The Caldwell Objects

93 & 101
And when it comes to its brightest stars, NGC
6752 ranks second, trailing behind only NGC
6397 (Caldwell 86) in Ara. Yet how many North
American observers have seen it? And how many
North American observers have seen another
Pavo prize, NGC 6744 (Caldwell 101), one of the
largest barred spirals known? Since NGC 6744
lies only 4° south of Caldwell 93, we will throw
order to the wind and explore both of these
Caldwell gems here.
James Dunlop discovered NGC 6752 from
Parramatta, New South Wales, listing it as the
295th object in his 1827 catalog of southern deepsky
objects. He described it thus:
[A] "pretty large and very bright nebula, 5' or 6'
diameter, irregular round figure, easily resolved into
a cluster of small stars, exceedingly compressed at
the centre. The bright part at the centre is occasioned
by a group of stars of some considerable magnitude
when compared with those of the nebula. I am
inclined to think that this may be two clusters in the
same line; the bright part is a little south of the centre
of the large nebula.
From the Cape of Good Hope, John
Herschel noted that the cluster's brighter stars
"run out in lines like crooked radii," while the
fainter ones "are massed together in and round
the middle."
Dunlop and Herschel both noted one of the
most singular and fascinating qualities of this
cluster: its extremely high degree of central
compression. The central condensation appears
to be about 2.5' in diameter, but this globular also
hides an extremely tiny inner core only 20"
across. While the cluster's true physical extent is
110 light-years, this tiny inner core spans but 1.3
light-years. NGC 6752 has provided astronomers
with one of the best examples of a remarkable
phenomenon called core collapse. Astronomers
reviewed more than
372
100 Hubble Space Telescope images of the cluster's
inner core and discovered that about 20 to 40
percent of all the stars there are binaries. The
fraction of binary stars in a globular star cluster
provides important insight into the cluster's
evolution. The orbital energy stored within
binary systems is the main source of a globular's
dynamical "heating," which prevents its core
from collapsing into a single massive object. But
in the case of NGC 6752, the intense gravitational
pull of so many stars in such a small volume
seems to be causing stars to converge on the
cluster's core like bees swarming to their hive.
Such a collapse may be the process responsible
for creating such exotic objects as X-ray pulsars,
blue stragglers, and possibly (though doubtfully)
black holes. Blue stragglers
Deep-Sky Companions: The Caldwell Objects