Smaller than ω Centauri, but even more beautiful in the telescope,
is the cluster 47 Toucani,[10] which to the unaided eye appears like
a fourth-magnitude star near the smaller Cloud of Magellan. The
long curve of Grus followed southwards leads to it. Nearly as many
stars as in ω Centauri, or about 9500, are here massed into a still
smaller space, so the cluster is brighter, and is “compressed to
a blaze of light” at the centre. The two sets of stars, which are
mingled together throughout, are of thirteenth to fifteenth and of
seventeenth magnitudes respectively. Herschel saw the inner denser part
rose-coloured while the outer was white, but the present writer could
not see this nor find anyone to confirm it to-day, possibly because
the refracting telescopes now so often used do not show colour so well
as large reflectors like Herschel’s. A double star of 11th magnitude,
which is conspicuous in Herschel’s drawing, is doubtless far outside
the cluster, and only appears projected against it by perspective.
[10] Also named ξ Toucani.
Near β Aquarii there shines with the light of a sixth-magnitude star
another “magnificent ball of stars” which has been compared to “a heap
of fine sand.” It is named 2 M Aquarii.
Over seventy of these tightly packed balls of stars are known, even
counting only the brightest, and their distribution is rather curious.
A large number (about twenty) occur in the Clouds of Magellan, and more
than half of the seventy are in the Milky Way, not scattered evenly
along its course, but almost if not entirely confined to its southern
part, and chiefly gathered in a great group in its brightest portion,
where it passes through Sagittarius, Ophiuchus, and Aquila. Here they
are mingled with—or perhaps projected against—numerous stars of the
same magnitudes; but many balls are also found outside the Milky Way,
widely scattered, and in these parts of the sky there are relatively
few of the faint-magnitude stars which compose all the globular
clusters. 47 Toucani, for instance, though it is near the small
Magellanic Cloud, stands quite apart from it, isolated in a black sky.
We do not know the distances of any of these balls of stars. Those
which have been examined spectroscopically shine like Canopus—that is,
they are of a type intermediate between Sirius and our sun—but the
chief light comes, of course, from the brighter stars, and it may be
that the fainter stars mingled with them belong to a different type.
A remarkable fact lately discovered is that many globular clusters—but
not all—contain a large number of variable stars. These vary in light
in a period of about a day and have a range of about one magnitude.
They are not of the Algol type, nor quite of the usual “short-period”
types, and it is not yet clear what is the cause of variation, though
it seems probable that “cluster-variables” are double stars.