors and inefficient speculum-mirror
reflectors, generally at magnifications ranging
from 60x to 138x. Roger Clark states that the
optimum magnification for detecting NGC 752 is
7x with a 2-inch telescope and 27x with an 8inch.
Clearly Messier was at a disadvantage. I
can imagine him sweeping past NGC 752 and
noticing a rich field of stars but moving on
because the object did not look fuzzy. Of course,
it's also possible that his telescope didn't provide
a large enough field of view to make NGC 752
stand out as a discrete entity.
Brent Archinal has acquired an interesting
naked-eye perspective on this cluster. While he
was attending the Black Forest Star Party in
north-central Pennsylvania, he set out to look for
M33 with the naked eye — a time-honored test of
sky darkness and transparency "This is definitely
one of the darkest sites I've seen in the eastern
U.S.," he said, "second only to Spruce Knob, West
Virginia. Something I noticed that might be
worth pointing out is that if one tries to casually
observe M33 with the unaided eye, it is easy to
confuse it with NGC 752, which is in the vicinity
and much easier to see."
Unlike most open clusters, which are tens or
hundreds of millions of years old, NGC 752 is
about 2 billion years old. This places NGC 752
among the oldest open clusters known, though it
is much younger than 5-billion-year-old NGC
188 (Caldwell 1). Most of the 77 or so measured
stars apparently populating NGC 752 are F-type
subgiants that have moved off the main sequence
and are evolving toward gianthood. An 18-yearlong
radial-velocity survey has confirmed that 15
red giants are members of the cluster. Rosat
detected 49 X-ray sources in the cluster region,
seven of which have been tentatively matched up
with visible cluster members; three of these latter
ob-jects are short-period binaries, one is a blue
strag-
The Caldwell Objects
28
gler, and one a so-called rapid rotator. These
discoveries all paint a picture of a cluster whose
stars have had considerable time to follow
various evolutionary paths.
NGC 752's very open nature itself hints at
the cluster's seniority. Unlike a globular cluster,
whose hundreds of thousands of stars exert a
mutual gravitation that keeps them moving in a
tight pack, an open cluster tends to loosen as it
ages. Stars lining the cluster's edge are lost to the
gravitational pulls of passing stars, molecular
clouds, and other clusters. These "lost" suns
blend into the populated stellar streets of the
Milky Way, where their identities eventually
fade beyond recovery.
At 23x in the 4-inch NGC 752 seems to fill
the field of view but, as alluded to above, it is
hard to know where the cluster's boundaries end.
The cluster has no central concentration; in fact, I
see a curious double nature to it, each section
seemingly a very wide open cluster of its own.
To me these groupings look like celes-
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