The Caldwell Objects

the eye ever inward and takes your mind on a
fantastic journey. With the help of the telescope I
felt like I was in the cluster.
The horizontal-branch magnitude of 47
Tucanae is a reasonable 14.1, so the cluster can be
resolved in a 4-inch telescope. When I studied 47
Tucanae in an 8-inch from Wellington in 1997,1
noticed something peculiar, something I have not
read much about in the observing literature: the
cluster is not perfectly round but elliptical,
extending from north to south. Furthermore, its
interior is not as uniform as it appears at first
glance. With quiet scrutiny I detected at least
eight clearly defined spidery arms. Barbara
Wilson had a similar impression when she
observed 47 Tucanae on November 4, 1994, from
Tacna, Peru, under the darkest, steadiest, and
most transparent skies she had experienced thus
far. "The core is a brilliant crowded mass of
stars," she penned, describing the view through
her 7-inch telescope, "and the cluster has an
almost delicate appearance, with spiraling lines
of stars emanating in all directions. In a 20-mm
Nagler [eyepiece], the cluster resolves to the core,
some stars are reddish in appearance in [the]
center, and the cluster's center is sharply brighter
than the outer
104 & 106
portions. This is the finest globular I've seen."
While living in New Zealand in 1999, William
Sheehan had the opportunity to view 47 Tucanae
with the 1-meter telescope at the Mount John
Observatory. "At 48x," he says, "one immediately
notices the intensely concentrated nucleus with a
large spreading field of stars decreasing in
density away from the center. It is extremely
bright, in fact the brightest globular available to
the eye, inasmuch as Omega [Centauri] is
diffusely spread and has [a] low surface
brightness. At higher power the brilliance of the
globular seems to increase as it fills the whole
field of view and the core becomes ever more
condensed."
We often hear about the colors of globular
clusters. To my eyes, none makes a stronger
impression of color than 47 Tucanae. To most
observers 47 Tucanae displays a yellowish cast,
as its spectral type would suggest. But can that
color really be perceived at the eyepiece? The
cluster's concentrated core, it seems, should be
bright enough to stimulate the eye's colorsensitive
cones. If so, what color should we see?
David Malin, of the Anglo-Australian Observatory,
writes: "Though the light of globular
clusters is dominated by so-called 'red' giant
stars, their colour is no redder than a domestic
tungsten lamp, so the true colour of 47 [Tucanae]
is close to . . . pale yellow."
Globular clusters harbor some of the oldest
stars known in our galaxy. But when the Hubble
Space Telescope peered into the heart of 47
Tucanae, it revealed that the cluster's entire core
is crowded with mysterious "blue stragglers" —
stars that are bluer and brighter than the other
cluster members, as if somehow they were born
more recently than their siblings. Blue stragglers
were discovered about a half century ago and
were enigmatic until recently. However, in
recent years astronomers have become
convinced that these stellar non-
The Caldwell Objects 415