THE GALAXY OR MILKY WAY
Grandest of all the problems that have occupied the mind of man is the
distribution of the stars throughout space. To the earliest astronomers
who knew nothing about the distances of the stars, it was not much of a
problem because they thought all the fixed stars were attached to a
revolving sphere, and therefore all at essentially the same distance; a
very moderate distance, too. Even Kepler held the idea that the
distances of individual stars from each other are much less than their
distances from our sun.
Thomas Wright, of Durham, England, seems to have been the first to
suggest the modern theory of the structure of the stellar universe,
about the middle of the eighteenth century. His idea was taken up by
Kant who elaborated it more fully. It is founded on the Galaxy, the
basal plane of stellar distribution, just as the ecliptic is the
fundamental circle of reference in the solar system.
What is the Galaxy or Milky Way?
Here is a great poet's view of the most poetic object in all nature:
A broad and ample road, whose dust is gold,
And pavement stars, as stars to thee appear
Seen in the Galaxy, that Milky Way
Which nightly as a circling zone thou seest
Powder'd with stars.
_Milton, P. L._ vii, 580.
Were the earth transparent as crystal, so that we could see downward
through it and outward in all directions to the celestial sphere, the
Galaxy or Milky Way would appear as a belt or zone of cloudlike
luminosity extending all the way round the heavens. As the horizon cuts
the celestial sphere in two, we see at anyone time only one-half of the
Milky Way, spanning the dome of the sky as a cloudlike arch.
As the general plane of the Galaxy makes a large angle with our equator,
the Milky Way is continually changing its angle with the horizon, so
that it rises at different elevations. One-half of the Milky Way will
always be below our horizon, and a small region of it lies so near the
south pole of the heavens that it can never be seen from medium northern
latitudes.
Galileo was the first to explain the fundamental mystery of this belt,
when he turned his telescope upon it and found that it was not a
continuous sheet of faint light, as it seemed to be, but was made up of
countless numbers of stars, individually too faint to be visible to the
naked eye, but whose vast number, taken in the aggregate, gave the
well-known effect which we see in the sky. In some regions, as Perseus,
the stars are more numerous than in others, and they are gathered in
close clusters. The larger the telescope we employ, the greater the
number of stars that are seen as we approach the Galaxy on either side;
and the farther we recede from the Galaxy and approach either of its
poles fewer and fewer stars are found. Indeed, if all the stars visible
in a 12-inch telescope could be conceived as blotted out, nearly all the
stars that are left would be found in the Galaxy itself.
The naked eye readily notes the variations in breadth and brightness of
the galactic zone. Nearly a third of it, from Scorpio to Cygnus, is
split into two divisions nearly parallel. In many regions its light is
interrupted, especially in Centaurus, where a dark starless region
exists, known as the "coal sack." Sir John Herschel, who followed up the
stellar researches of his father, Sir William, in great detail, places
the north pole of the Galactic plane in declination 37 degrees N., and
right ascension 12 h. 47 m. This makes the plane of the Milky Way lie at
an angle of about 60 degrees with the ecliptic, which it intersects not
far from the solstices.
Now Kant, in view of the two great facts about the Galaxy known in his
time, (1) that it wholly encircles the heavens, and (2) that it is
composed of countless stars too faint to be individually visible to the
naked eye, drew the safe conclusions that the system of the stars must
extend much farther in the direction of the Milky Way than in other
directions.
This theory of Kant was next investigated from an observational
standpoint by Sir William Herschel, the ultimate goal of whose
researches was always a knowledge of the construction of the heavens.
The present conclusion is that we may regard the stellar bodies of the
sidereal universe as scattered, without much regard to uniformity,
throughout a vast space having in general the shape of a thick watch,
its thickness being perhaps one-tenth its diameter. On both sides of
this disk of stars, and clustered about the poles of the sidereal system
are the regions occupied by vast numbers of nebulae. The entire visible
universe, then, would be spheroidal in general shape. The plane of the
Milky Way passes through the middle of this aggregation of stars and
nebulae, and the solar system is near the center of the Milky Way.
Throughout the watch-form space the stars are clustered irregularly, in
varied and sometimes fantastic forms, but without approach to order or
system. If we except some of the star groups and star clusters and
consider only the naked-eye stars, we find them scattered with fair
approach to uniformity.
[Illustration: STAR CLOUDS AND BLACK HOLES IN SAGITTARIUS. The dark
rifts and lanes resemble those in the nearby Milky Way. (_Photo,
Yerkes Observatory._)]
[Illustration: THE GREAT NEBULA OF ANDROMEDA, LARGEST (APPARENTLY)
OF ALL THE SPIRAL NEBULAE. This nebula can be seen very faintly
with the naked eye, but no telescope has yet resolved it into
separate stars. (_Photo, Yerkes Observatory._)]
The watch-shaped disk is not to be understood as representing the actual
form of the stellar system, but only in general the limits within which
it is for the most part contained.
A vigorous attack on the problem of the evolution and structure of the
stellar universe as a whole is now being conducted by cooperation of
many observatories in both hemispheres. It is known as the Kapteyn "Plan
of Selected Areas," embracing 206 regions which are distributed
regularly over the entire sky. Besides this a special plan includes
forty-six additional regions, either very rich or extremely poor in
stars, or to which other interest attaches.
Of all investigators Kapteyn has gone into the question of our precise
location in the Milky Way most thoroughly, concluding that the solar
system lies, not at the center in the exact plane, but somewhat to the
north of the Galaxy. Discussing the Sirian stars he finds that if stars
of equal brightness are compared, the Sirians average nearly three times
more distance from the sun than those of the solar type. So, probably,
the Sirians far exceed the Solars in intrinsic brightness. Farther,
Kapteyn concludes that the Galaxy has no connection with our solar
system, and is composed of a vast encircling annulus or ring of stars,
far exceeding in number the stars of the great central solar cluster,
and everywhere exceedingly remote from these stars, as well as differing
from them in physical type and constitution. So it would be mainly the
mere element of distance that makes them appear so faint and crowded
thickly together into that gauzy girdle which we call the Galaxy.
The Milky Way reveals irregularities of stellar density and star
clustering on a large scale, with deep rifts between great clouds of
stars. Modern photographs, particularly those of Barnard in Sagittarius,
make this very apparent. Within the Milky Way, nearly in its plane and
almost central, is what Eddington terms the inner stellar system, near
the center of which is the sun. Surrounding it and near its plane are
the masses of star clouds which make up the Milky Way. Whether these
star clouds are isolated from the inner system or continuous with it, is
not yet ascertained.
The vast masses of the Milky Way stars are very faint, and we know
nothing yet as to their proper motions, their radial motions, or their
spectra. Probably a few stars as bright as the sixth magnitude are
actually located in the midst of the Milky Way clusters, the fainter
ninth magnitude stars certainly begin the Milky Way proper, while the
stars of the twelfth or thirteenth magnitude carry us into the very
depths of the Galaxy.
It is now pretty generally believed that many of the dark regions of the
Milky Way are due not to actual absence of stars so much as to the
absorption of light by intervening tracts of nebulous matter on the
hither side of the Galactic aggregations and, probably in fact, within
the oblate inner stellar system itself. Easton has made many hundred
counts of stars in galactic regions of Cygnus and Aquila where the range
of intensity of the light is very marked; in fact, the star density of
the bright patches of the Galaxy is so far in excess of the density
adjacent and just outside the Milky Way, that the conclusion is
inevitable that this excess is due to the star clouds.
Of the distance of the Milky Way we have very little knowledge. It is
certainly not less than 1,000 parsecs, and more likely 5,000 parsecs, a
distance over which light would travel in about 16,000 years. Quite
certainly all parts of the Galaxy are not at the same distance, and
probably there are branches in some regions that lie behind one another.
While the general regions of the nebulae are remote from the Galactic
plane, the large irregular nebulae, as the Trifid, the Keyhole, and the
Omega nebulae, are found chiefly in the Milky Way.
In addition to the irregular nebulae many types of stellar objects appear
to be strongly condensed toward the Milky Way, but this may be due to
the inner stellar system, rather than a real relation to Galactic
formations. Quite different are the Magellanic clouds, which contain
many gaseous nebulae and are unique objects of the sky, having no
resemblance to the true spiral nebulae which, as a rule, avoid the
Galactic regions. Worthy of note also is the theory of Easton that the
Milky Way has itself the form of a double-branched spiral, which
explains the visible features quite well, but is incapable of either
disproof or verification. The central nucleus he locates in the rich
Galactic region of Cygnus, with the sun well outside the nucleus itself.
By combining the available photographs of the Galaxy, he has produced a
chart which indicates in a general way how the stellar aggregations
might all be arrayed so as to give the effect of the Galaxy as we see
it.
Shapley, at Mount Wilson, has studied the structure of the Galactic
system, in which he has been aided by Mrs. Shapley. An interesting part
of this work relates to the distribution of the spiral nebulae, and to
certain properties of their systematic recessional motion, suggesting
that the entire Galactic system may be rapidly moving through space.
Apparently the spiral nebulae are not distant stellar organizations or
"island universes," but truly nebular structures of vast volume which in
general are actively repelled from stellar systems. A tentative
cosmogonic hypothesis has been formulated to account for the motions,
distribution, and observed structure of clusters and spiral nebulae.
An additional great problem of the Galaxy is a purely dynamical one.
Doubtless it is in some sort of equilibrium, according to Eddington,
that is to say, the individual stars do not oscillate to and fro across
the stellar system in a period of 300 million years, but remain
concentrated in clusters as at present. Poincare has considered the
entire Milky Way as in stately rotation, and on the assumption that the
total mass of the inner stellar system is 1,000,000,000 times the sun's
mass, and that the distance of the Milky Way is 2,000 parsecs, the
angular velocity for equilibrium comes out 0".5 per century. That is to
say, a complete revolution would take place in about 250 million years.