The Caldwell Objects

ly east of NGC 362 we find the magnitude-12.3
galaxy NGC 406, a tiny spindle just 2.9' long. In
photographs it displays two peculiar spiral arms
that are festooned with bright knots. How large a
telescope is needed to see the knots?
Next to Omega Centauri (Caldwell 80), 47
Tucanae is the brightest globular in the heavens.
But Omega Centauri's top ranking seems
insignificant once you realize that it is only 0.05
magnitude (5 percent) brighter than 47 Tucanae.
The magnificent 47 Tucanae is also 1.25
magnitudes (3 times) brighter than the brightest
Messier globular, M22 in Sagittarius, and its
diameter is 30 percent larger than M22's. The
cluster's true physical size is 214 light-years —
nearly twice that of NGC 362 — and its apparent
diameter is some 60 percent larger than the full
Moon's. Like NGC 362, 47 Tucanae is a relatively
metal-rich cluster, with 1/6 as much iron in each
of its stars as there is in our Sun. Its integrated
spectral type is a yellowish G4. Unlike NGC 362,
47 Tucanae is receding from us, at a paltry 19 km
per second.
Many works say that 47 Tucanae was not
discovered by astronomers before 1751, when
104 & 106
Abbe Nicolas Louis de Lacaille swept up this
spectacular object and kicked off his 1755 catalog
with it. After seeing it through a ½-inch 8x
telescope he said 47 Tucanae was "like the
nucleus of a fairly bright comet." (Ironically he
placed it in his Class I, "nebulae without stars.")
But 47 Tucanae does have a history that predates
Lacaille's telescopic observation. The cluster
appears as a "star" just west of Nubecula Minor
(the Small Magellanic Cloud) on Bayer's
Uranometria, which was first published in 1603.
Bayer never did see that "star" himself, and his
information on far-southern stars came not from
astronomers but from explorers who journeyed
south of the equator. Indeed, the first catalogs of
southern stars were those of the Dutch navigators
Pieter Dirckszoon Keyser (published around
1595, and now missing) and Frederick de
Houtman (published in 1603). Had these
explorers possessed a keener understanding of
the heavens, they undoubtedly would have
noticed that the "star" just west of the Small
Magellanic Cloud might appear swollen to the
attentive naked eye. (I have seen it this way from
Africa, with the "star" only 10° above the horizon,
and I'm certain many aboriginal skywatchers
would have done the same.) Astronomers did not
study the southern sky in any detail until a 20year-old
Edmond Halley went to St. Helena in
1676 with the sole purpose of cataloging the
southern stars. Halley, who had "poor eyesight,"
spent 18 months on that remote South Atlantic
island, plagued by inclement weather, an
arrogant governor, and harsh living conditions.
In the end Halley cataloged only 341 stars. He did
discover the diffuse nature of Omega Centauri
(Caldwell 80), however, and given more clear
nights he undoubtedly would have discovered
the fuzzy nature of 47 Tucanae before Lacaille
did.
In the 19th century Benjamin Apthorp
Gould (1824-1896), the founder and first direc-
The Caldwell Objects 413