The Caldwell Objects

have been identified in NGC 6752's core.
Astronomers have had great success in probing
the depths of NGC 6752's extremely tight core
because it is one of the closest globulars to us. Its
computed distance of 13,000 light-years makes it
about 1,700 light-years closer than 47 Tucanae
(Caldwell 106). NGC 6752 lies 17,000 light-years
from the center of our galaxy. With an age of
perhaps 15 billion years, the cluster is quite old,
and, like most
globulars, it is metal-poor; each of its stars has,
on average, 1/36 as much iron (per unit of hydrogen)
as does our Sun (a fairly typical value for a
globular cluster). Expert William E. Harris lists its
overall spectral type as F4 or F5. From 1897 to the
1980s, only two variable stars were known in the
cluster. But that number has risen sharply. When
the cluster was monitored in 1996 and 1997 with
the 1-meter Swope Telescope in Chile, 11 new
variables were identified.
From Hawaii NGC 6752 can rise as much as
10° above the southern horizon. It is visible
(albeit with effort) to the unaided eye and is
The Caldwell Objects
93 & 101
splendidly apparent in 7x50 binoculars. It lies just
3° northeast of 4th-magnitude Lambda (λ)
Pavonis and 1½° east-northeast of yellow, 5thmagnitude
Omega (ω) Pavonis. There is nothing
else in the area to mistake it for. Its small, pale
disk of white light suddenly swells to nearly ¾
the size of the full Moon when averted vision is
used. A magnitude-7.4 star lies 5' southsouthwest
of the cluster's core.
At 23x in the 4-inch Genesis the cluster is
amazingly bright and
round, and its outer halo
seems to sweep across
the light of the 7.4magnitude
star like
windblown sand. When
you look at this part of
the cluster with averted
vision, then directly, and
repeat the process, the
"dust" seems to flicker
eerily. The cluster's tight
inner core gradually
extends outward into an
irregular shell of
scintillating light. At 72x
NGC 6752 is most
remarkable. The soft
glow of count
less suns snaps into view. The core has a dense
cross of bright members whose magnitudes range
from 10.5 to 12. The cross's arms are oriented
northeast-southwest and northwest-southeast.
The northeastern arm loops to the north, forming
a starry atoll that encircles a dark lagoon. Three
weaker arms extend toward the south and
southeast, forming a letter E. Most curiously, the
brighter members of these arms appear to cluster
away from the core. Seeing this helped me to
understand how Dunlop could have felt that
NGC 6752 might be two different clusters along
the same line of
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