1921.] 
Astronomical Notes. 
135 
ASTRONOMICAL NOTES. 
The Spiral Nebulae. 
By A. C. Gifford. 
In his monograph on Stellar Movements and the Structure of the Universe, 
page 244, Professor A. S. Eddington says :— 
“ The two arms of the spiral have an interesting meaning for us in 
connection with stellar movements. The form of the arms—a logarithmic 
spiral—has not as yet given any clue to the dynamics of spiral nebulae. But, 
though we do not understand the cause,- we see that there is a widespread 
law compelling matter to flow in these forms. It is clear, too, that either 
matter is flowing into the nucleus from the spiral branches or it is flowing out 
from the nucleus into the branches. It does not at present concern us in 
winch direction the evolution is proceeding. In either case we have currents 
of matter in opposite directions at the points where the arms merge in the 
central aggregation. These currents must continue through the centre, 
for, as will be shown in the next chapter, the stars will not interfere with 
one another’s paths. Here, then, we have an explanation of the prevalence 
of motions to and fro in a particular straight line ; it is the line from which 
the spiral branches start out. The two star streams and the double- 
branched spirals arise from the same cause.” 
The difficulty which Eddington feels in explaining the spiral form 
appears to be due to the two assumptions which he makes in the above 
statement of the case. The first of these is that the motion of the stars 
composing the nebula is along the spiral arms in one direction or the 
other ; the second that the line of relative motion of the two streams 
through the nucleus is the line from which the spiral branches start. The 
truth of the first of these assumptions is extremely unlikely, as it would 
require the action of a different law of force, the inverse-cube law ; and 
the assumption is unnecessary, as the spiral form can be more simply 
explained in terms of gravitation alone. If the explanation here suggested 
is accepted we shall find that the motion of the individual stars, instead 
of being along the spiral arms, is initially approximately perpendicular 
to them, and that the line of relative motion within the nucleus is initially 
at right angles to the line joining the points from which the spiral branches 
start. 
The suggested explanation is that the spirals owe their form as well 
as their visibility to the encounter and interpenetration of cosmic systems, 
and the persistence of angular momentum in the resulting system, in virtue 
of which the outer stars apparently lag behind whilst the inner ones forge 
ahead. 
The evidence afforded by recent observations tends to prove more 
and more completely that the spiral nebulae are distant cosmic systems far 
beyond the Milky Way, and that the latter is itself just that particular 
spiral in which our solar system is situated. The reasoning leading to this 
conclusion is admirably summed up by H. D. Curtis in his lecture on the 
nebulae ( Adolfo Stahl Lectures), p. 108, and further evidence is given in 
the Lick Observatory Bulletins, Nos. 297 and 300. 
