The Caldwell Objects

core and limber arms, which, he wrote, merged
"into the rich field with long irregular rays." He
called the entire spectacle "rich and delicate" as
seen through his 4-inch telescope.
At 72x the cluster's central bar appears
sharply uniform, with three or four horizontal
rows of stars separated by prominent dark lanes,
like those of M25 in Sagittarius. In fact,
on really crisp, dark nights the entire cluster
appears disrupted by intense black fissures lined
with rays of sunlight. All kinds of geometric
patterns can be imagined in the seemingly
regimental array of stars — a triangle to the east,
a straight line to the south, an ellipse to the west.
Using my imagination, the bright core looks like
the body of a rat, complete with head, feet, and a
tail stretching to the southwest. And the stellar
spectacle increases with aperture. Through her
17½-inch at 166x, Barbara Wilson finds NGC 2360
one of the nicest clusters in Canis Major, being
"round and rich with bright straight star chains,
pentagon shaped." Brent Archinal tallies 91 stars
The Caldwell Objects
58
in the cluster, which spans 15 light-years of
space. Although this seems rather sparse, it is
similar to the star count in, say, M26 in Scutum,
which measures 12 light-years across and has a
total population of about 120 suns. NGC 2360's
brightest star shines at magnitude 10.4, while its
faintest member is a dim 17th magnitude.
As with many clusters in star-rich regions of
the Milky Way, it is hard to define NGC 2360's
boundaries; the cluster lies but 1½° west of the
galactic equator, in a region wealthy with 10thto
12th-magnitude suns. Indeed, in his Celestial
Objects for Common Telescopes Rev. T Webb .
describes NGC 2360 as a "[b]eautiful [cluster]...
melting into a very rich neighbourhood, as
though the Galaxy were approaching us."
Webb's description may also allude to the
cluster's three-dimensional quality.
Despite its visual splendor the cluster is
aged. In the October (I) 1990 issue of Astronomy
and Astrophysics, Swiss astronomers Jean-Claude
Mermilliod and Michel Mayor estimate the
cluster's age at about 2.2 billion years — about
halfway between the ages of the Hyades
(Caldwell 41) and M67 in Cancer. At this
advanced age NGC 2360 has formed a sizable
population of red-giant stars, which stand out on
the cluster's color-magnitude diagram.
NGC 2360 is also about 2½° west of the
Puppis border and some 5° west-southwest of
M46 and M47, the famous open-cluster couple in
Puppis. Less than 1° southeast of NGC 2360
you'll find another bright stellar treasure, R Canis
Majoris, an Algol-type eclipsing binary that dips
from magnitude 5.7 to magnitude 6.3 once every
1.14 days.
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