266 



SCIENCE. 



[N. S. Vol. XXII. No. 557, 



from gas. Spots of intenser light are ob- 

 served in less brilliant regions ; clusters of 

 stars are sometimes imbedded in nebulos- 

 ity, while in other cases each individual 

 star of a cluster stands out clear by itself. 

 These and other observations force on us 

 the conviction that the wispy clouds repre- 

 sent the earliest stage of development, the 

 more condensed nebulae a later stage, and 

 the stars themselves the last stage. This 

 view is in agreement with the nebular hy- 

 pothesis of Laplace, and we may fairly con- 

 jecture that chains and lines of stars repre- 

 sent preexisting streaks of nebulosity. 



Change is obviously in progress every- 

 where, as well in each individual nebula 

 and star as in the positions of these bodies 

 relatively to one another. But we are un- 

 able even to form conjectures as to the 

 tendency of the evolution which is going 

 on. This being so, we can not expect, by 

 considering the distribution of stars and 

 nebulfe, to find many illustrations of the 

 general laws of evolution which I have at- 

 tempted to explain; accordingly I must 

 confine myself to the few cases where we at 

 least fancy ourselves able to form ideas as 

 to the stages by which the present condi- 

 tions have been reached. 



Up to a few years ago there was no evi- 

 dence that the law of gravitation extended 

 to the stars, and even now there is nothing 

 to prove the transmission of gravity from 

 star to star. But in the neighborhood of 

 many stars the existence of gravity is now 

 as clearly demonstrated as^ within the solar 

 system itself. The telescope has disclosed 

 the double character of a large number of 

 stars, and the relative motion of the pairs 

 of companions has been observed with the 

 same assiduity as that of the planets. 

 When the relative orbit of a pair of binary 

 or double stars is examined, it is found 

 that the motion conforms exactly to those 

 laws of Kepler which prove that the 

 planets circle round the sun under the ac- 



tion of solar gravitation. A leading char- 

 acteristic of all these double stars is that 

 the two companions do not differ enor- 

 mously in mass from one another. In this 

 respect these systems present a strongly 

 marked contrast with that of the sun, at- 

 tended as it is by relatively insignificant 

 planets. 



In the earlier part of my address I 

 showed how theory indicates that a rotating 

 fluid body will as it cools separate into two 

 detached masses. Mathematicians have 

 not yet been able to carry their analysis 

 far enough to determine the relative magni- 

 tudes of the two parts, but as far as we 

 can see the results point to the birth of a 

 satellite whose mass is a considerable frac- 

 tion of that of its parent. Accordingly See 

 (who devotes his attention largely to the 

 astronomy of double stars), Alexander 

 Roberts and others consider that what they 

 have observed in the heavens is in agree- 

 ment with the indications of theory. It 

 thus appears that there is reason to hold 

 that double stars have been generated by 

 the division of primitive and more diffused 

 single stars. 



But if this theory is correct we should 

 expect the orbit of a double star to be ap- 

 proximately circular; yet this is so far 

 from being the case that the eccentricity 

 of the orbits of many double stars exceeds 

 by far any of the eccentricities in the solar 

 system. Now See has pointed out that 

 when two bodies of not very unequal 

 masses revolve round one another in close 

 proximity the conditions are such as to 

 make tidal friction as efificient as possible 

 in transforming the orbit. Hence we seem 

 to see in tidal friction a cause which may 

 have sufficed not only to separate the two 

 component stars from one another, but 

 also to render the orbit eccentric. 



I have thought it best to deal very briefly 

 with stellar astronomy in spite of the im- 

 portance of the subject, because the direc- 



