Endlessly varied is the grouping of the stars. We find solitary stars,
twin stars, large stars with small companions, bright stars with
faint companions, stars alike in colour and type, stars in strongly
contrasted colours, one young, the other aged, stars with invisible
companions, couples close enough to touch one another, and couples
so distant that the satellite takes more than a century to revolve
round its primary. One or even both components of a bright pair may be
themselves divisible into a closer pair, sometimes into a whole group
of stars.
More beautiful and wonderful still are the clusters of stars of various
tints and magnitudes, where scores, even hundreds, are gathered
together in what to the eye is a tiny patch, though it may in fact take
light many years to cross from one side of it to the other.
The finest example in the heavens of this kind of cluster is Kappa
Crucis, near β in the Southern Cross.[8] It is just visible to the
naked eye as a small star, and in a binocular the main star is seen
to be surrounded by a number of others; in a telescope it is a
glorious sight. Orange and red stars are easily distinguished in the
brilliant throng, even if we have not Herschel’s eye for colour and
fail to discriminate the “greenish-white, green, red, blue-green,
and ruddy” which made up what he likened to “a superb piece of fancy
jewellery.” He charted over a hundred stars of all magnitudes from
7 to 17. Herschel’s observations at the Cape were made between 1834
and 1838. When Mr. Russell charted the stars of Kappa Crucis at the
Sydney Observatory in 1872, he found 25 that had not been recorded by
Herschel, although the great reflector was much larger than the Sydney
instrument; many of Herschel’s stars had drifted, and five could not be
found at all. If changes so striking as this take place in less than
forty years, “it is evident,” as Russell observed, “that more attention
should be bestowed on clusters.”
[8] It is easy to remember the names of the stars in the Southern
Cross. Begin at the foot, which is obviously the brightest, and count
round the Cross in clockwise direction α, β, γ, δ. κ is beyond β in a
line with γ, β.
Quite a dozen star-clusters of this kind are visible to the naked eye
in the southern hemisphere, the most striking being in Scorpio, where
it shines as a conspicuous silvery spot just beyond the Scorpion’s
tail, midway between κ Scorpii and γ Sagittarii. It is named M 7,
which means the seventh in Messier’s list of clusters and nebulae.
In a binocular it is seen to be a group of very many stars, some
close together, others scattered. Lacaille, with his little half-inch
telescope, counted from 15 to 20 stars, and Herschel, with his large
reflector, estimated the number at 60. At Cordoba Observatory no less
than 139 were catalogued.
M 6, a little north and west of M 7, is also visible to the naked eye
as a nebulous patch, and is a fine cluster. About 50 stars between
magnitudes eight and twelve have been photographed, and there are
doubtless many more of lesser brightness.
Look also just above the naked-eye double ζ in Scorpio, and see what
appears to be a hazy star. A binocular separates this into a number of
small stars, and 150 have been photographed, of magnitudes seven to
twelve. It is known as h 3652 or N.G.C. 6231, the former being the
number in Sir John Herschel’s catalogue, the latter in Dreyer’s New
General Catalogue of star-clusters and nebulae.
A little south of Sirius is a patch of nebulous light which shows as
stars in a binocular. This was registered by Messier in 1764 as a “mass
of small stars,” and is known as M 41. Webb saw the brighter stars
arranged in curves and a ruddy star near the centre. One hundred and
forty-four stars were registered by Gould, of which only five are as
bright as eighth magnitude.
In the neighbourhood of the Cross there are quite a number of large
bright clusters, for they must be large and bright above the average
to be seen by the naked eye. A line passing through the shorter arm
of the False Cross—_i.e._ from δ Velorum to ι Carinae, the naked-eye
double—and continued for an equal distance beyond, leads to a white
oval patch which is plainly visible to the naked eye, and in the
binocular appears like a few stars sparkling on a nebulous background.
With higher powers the background also is resolved into stars, of which
there are some two hundred of the fifteenth magnitude and brighter up
to the eighth. This is N.G.C. 3114.
In the same direction is θ Carinae, a bright Orion-type star with
numerous small stars crowding close to it. This is a very lovely group
in a good binocular. It contains about twenty stars of magnitudes
between three and eight, and with high powers appears as a brilliant,
loosely scattered cluster covering a portion of sky equal in breadth to
twice the sun’s apparent diameter.
Very similar is a cluster close to the bright and richly coloured star
X Carinae. It looks like an elliptical nebula in a binocular, with a
few stars scattered over it. Two hundred have been photographed.
Near ε Carinae, the beautiful ruddy star at the foot of the False
Cross, is yet another most beautiful cluster, which contains about
fifty stars of the ninth magnitude and brighter. It is visible to the
naked eye, but when a telescope is turned upon it the brilliancy is
startling. Radiant stars are scattered all over the field.
Just east of π Puppis, the top of the poop of Argo, is a
fourth-magnitude star C of a bright orange colour, and round it is a
cluster which Gould describes as “extremely impressive to the naked
eye.” Ninety-two stars show on his photograph.
Close to δ Velorum (in the False Cross) is the star ο Velorum. Even to
the naked eye the bright star (3½ mag.) is seen to be surrounded by
a cluster of faint stars, and in a binocular it is a splendid sight.
The number of stars is small, but they are bright, ten being above the
ninth magnitude.
Lastly we may mention a remarkably fine cluster which is visible in
a binocular though not to the naked eye, in the Centaur, among a
stream of small stars between the Cross and η Argūs. Dunlop called it
“a pretty large cluster of stars of mixt magnitudes”; Brisbane, “a
prodigious number of small stars very close together.”[9] There are at
least a hundred above the sixteenth magnitude.
[9] These two astronomers observed at Paramatta, New South Wales, in
the early part of the nineteenth century.
It should be noticed that nearly all these clusters are in the Milky
Way, and the rest are very near it, for this is characteristic of
star-clusters like these, viz. irregular groups containing mixed
magnitudes of stars. In a list of all bright objects of this kind, only
two are found as far as 30° away from the middle line of the Galaxy,
while 89 are within 30° north or south of it. There are besides 38 in
the Clouds of Magellan, which resemble the Milky Way in constitution.
Many clusters are simply unusually dense portions of the Milky Way, and
we may almost say that this type forms part of it and of the Magellan
Clouds.