The Caldwell Objects

ought it to our attention in the February 1985
issue of the Journal for the History of Astronomy.
Hodierna, an astronomer at the court of the Duke
of Montechiaro, used a simple 20x Galilean
refractor for his observations. Most interesting,
Hodeirna also predated Herschel with a
"plurality of worlds" concept, for he believed that
all nebulous objects were clusters of stars that
astronomers had resolved to various degrees.
(Actually, this belief has roots that stretch back to
Galileo, who had resolved the Milky Way, and
"nebulous patches" within the Milky Way, into a
number of stars.)
Both William and John Herschel singled out
NGC 2362, calling it a "beautiful cluster"
and a "fine cluster," respectively. In his 1844
Cycle of Celestial Objects, Adm. William Henry
Smyth describes it under the heading of 30
Canis Majoris, writing "a star with a compan-
ion [29 Canis Majoris] in cluster . . . on the
Greater Dog's back The whole has a beau-
tiful appearance, the bright white star [30
Canis Majoris] being surrounded by a rich
gathering of minute companions, in a slightly
elongated form, and nearly vertical position."
Several sources list NGC 2362 as a star cluster
and a nebula. Sky Catalogue 2000.0 lists NGC 2362
in its section on open clusters
64
and again in its section on bright nebulae. But
look at the NGC description in the table above.
Note that there is no mention of nebulosity. Sky
Catalogue2000.0 also includes aTrumpler code for
the open cluster and that code includes the letter
η for nebula. This, too, is a mystery, because in
his original 1930 work (Lick Observatory Bulletin
Vol. 14, No. 420) Robert J. Trumpler did not use
an n. In other words, Trumpler did not see any
nebulosity in NGC 2362. In fact, color-magnitude
diagrams of the cluster reveal litde if any
reddening or absorption.
In 1949 the late Harold Johnson of Washburn
Observatory observed NGC 2362 at the urging of
Walter Baade, who pointed out that "there is no
discernible nebulosity around the cluster."
Johnson's photometric and visual observations,
made with the 100- and 60-inch reflectors on
Mount Wilson, confirmed this fact. "There is little
reason," Johnson wrote, "to suspect the existence
of nebulosity in NGC 2362; an examination of the
original negative [taken with the 100-inch
reflector] likewise shows no nebulosity. The
visual appearance with the 100-inch telescope is
in agreement with the conclusion that the amount
of nebulosity associated with the cluster is
negligible." More recently, in a 1991 Astronomical
Journal paper, David Wilner (University of
California, Berkeley) and Charles Lada (Harvard-
Smithsonian Center for Astrophysics) referred to
NGC 2362 as "a rich young star cluster devoid of
natal gas and dust." Other researchers have come
to the same conclusion.
Yet there is nebulosity around the Tau Canis
Majoris Cluster. The confusion, says Lada, stems
from the fact that a bubble of gas does surround
the cluster, but it is about 5° across — vastly
wider than the cluster. Sharp-less 310, as the
bubble is called, is a very-low-density
filamentary HII region that appears on the redsensitive
National Geographic So-
The Caldwell Objects 255