The Caldwell Objects

Omega Centauri is generally regarded as the
night sky's preeminent globular star cluster. But
even that is debatable. Okay, now pick your jaw
up off the floor. I haven't lost my mind. Let's look
at the statistics. Omega Centauri is the brightest
globular star cluster, but just barely so. Although
many sources show it as bright as magnitude 3.7
or 3.5, my naked-eye estimate of magnitude 3.9 is
not so optimistic; my estimate also agrees with
the value listed in Brian Skiff's Observational Data
for Galactic Globular Clusters, which was
published in the Webb Society Quarterly Journal for
January 1995 and has since been updated on the
World Wide Web. If we accept a magnitude of 3.9
for the cluster, Omega Centauri has the same
apparent brightness as 47 Tucanae (Caldwell
106). So which is the greater globular? Skiff says,
"Some prefer the sheer richness of Omega Cen,
and others the star-density and remarkable
(indeed unique) structure of 47 Tuc— Both are
luminous objects with large numbers of stars, as
anyone who has observed them can confirm, and
their combined light sends them to the top of the
total magnitude list. But because of their distance,
the brightest [individual] stars [in these clusters]
are not as bright as [stars in] nearer clusters."
Indeed, when ranked by the brightnesses of
their constituent stars, both Omega Centauri and
47 Tucanae fall short of the top. Omega
Centauri's brightest stars shine at an impressive
magnitude 11.5, but five other globular clusters
beat it in that category, while seven clusters beat
out 47 Tucanae. When ranked by horizontalbranch
magnitude (an indirect measure of the
cluster's resolvability), three clusters beat out 47
Tucanae, while seven clusters are more readily
resolved than Omega Centauri in small
telescopes. Of course, a cluster's greatness is
highly subjective; it's all in the eye or the heart of
the beholder. Observing is a
80
highly personal experience, and we all react
differently to what we see through a telescope.
And even one observer's reaction will differ with
telescope aperture. But if we were to magically
place all the globulars at the same distance and
look at them side by side, we'd find that Omega
Centauri is the largest and most massive of them
all. It is, in fact, the biggest globular in our Milky
Way galaxy. The cluster spans more than 280
light-years of space, contains several million
stars, and has a mass of about 5 million Suns,
making it about 10 times more massive than
other big globulars and a near match for some
dwarf galaxies.
Omega Centauri is also one of the oldest
objects in the Milky Way; its age is comparable to
that of the universe itself. On average, each of the
cluster's member stars contains about 1/40 as
much iron as does our Sun — a pretty typical
metallicity for a globular cluster. Its overall
spectral type is F5, and its radial velocity is 230
km per second.
In 1999 Young-Wook Lee (Yonsei University,
South Korea) and five colleagues studied
50,000 of Omega Centauri's stars with the 1meter
reflector atop Cerro Tololo in Chile. They
found several stellar populations that had
formed in distinct bursts over a 2-billion-year
period. The scientists speculate that the
prolonged starburst activity would make sense if
Omega Centauri were the remnant nucleus of a
small galaxy that merged with our Milky Way If
true, this would make Omega Centauri akin to
the globular cluster M54, which appears to be the
nucleus of the Sagittarius Dwarf Galaxy now
being consumed by the Milky Way.
In 7x35 binoculars Omega Centauri is
magnificent, a bright, distended cloud of pseudocometary
fluff. It lies immediately northeast of
a bowl of 7th-magnitude suns in a 2°-long
dipperlike asterism. The cluster swells dramatically
when one switches from naked-eye
The Caldwell Objects 321