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The Caldwell Objects

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elongated mass beside it.<br />

As noted above, of the remaining objects in<br />

Messier's first list of 41 noncometary objects, all<br />

but one (the double star M40) lie within the<br />

magical comet haystack at various times of the<br />

year. But even M40 appears to be an explainable<br />

exception. After searching through the lists of<br />

previously reported nebulae to make his list as<br />

complete as possible, Messier set out to locate<br />

each of these objects, some of which he could not<br />

find. "He therefore published as part of his<br />

memoir," Gingerich writes, "a number of objects<br />

previously reported, but which he could not find.<br />

He realized that many of them were nebulous to<br />

the naked eye but were mere asterisms when<br />

examined with a telescope." This left Messier<br />

with a list of 39 cometlike objects. Given Messier's<br />

penchant for rounding out his lists, he included<br />

in his "final" listing of 1764 the "nebula" that had<br />

fooled not him, but Johannes Hevelius in 1660<br />

(namely, M40). <strong>The</strong> "nebula," as Messier pointed<br />

out, is merely "two stars very close to one another<br />

and very faint." As many comet hunters today<br />

know, close pairs of stars seen at low magnifications<br />

can look remarkably like dim comets.<br />

Messier's choice to include M40 was not only<br />

logical but educational — a warning, if you will,<br />

to comet hunters that there are things other than<br />

nebulae and star clusters that can masquerade as<br />

comets.<br />

In January 1765, just as Messier had finished<br />

rounding out his list, he discovered, by chance<br />

(not in the course of a systematic comet search) a<br />

cluster of stars (M41) just south of brilliant Sirius.<br />

And that is why, four years later, he decided<br />

once again to round off his list. <strong>The</strong> inclusion of<br />

M42 seems logical because of M43, the small<br />

nebula north of M42 that has a round cometlike<br />

head, starlike nucleus, and "tail" of nebulosity.<br />

Both the Praesepe and the Pleiades star clusters<br />

lie near-<br />

ly on the ecliptic. Although they would not necessarily<br />

be problematical to telescopic comet<br />

hunters (being bright and easily resolvable into<br />

stars), they still might confuse someone looking<br />

in the comet haystack with the unaided eye in the<br />

twilight.<br />

<strong>The</strong> Double Cluster, then, is the only celestial<br />

object visible from Messier's latitude and known<br />

since classical times that is not included in the<br />

comet hunter's original list of 45 objects. <strong>The</strong><br />

remaining 65 objects in Messier's final catalog are<br />

not part of the mystery." [<strong>The</strong>] first group of [45]<br />

nebulae was the only part of the catalogue<br />

prepared for its own sake," Gingerich explains;<br />

"most of the subsequent objects were found and<br />

reported in comet searches, so that the remainder<br />

of the catalogue is less systematic than the first<br />

part."<br />

So why not include the Double Cluster?<br />

From Messier's latitude of nearly 50° N, the<br />

Double Cluster never sets. Even at lower culmination<br />

it would still be almost 20° above<br />

Messier's northern horizon. Its far northerly<br />

declination of nearly +60° places it well outside<br />

of the comet haystack during most of the year.<br />

Only in the late spring and early fall does it lie on<br />

the far northern fringe of the comet haystack.<br />

Furthermore, in the twilight, not only is the<br />

Double Cluster obviously a pair of close objects<br />

(a dead giveaway), its appearance coincides with<br />

that of Cassiopeia's brightest stars. Even in the<br />

unlikely event that Messier were to spot its glow<br />

in the early evening twilight, by nightfall he<br />

certainly would have recognized it. Nor is the<br />

Double Cluster a problem when it lies close to<br />

the comet haystack in the morning sky. Predawn<br />

comet searches begin before the onset of twilight.<br />

<strong>The</strong> observer starts by looking 45° above the<br />

horizon and works his way toward the Sun.<br />

Since the Double Cluster is cir-cumpolar, its<br />

presence and identity would be obvious in a<br />

dark sky. Furthermore, as the<br />

452 Deep-Sky Companions: <strong>The</strong> <strong>Caldwell</strong> <strong>Objects</strong>

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