PEOGRESS IN ASTROPHYSICS — ABBOT. 193 



present positions. Tliey may have originated from comparatively few great 

 separate collections of matter in or near the plane of the Milky Way. The 

 variety of motions which we observe in the stars in one of these apparent 

 groups might, perhaps, have originated from the influence of the passing of 

 many individual stars through the immense volume of space occupied by the 

 group. The absence of class B stars in our vicinity may indicate primeval 

 vacancy in this region, or the developmeiit of the stars in this region to an 

 effective age beyond that corresponding to the class B spectrum. 



Kapteyn has said : 



As the younger the stars are the smaller are their internal motions, it follows 

 at on'^ that from whatever matter our youngest stars — the helium stars* — 

 may ha j been evolved, that matter must have in all probability still smaller 

 internal motion. Let us call this matter primordial matter. As the internal 

 velocity of the helium stars is already so very small, we come to the conclu- 

 sion that primordial matter must practically have hardly any other motion 

 than the motion of the cloud to which it belongs. 



The statements quoted above, and many others which might be 

 quoted from astronomical literature, lead us to the conclusion that 

 their writers assume the following evolution of the universe, begin- 

 ning from the nebula, and proceeding with passing time to the stages 

 of the classes B, A, F, G, K, M in spectra. Originally the matter 

 had very low velocity in space, and as the stars were formed and 

 grew in age their velocity became greater and greater. Whatever 

 the drift which the original primordial matter may have had, the 

 formation of the stars and the gravitation which they mutually 

 exert, together with their increasing velocity in space, tended to 

 alter the motions of the stars from a slow drift in some particular 

 direction to a much more rapid progress of individual stars in every 

 conceivable direction. This motion naturally took the stars of the 

 later types farther and farther from the orjginal seat of the primor- 

 dial matter, so that now, although we find the class B stars still 

 mainly confined to the neighborhood of the Millcy Way, yet for 

 other types of stars the dispersion has gone farther and farther. 

 For stars similar in constitution to our sun, and naturally of the 

 same order of age as the sun, the circumstances of the wandering 

 have naturally been much the same, so that we find the stars of ap- 

 proximately the spectral class of our sun to be, on the whole, in the 

 less remote parts of space. When, however, we consider the stars 

 of most advanced type, of spectral class M, whose wanderings have 

 continued for the most untold ages, we find these stars as a class in 

 the more remote parts of the universe. 



Although this speculation is supported by a good many facts of 

 observation yet it is only fair to state that there are astronomers 

 of very high eminence who consider either that the time is not ripe 

 for such speculations, or that the evidence may equally well be 



1 Class B. 

 44863°~SM 1913 -13 



