The Caldwell Objects

galaxy maintained their relative spatial relationship
no matter how I moved the telescope.
NGC 5102 is regarded as a classical example of a
type-S0l lenticular galaxy. It spans 24,000 lightyears
and is receding from us at 460 km per
second. We see the galaxy inclined 19° from edge
on. Under moderate magnification, the galaxy
displays a tack-sharp nucleus in a bright and
obvious saucer of light; a larger, uniform oval
glow surrounds both these features. Longexposure
photographs show that the lens may be
dappled with dust, and one can try to imagine
other dusty forms in the outer disk as well. A
1993 Astrophysical Journal Supplement Series article
classifies NGC 5102 as a very blue lowluminosity
galaxy. In addition to an underlying
population of old, red, metal-poor stars, the
galaxy contains an abundance of hot, young,
metal-rich stars. Together with infrared
observations gathered in the 1980s, this fact has
led astronomers to suggest that a burst of star
formation occurred in the galaxy about 400
million years ago.
18
NGC 6723
Type: Globular Cluster
Con: Sagittarius
RA: 18 h 59.6 m
Dec: -36° 38'
Mag: 7.0
Diam: 13'
Dist: 29,000 light-years
In the case of Caldwell 68 (NGC 6729), the R
Coronae Australis Nebula, Patrick Caldwell-
Moore seems to have missed the forest for a tree,
because a mere 30' northwest of this rather
obscure nebula lies the glorious Sagittarius
globular cluster NGC 6723 — one of the most
spectacular objects of its kind in the
442
night sky. (Of course, the Caldwell Catalog is not
a list of the sky's most spectacular non-Messier
objects but of Moore's favorite non-Messier
objects, whether he chose them on the basis of
visual appeal or astrophysical interest.) NGC
6723 marks the apex of a near-perfect isosceles
triangle with Zeta (ζ) and Epsilon (ε) Sagittarii,
which lie 6¾° to the north-northeast and 7½° to
the northwest of the cluster, respectively. It also
just skirts the northern boundary of Coronae
Australis, so it is well within the reach of most
Northern Hemisphere observers.
Shining at 7th magnitude, the cluster is a tad
faint to be detected with the unaided eye, but it is
so concentrated that someone with young eyes
should take up the challenge, especially if
attending one of the more southerly star parties,
such as the Texas Star Party. Needless to say the
object is a cinch in 7x35 binoculars and a fine
sight in the smallest of telescopes. The cluster's
brightest star shines at magnitude 12.8 and at
low power the remainder form a sizzling haze of
tiny suns. Christian Luginbuhl and Brian Skiff
say that in a 60-mm refractor the cluster looks
similar to M4 in Scorpius. I came to the same
conclusion independently after I resolved the
needle-like row of stars at the cluster's core. At
high power NGC 6723 looks like a blizzard of
starlight. Look for dark lanes running through its
core and fringes.
Deep-Sky Companions: The Caldwell Objects