If Kappa Crucis is the finest irregular star-cluster in the sky,
Omega Centauri is undoubtedly the largest and most splendid of all
the globular star-clusters, for its diameter is more than twice the
diameter of the famous northern cluster in Hercules. It is easily
found, being nearly in line with δ and γ Centauri (the two conspicuous
stars just north of the Cross) and a little further from γ than γ
is from δ. The cluster looks to the naked eye just like a tailless
comet, and was mistaken for one by the author when first seen. In a
binocular it is quite round, the soft milky light growing gradually
brighter towards the centre, but without the slightest suggestion of
irregularity, and no appearance of stars. It must have been seen by
the early navigators who named the southern constellations, but it was
first discovered as a star-cluster by Halley in the island of St.
Helena. Most amazing is its appearance in a telescope, for the milky
disc breaks up into thousands of tiny points of light, densely crowded,
all alike, innumerable.
“This most glorious object,” as Herschel calls it, “the noble globular
cluster ω Centauri, beyond all comparison the richest and largest
object of the kind in the heavens,” is evidently quite distinct from
κ Crucis and clusters like those described in the last chapter. In
form it is circular, and the condensation towards the centre suggests
that it is spherical. There are some scattered members of the group
lying outside the bright crowded sphere. The stars are also immensely
more numerous and more closely packed than in the irregular clusters,
their total number being estimated at 10,000, and 6000 have been
actually counted on photographs, and all these in a space which looks
little larger than that occupied by the sun in the sky. Another
striking difference is that, instead of bright and faint stars mingled
together, here they are all nearly alike and very minute. Curiously
enough, it is found that they belong to two magnitudes, and two only,
the thirteenth and fifteenth, and this seems to be a feature of all
globular clusters, as well as the form and the dense crowding of the
stars. Herschel at first thought the stars in ω Centauri “singularly
equal, and distributed with the most exact equality, the condensation
being that of a sphere equally filled.” But he immediately adds:
“Looking attentively, I retract what is said about the equal scattering
and equal sizes of the stars. There are two sizes ... without greater
or less, and the larger stars form rings like lace-work on it.” In
his later notes he is again doubtful, for he thinks that the effect
may be optical, and the larger stars only knots of faint stars; but
photography has settled the question in our day.
Yet another point of difference between globular and irregular clusters
is that the latter often have wisps of nebulosity clinging about them,
but globular clusters are entirely free from it.
[Illustration: THE STAR-CLUSTER 47 TOUCANI