EVOLUTION OF STARS—-ABBOT 183 
that these arms must separate into knots of starlike dimensions, which 
indeed agrees with the appearance of many spiral nebulae. 
Some of the arm-knots, being too massive for existence as single 
stars, would, as Jeans shows, form double or even quadruple star 
systems. This also is verified by observation. For among the stars 
of our galaxy at least one-third are multiple stars. His analysis 
seems to lead very satisfactorily toward the phenomena of stars as 
they have been discovered in the spiral nebulae and in our own 
galaxy, which may be taken as a sample of such a nebula. 
16. But what of the sun’s family, the planets and their moons? 
Are these also to be regarded as agglomerations of the arms of a 
small spiral nebula? Apparently not. Certain dynamical difficul- 
ties stand in the way of accepting the present arrangement of mass 
in the solar system as a result of such a process. 
Jeans adopts a similar device to that proposed by Professors 
Chamberlin and Moulton of the University of Chicago, who con- 
ceived that at some ancient epoch another star came so near to our 
sun as to raise upon it ropelike tides. The two stars separated 
again in their rapid flights before our sun had been divested of much 
of his mass. From the ropes of matter thrown off by this tidal en- 
counter were concentrated, it is conceived, the planets and their 
satellites. Some of the matter given off in the encounter remains un- 
gathered in the meteors and minor planets, and among the curiosi- 
ties of condensation are the beautiful rings of Saturn. 
17. Such are the present views of the evolution of galaxies, of 
multiple stars and of planetary families. Though they do not pre- 
tend to explain the original creation, they harmonize, far better than 
I can take note of in this brief account, great numbers of the phe- 
nomena known to mathematicians, chemists, physicists, and astrono- 
mers, and indicate a gradual progress in events which may well be 
called stellar evolution. Although its march is far too long to be 
followed in the span of human life, yet the heavens present so many 
cases of objects in every state of progress along the majestic course, 
that the summation of cases may be a satisfactory substitute for the 
continuity of action which in finite human life we can not perceive. 
18. There remain some branches of the subject yet to touch upon. 
One of them relates to the motions of the stars and nebulae. With 
the diligent employment of great telescopes and spectroscopes, the 
distances of several thousand stars have been determined, and their 
motions to or from the observer, as well as their angular motions 
across our line of sight have been measured. By combining all these 
data, we arrive at length to know the actual motions in space of 
great groups of stars relative to each other and to our sun. Even for 
the spiral nebulae these facts are observed. Stromberg has ar- 
ranged it all in a great diagram. 
