The Caldwell Objects

94
visual majesty of the Milky Way surrounding it.
The Southern Cross marks the eastern end of a
swath of swirling madness running between 10
and 13 hours of right ascension and from -55° to -
65° in declination. This region contains about two
dozen open star clusters (including IC 2602, or
Caldwell 102, the Southern Pleiades), about a
dozen nebulae (including Caldwell 92, the Eta
Carinae Nebula), thick star clouds, and eerie
lagoons of darkness like the Coalsack (Caldwell
99). No other part of the visible heavens draws
more attention to itself than this. The Southern
Cross simply happens to be the most obvious
asterism in this Van Goghian celestial landscape.
And of all the clusters that dapple this
region, none has captured the imagination more
than the Jewel Box. Early explorers saw the Jewel
Box as a 4th-magnitude star and designated it
Kappa (κ) Crucis. Many believe that John
Herschel discovered the cluster because his
description of it led to the Jewel Box nickname:
"this cluster, which though neither a large nor a
rich one, is yet an extremely brilliant and
beautiful object when viewed through an
instrument of sufficient aperture to show
distinctly the very different colour of its
constituent stars, which give it the effect of a
superb piece of fancy jewellery." But credit for
the Jewel Box's discovery (if not its name)
belongs to Nicolas Louis de Lacaille, who designated
Kappa Crucis a "nebulous cluster" during
his 1751-53 exploration of the southern skies. The
cluster is the 12th object in his 1755 catalog,
where it is described as "five to six stars between
two mag. 6 stars."
That Lacaille saw the cluster as nebulous is
not surprising; he used tiny ½-inch telescopes.
Through such instruments any background of
faint stars would have appeared as a milky haze.
James Dunlop split that haze into individual
stars with his 9-inch telescope in Par-
ramatta, New South Wales. His notes: "κ Crucis
. . . is five stars of the 7th magnitude, forming a
triangular figure, and a star of the 9th magnitude
between the second and third, with a multitude
of very small stars on the south side."
At the heart of the Jewel Box John Herschel
noticed a bright "extremely red" ruby among a
smattering of emeralds "of different shades of
green." In all, Herschel found eight stars in which
the "colour is conspicuous." Many sources
identify this ruby star at the cluster's center as the
star Kappa Crucis, but it is not. That central star
(a type-M2Iab red supergiant known as SAO
252073) is too faint,
at magnitude 7.2, to have received a Bayer designation.
According to the Yale Bright Star Catalogue,
Kappa Crucis is HD 111973, the
magnitude 5.9 star at the southeastern end of a
pyramid of four 6th- to 8th-magnitude stars.
Kappa Crucis, then, would be a blue (type-B5Ia)
supergiant. Alas, the Yale Bright Star Catalogue
also is mistaken. The naked-eye observers who
first charted Kappa Crucis saw the entire cluster
as a naked-eye star, just as they did with Omega
Centauri. On many nights I have gone out and
looked at the Jewel Box with the naked eye, and I
have convinced myself that our astronomical
ancestors did not isolate HD 111973 from the rest
of the compact cluster. It
378 Deep-Sky Companions: The Caldwell Objects