The Caldwell Objects

elongated mass beside it.
As noted above, of the remaining objects in
Messier's first list of 41 noncometary objects, all
but one (the double star M40) lie within the
magical comet haystack at various times of the
year. But even M40 appears to be an explainable
exception. After searching through the lists of
previously reported nebulae to make his list as
complete as possible, Messier set out to locate
each of these objects, some of which he could not
find. "He therefore published as part of his
memoir," Gingerich writes, "a number of objects
previously reported, but which he could not find.
He realized that many of them were nebulous to
the naked eye but were mere asterisms when
examined with a telescope." This left Messier
with a list of 39 cometlike objects. Given Messier's
penchant for rounding out his lists, he included
in his "final" listing of 1764 the "nebula" that had
fooled not him, but Johannes Hevelius in 1660
(namely, M40). The "nebula," as Messier pointed
out, is merely "two stars very close to one another
and very faint." As many comet hunters today
know, close pairs of stars seen at low magnifications
can look remarkably like dim comets.
Messier's choice to include M40 was not only
logical but educational — a warning, if you will,
to comet hunters that there are things other than
nebulae and star clusters that can masquerade as
comets.
In January 1765, just as Messier had finished
rounding out his list, he discovered, by chance
(not in the course of a systematic comet search) a
cluster of stars (M41) just south of brilliant Sirius.
And that is why, four years later, he decided
once again to round off his list. The inclusion of
M42 seems logical because of M43, the small
nebula north of M42 that has a round cometlike
head, starlike nucleus, and "tail" of nebulosity.
Both the Praesepe and the Pleiades star clusters
lie near-
ly on the ecliptic. Although they would not necessarily
be problematical to telescopic comet
hunters (being bright and easily resolvable into
stars), they still might confuse someone looking
in the comet haystack with the unaided eye in the
twilight.
The Double Cluster, then, is the only celestial
object visible from Messier's latitude and known
since classical times that is not included in the
comet hunter's original list of 45 objects. The
remaining 65 objects in Messier's final catalog are
not part of the mystery." [The] first group of [45]
nebulae was the only part of the catalogue
prepared for its own sake," Gingerich explains;
"most of the subsequent objects were found and
reported in comet searches, so that the remainder
of the catalogue is less systematic than the first
part."
So why not include the Double Cluster?
From Messier's latitude of nearly 50° N, the
Double Cluster never sets. Even at lower culmination
it would still be almost 20° above
Messier's northern horizon. Its far northerly
declination of nearly +60° places it well outside
of the comet haystack during most of the year.
Only in the late spring and early fall does it lie on
the far northern fringe of the comet haystack.
Furthermore, in the twilight, not only is the
Double Cluster obviously a pair of close objects
(a dead giveaway), its appearance coincides with
that of Cassiopeia's brightest stars. Even in the
unlikely event that Messier were to spot its glow
in the early evening twilight, by nightfall he
certainly would have recognized it. Nor is the
Double Cluster a problem when it lies close to
the comet haystack in the morning sky. Predawn
comet searches begin before the onset of twilight.
The observer starts by looking 45° above the
horizon and works his way toward the Sun.
Since the Double Cluster is cir-cumpolar, its
presence and identity would be obvious in a
dark sky. Furthermore, as the
452 Deep-Sky Companions: The Caldwell Objects