CONCERNING THE ORIGIN OF SPIRAL NEBULÆ
I think it is very doubtful whether the spiral nebulæ have in general
been formed by the close approaches of pairs of stars, as the authors
have postulated for the assumed solar spiral.[2]
The distribution of the
spirals seems to me to negative the idea. To witness the close approach
of two stars we must look in the direction where the stars are. To the
best of present-day knowledge the stars are in a spheroid whose longer
axes are coincident with the plane of the Milky Way. If this is so,
the close approach of pairs of stars should occur preeminently in the
Milky Way, and we should find the spirals prevailingly in and near the
Milky Way. This is precisely where we do not find them. In fact,
they seem to abhor the Milky Way. The new stars, which are credibly
explained as the products of collisions of stars with nebulæ, are found
preeminently in the Milky Way and almost negligibly in the regions
outside of the Milky Way. Again, the spirals are believed to be, on
the whole, of enormous size. They are too far away to let us measure
their distances by the usual methods, and they move too slowly on the
surface of the sphere to have let us determine their proper motions.
Slipher's recent work with a spectrograph seems to show that the dozen
spirals observed by him are moving with high speeds of approach and
recession; from 300 km. per second approach in the case of the Andromeda
nebula to 1,100 km. per second recession in the case of several
objects. If the spirals are moving at random their speeds at right
angles to the line of sight must be even greater than their speeds of
approach and recession. Unless they are very distant bodies their proper
motions should be detected by observations extending over only a few
years. My colleague Curtis has this year compared recent photographs
of some 25 spirals with photographs of the same object made by Keeler
fifteen years ago. They reveal no appreciable proper motions, or rotations.
In this same interval Neptune has revolved more than 30°.
Slipher has recently measured the rotational speed of one "spindle''
nebula, believed to be a spiral. He finds it to be enormously rapid; no
motions in the solar system approach it in magnitude. The evidence
is to the effect that the spirals are in general very far
away;
[3] perhaps on or beyond the confines of our
stellar system, but not certainly so.
Accordingly, we are led to believe that the spirals studied thus far have
diameters 20 times or 100 times, or in some cases several thousand times,
the diameter of our solar system. It is difficult to avoid the conclusion
that in general they are immensely more massive than is our solar
system. The spiral which has been assumed as the forerunner of our
system must have been of diminutive size as compared with the larger
and brighter spirals which we see to-day.
We are sadly in need of information concerning the constitution of
the spiral nebulæ. Their spectra appear to be prevailingly of the solar
type, except that a very small proportion contain some bright lines in
addition to the continuous spectrum. So far as their spectra are concerned,
they may be great clusters of stars, or they may consist each of
a central star sending its light out upon surrounding dark materials
and thus rendering these materials visible to us. The first alternative
is unsatisfactory, for all parts of spirals have hazy borders, as if the
structure is nebulous or consists of irregular groups of small masses;
and the second alternative is unsatisfactory, for in many spirals the
most outlying masses seem to be as bright as masses of the same areas
situated only one half as far from the center, whereas in general the
inner area should be at least four times as bright as the outer area.
All astronomers are ready to confess that we do not know much about
the conditions existing in spiral nebulæ.
