The Caldwell Objects

81
NGC 6352
Type: Globular Cluster
Con: Ara
RA: 17 h 25 m 29.2 s
Dec: -48° 25' 22"
Mag: 8.0
Diam: 7.1'
Dist: 18,600 light-years
Disc: James Dunlop, included in his 1827 catalog
H ERSCHEL: None.
GC: None.
NGC: Cluster (not nebula), pretty faint, large.
S INCE GLOBULAR CLUSTERS ARE THE OLDEST
objects in the Milky Way, they have become
proving grounds for theories about the evolution
of our galaxy and the age of the universe. By
studying the ages, positions, motions, and
chemical compositions of these fantastic stellar
agglomerations, astronomers have determined
that two groups of globular clusters exist in our
galaxy. One is huddled near the flat galactic
plane; the other is a spherical-halo clan. The
"thick disk" clusters that constitute the first group
typically have more "metals" (heavy chemical
elements) than those in the halo. Yet NGC 6352,
an inner-halo object that lies 10,800 light-years
from the galactic center, is one of the most metalrich
globulars known. Typically a halo globular
might have about 1/100 as much iron (relative to
hydrogen) as does our Sun, but the stars in NGC
6352 have one-fifth as much. Clusters like NGC
6352, then, might be connecting links between
halo globulars and old open clusters like M67 in
Cancer and NGC 188 (Caldwell 1) in Cepheus.
As reported in a 1995 Astrophysical Journal
324
81
paper, Hubble Space Telescope observations of
NGC 6352 have revealed it to be very similar to
the heavens' prototypical disk globular cluster, 47
Tucanae (Caldwell 106); the clusters are similar in
age (about 14.5 billion years) and metallicity. In
the opinion of the HST study's authors, this
suggests that the thick-disk system of globulars is
older than the system populating the halo. This
contradicts a widely held consensus that the halo
clusters are older than those in the thick disk (see
the Caldwell 73 discussion starting on page 288).
If it is true, however, the thick-disk globulars
probably formed along with the rest of the Milky
Way's disk, while the halo clusters formed in
satellite galaxies that the Milky Way later
cannibalized. Dramatic evidence of this kind of
merger was provided in 1994 by the discovery of
a dwarf spheroidal galaxy in Sagittarius. That
galaxy appears to be undergoing tidal disruption
and is depositing its stars and globular clusters
into the Milky Way's halo. More likely is a hybrid
scenario in which some halo globulars are
primordial while others have been accreted along
with the
Deep-Sky Companions; The Caldwell Objects