150 STELLAE EVOLUTION. 



available were too crude to give aeciirate positions of the heavenly 

 bodies. Even Copernicus, though he estal)lished the sun at the center 

 of our system, and thus paved the way for the nel)uiar iiypothesis, 

 retained the epic^n-les of the Greeks. Kepler, ))asino- his investiga- 

 tions upon the observations of Tycho Brahe, proved that the planets 

 move in ellipses with the sun at the focus, and reuioved all vestige of 

 doul)t as to the general plan of the solar system. The harmony which 

 characterizes the motions of the planets and a knowledge of the ell'ect 

 of gravitation led Kant to formulate an explanation of the origin of 

 the solar system, which subsequently found more perfect expression 

 in the nebular hypothesis of Laplace. 



In this h3'pothesis Laplace seeks to account for the formation of 

 the sun and planets through the contraction of a vast nebulous cloud 

 which once tilled -the entire solar system, extending to the orbit of 

 Neptune. This mass, which he considered to be tiery hot, vyas sup- 

 posed to be in rotation. As it cooled, through radiation into space, it 

 contracted toward the center. The result of this contraction was to 

 increase the velocity of rotation, and when through increasing velocitj^ 

 the centrifugal force at the periphery counterbalanced the attraction 

 of the central mass a ring was thrown oil'. Further contraction 

 resulted in the formation of other rings, in each of which the mattin* 

 collected about its densest part, and thus produced a planet. Before 

 the}^ had time to cool these planets in turn threw oil' rings, which, 

 with the single exception of Saturn's ring system, condensed into 

 satellites. 



This celebrated hypothesis, though unsupported ])y mathematical 

 proof, has occupied n dominant position since the time of its pul)lica- 

 tion, more than a century ago. It has been subjected to much criti- 

 cism, but most of the objections raised by Faye and others have been 

 met by modifications of the hypothesis. C^f late it has encountered 

 fresh attacks on the part of Chaml)erlin and Moulton, and it now 

 seems doul)tful whether it will be possible to overcome their criticisms, 

 which ar(» ])ased on dynamical considerations. It may ]5rove to be 

 sufficient, however, to foi'sake the lenticular mass of va])()r predicated 

 by Laplace in favor of the spiral form which Kecder has shown to 

 characterize so many nebuhe. 



The nebular hypothesis seeks to account for a system like our own, 

 wherein a ccMitral sun is surrounded by planets and satellites, originally 

 self-luminous, l>ut ultimately cooled to the point where they are lumi- 

 nous only through reflected light. The stars are so distant from us 

 that an}' jdanets which may attend tlu^ni are beyond the reach of the 

 most powerful telescopes. In some of the planetary and spiral nebuhe, 

 such as the Great Nebula in Andromeda (PI. I), we perhaps observe 

 the earlier stages of the process of condensation, l)ut no distinct (>vi- 

 clence of progressive change has y(^t ])een gathei(>d from telescopic 



