OEIGIN OF THE SOLAR SYSTEM JEANS 149 



which the practical astronomer is very familiar. He finds that 

 a very large proportion — perhaps about one-half — of the stars in 

 the sky are binary, and he can detect an evolutionary sequence in 

 these binary stars. The sequence starts with the formation just 

 described in which the two constituent stars are almost in contact. 

 As it progresses the stars move ever farther and farther apart, while 

 the eccentricity of their orbits increases. Theory indicates that the 

 process of fission which has broken up the original star into two 

 constituents may repeat itself in either or both of these constituents, 

 so that the final product may be a " multiple " star of either three or 

 four constituents. Prof. H. N. Kussell, investigating this question 

 theoretically, found that certain numerical relations must hold be- 

 tween the relative distances of the various constituents of a mul- 

 tiple star; he also showed that the predictions of theory are con- 

 firmed quantitatively by observation. 



So far, then, theory and observation have gone hand in hand. 

 We have traced the evolution of astronomical matter through stages 

 of ever-increasing density, from the most tenuous of nebulae to the 

 densest of multiple stars, and at almost every stage observation has 

 confirmed the predictions of theory. Not all astronomical matter 

 will traverse the whole length of this evolutionary course. The 

 driving force on this course is increase of rotation consequent on the 

 shrinkage produced by emission of radiation. When the shrinkage 

 has proceeded a certain length solidification sets in ; the rotation can 

 increase now no further, and evolution, in the physical sense, stops. 

 The distance along the course to which any particular system pro- 

 ceeds depends in effect on the amount of rotation with which it was 

 originally endowed. Let a nebula begin its career with absolutely 

 no rotation and it will remain sperical in shape throughout its whole 

 career, ending merely as a cold non-radiating, but always spherical, 

 mass. Such a nebula never even gets away from the starting-post. 

 It is true that this is not a likely event, but for aught we Imow many 

 a nebula may freeze and die before reaching the critical configura- 

 tion (pi. 1, fig. 1) at which the birth of stars first commences. 

 Similarly many of the stars may become cold and so cease to develop 

 without ever attaining the stage at which binary systems are formed. 

 In the same way many binary systems must fail to develop into 

 multiple systems. Here again observation is with us; there are ten 

 times as many purely binary systems loiown as there are multiple 

 systems which have proceeded beyond the binary stage. Theory has 

 traced out for us the whole length of the evolutionary course, but 

 theory and observation agree that not many systems stay out the 

 whole course. 



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