The Caldwell Objects

original with Bayer, but goes back to the Italian
scientist Alessandro Piccolomini (1508-1578),
who was Archbishop of Patras. His book De le
Stelle Fisse, which went through several edi-
tions between 1540 and 1579, contains 48
woodcut constellation maps in which the stars
are labeled with [Roman] letters _ This paral-
lelism between Bayer's sequence of Greek let-
ters and Piccolomini's Roman ones suggests
that the German cartographer sometimes bor-
rowed from his Italian predecessor." His lack
of originality aside, Bayer's Greek-letter desig-
nations are in universal use today, and in most
cases they reflect how each constellation's
stars are ranked in brightness. When mistaken
for a star, Omega Centauri is outshone by 16
others in Centaurus, and thus it is labeled with
the last letter in the Greek alphabet. (Of course
we wouldn't call this cluster a "star" today, but
pretelescopic astronomers understandably
did so.)
Bayer included Omega Centauri in his
Uranometria of 1603 because Ptolemy had cataloged
it in his Almagest of 150 A.D. Ironically,
there is some question as to whether Ptolemy had
even made the observations that appear in that
latter work; instead, he may have "borrowed"
from Hipparchus and others in order to advance
his theories. That aside, Omega Centauri is bright
and obvious enough that aboriginal skywatchers
throughout time must have seen its
luminescence, though they presumably wouldn't
have known just what kind of celestial
magnificence blazed before them.
Omega Centauri remained a "star" until 1677,
when Edmond Halley rediscovered it from St.
Helena, recording it as a nonstellar object "in the
horse's back." Halley later included Omega
Centauri in a list of six "luminous spots or
patches" published in 1716 in the Philosophical
Transactions of the Royal Society. Halley's
contribution was titled "An Account of
several Nebulae or lucid Spots like Clouds, lately
discovered among the Fixt Stars by help of the
Telescope." Of this new object he writes (in the
third person):
The fourth [luminous spot or patch] was found by
M. Edm. Halley in the Year 1677, when he was
making the Catalogue of the Southern Stars. It is
in Centaur, that which Ptolemy calls [the one who
emerges from the (horse's) back] which He names
in dorso Equino Nebulus [Nebula on the back of the
horse] and is Bayers Omega; It is in appearance
between the fourth and fifth Magnitude, and
emits but a small Light for its Breadth, and is
without a radiant Beam; this never rises in
England, but at this time its Place is [Scorpio] 5
deg ¾ with 35 deg ⅕ South Lat.
80
Many sources erroneously credit Halley
with first discerning Omega Centauri's cluster
nature, but he never did so. Nor did the Swiss
astronomer and mathematician Philippe Loys de
Cheseaux (1718-51). De Cheseaux included
Halley's observations in his list of 21 nebulae,
which were presented to the French Academy of
Sciences in 1746, investigated by Guillaume
Bigourdan in 1884, and published as part of a
review of nebulous objects by Bigourdan in the
1892 Annales de VObservatoire de Paris. Of the first
14 objects in de Cheseaux's list Bigourdan writes:
"I begin with those which, viewed by telescope,
are found to be simple clusters of stars." But
Omega Centauri does not appear in that section.
Rather, it is listed in the second part of
Bigourdan's list, which contained "the nebulae
properly so called . . . which, when observed with
much larger telescopes, always appear like white
clouds." Bigourdan described Omega Centauri
as" [that nebula] in Centaurus, discovered by Mr.
Halley." Although Bigourdan made an effort to
observe most of the 21 neb-
The Caldwell Objects 319