OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY
NGC 5139 (OMEGA CENTAURI),
THE GREAT GLOBULAR CLUSTER IN
CENTAURUS
R.A. J3h 26.8m, Dec. --47° 29' (2 000.0)
Technical. Omega Centauri is without
doubt the grandest globular cluster in
sky. I t has the larges t true diameter of any
measured (620 light-years) and also "If-'jJ"lIl1
to be one of the closes t known (16 500
years). It is about 15 billion years old,
Iy the age of our entire Galaxy. Omega
tauri contains an estimated one million s
It is one of the most massive clusters
with about 500 000 times the mass of
Sun. In addition, it is one of the fastest
ing globulars. As a result it is decidedly
tical, about 25 percent longer on one axis.
are fortunate to have it as such a
neighbor.
Like other globular clusters, Omega
tauri is deficient in heavy elements and
tains no measurable gas or dust. Since
cluster's stars are nearly as old as the
verse, they formed out of material
fresh from the Big Bang: hydrogen
helium, with only small traces of heavy
ments that were synthesized inside nr"\I1,rIllO
generations of stars.
The lack of gas and dust is explained
the cluster's orbit. A detailed study of the
motions of individual stars in the cluster
allowed an accurate determination of the entire
cluster's motion and hence its orbit
around and through our Galaxy. Omega
Centauri follows a highly elliptical orbit
around the center of our Galaxy that brings it
to within 6 200 light-years of the galactic nucleus
and as far away as 21 000 light-years. Its
many passes through the plane of our
have swept it clean of gas. This orbit is typ!cal
of other globulars. Since they have no m­
terstellar gas, they have not been able to produce
new stars for billions of years.
Omega Centauri was catalogued by
Claudius Ptolemy over 1,800 years ago. In
1603 it was again listed as a star, this time by
Johann Bayer, who gave it the Greek letter
Omega in his sky atlas Uranometria. Edmond
Halley seems to have been the first to recog­
high in the sky, it definitely appears larger
nize the object as a cluster, in 1677. When
than a star and quite fuzzy to the naked eye.
V· ual. Omega Centauri is beyond com-
IS
Its total visual magnitude of3.65 makes
p a t r brightest globular in the sky. With a
It .
h
r:
d' la meter of 30 arc-mmutes, . t e mean sunace
brightness is 19.6 magmtu d es per square arcond.
Only the cluster 47 Tucanae far to
sec
the south can come c I ose to ' Its Vlsua . I
splendor.
Through the 8-inch telescope under moderate
skies, Omega Centaui was awe­
. spiring even when very,low m the sky. At
;owers of more than 100 X it was resolved to
the center. Countless stars filled the field of
The star density increases only slowly toward
the center. The cluster's bright central
portion is 6 to 7 arc-minutes across, and the
maximum extent observed through the 8-
inch was about 13 arc-minutes. On deep-sky
photographs Omega Centauri is well over
one degree in diameter, though visually when
high in the sky it appears about 30 arc­
medium-power Erfle eyepieces.
minutes across.
East of the center are two U-shaped strings
of stars with the Us connected at their bottoms.
Farther east, and above and below the
double U, are two strings of stars that extend
farther eastward still. The southernmost is an
arc about 3.5 arc-minutes long. In the accompanying
drawing, only a few of the brightest
tions. Plotting thousands of stars by eye is
stars are in their geometrically correct posi­
hardly feasible!
The brightest stars in Omega Centauri are
of 11 th magnitude, so the cluster can be at
least partially resolved with telescopes as
small as 2 inches, when it is high in the sky.
These bright stars are red giants, and their
color may be detectable in large amateur telescopes.
Omega Centauri contains over 160
known variable stars, second only to M3.
The well-known globular M13 (NGC
6205) in Hercules appears miniscule in comparison
with Omega Centauri. From low
northern latitudes, such as in Hawaii, M13
and Omega Centauri are above the horizon
at the same time, so the two can be compared.
Such a comparison can be made here:
the drawing of Omega Centauri and that of
Ml3 on page 186 are at the same scale. Note
that Ml3 appeared only half the size ofOmega
Centauri through the 8-inch -even though
the latter was very low in the sky, while the
A VISUAL ATLAS OF DEEP-SKY OBJECTS
former was observed nearly overhead. This is
not intended to denigrate the splendor of
M13, but rather to put it in perspective for
northern observers for whom Omega Centauri
is one of those tantalizing, legendary
objects on or below the southern horizon.
Since M 13 is called the Great Globular Cluster
in Hercules, perhaps Omega Centauri
should be named the "Super Great Cluster in
Centaurus" .
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