Like a great river returning into itself, the Galaxy encircles the
starry heavens, and those who know only its northern course have no
idea of its brilliance and wonderful complexity in its brightest part.
Its light is soft, milky, and almost uniform, between Cygnus and
Sirius, but when it enters Argo it becomes extremely broad, and spreads
out like a river on a flat marshy plain, in many twisting channels with
spaces between. Where Canopus shines on the bank there is a narrow
winding ford right across its whole breadth, as if a path had been made
by the crossing of a star.
After this it suddenly becomes extremely narrow, but so bright that all
the light which was shining in the broad channel seems to be condensed
in this narrow bed. In the brightest, richest part the Great Nebula of
Argo is easily distinguished by the naked eye. Contrasting with this
and other bright condensations are black gaps, the largest and blackest
of which is the well-known Coal-Sack near the Southern Cross.
[Illustration: THE MILKY WAY IN SCORPIO, LUPUS, AND ARA
Photographed at Hanover, Cape Colony,
by Bailey and Schultz]
The river now divides. One short stream, which goes north from Centaur
towards Antares, is faint and soon lost; but another northern stream
is so bright and so persistent that from Centaur to Cygnus we may say
that the Galaxy flows in a double current. This northern portion forms
first the smoke of the Altar on which the Centaur is about to offer
the Beast, then passes through the Scorpion into the Serpent-Holder,
and here, between η Ophiuchi and Corona Australis, the double stream
has its greatest width. The northern division soon grows dim and seems
to die out, but begins again near β Ophiuchi, and, curving through a
little group of stars, passes through the head of the Eagle and forms
an oval lagoon in the Swan.
The southern stream passes through the Scorpion’s Tail into
Sagittarius, then through the Eagle and the Arrow till it flows close
beside the northern stream in the Swan, and finally rejoins it in a
bright patch round α Cygni. Except just here it is much brighter than
the northern stream, and its structure is even fuller of wonderful
detail than in Argo. In Sagittarius it consists of great rounded
patches with dark spaces between. The brightest of these contains the
star γ Sagittarii; then follows a remarkable region of small patches
and streaks, the portion passing through Sagittarius and Aquila being
thickly studded with nebulae. This is followed by another bright patch,
rivalling that round γ Sagittarii, which involves the stars λ and 6
Aquilae.
This ends the most brilliant and wonderful part of the Milky Way. When
well seen, as we see it in the south, it recalls Herschel’s words,
written at the Cape when it came into view in his telescope:
“The real Milky Way is just come on in great semi-nebulous masses,
running into one another, heaps on heaps.” And again: “The Milky Way is
like sand, not strewed evenly as with a sieve, but as if flung down by
handfuls, and both hands at once.”
What is it? The ancients thought it the pathway of departed spirits,
or fiery exhalations from the earth imprisoned in the skies, or a
former road of the sun through the stars. But Democritus and some
other inquiring Greeks believed it to be the shining of multitudes
of stars too faint and too close together to be seen separately, and
we know this to be the truth. We know also, from simply counting the
stars in different regions of the sky, that their numbers increase
regularly as we go from north or south towards the Milky Way, and stars
of all magnitudes are most abundant within its course. We saw also that
star-clusters and certain kinds of nebulae frequent it, while other
kinds avoid it, and that blue and white stars are the most abundant
near it, and tend to move through space in planes parallel with it,
while the redder stars are scattered and move about in all directions.
Facts like these lead astronomers to believe that the Milky Way has a
definite relation with all the visible universe, that even the most
distant nebula is not an outlying universe apart from ours, but all are
parts of one vast stellar system.
It is possible that the Milky Way, which we see as a great circle,
double in one part, is really an immense spiral, and that we are
nearest one curve of it, the great southern division which looks so
bright. It may be that the spiral nebulae, vast though they are in
terms of earthly measurement, are tiny models of one tremendous spiral
which enfolds the universe with its coils.
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