THE ORIGIN OF THE EARTH 421 



It had been held that the matter of a nebula, if formed of particles 

 revolving independently about their common center of gravity, 

 could not be gathered into planets without giving these a backward 

 rotation, at least normally; but the six inner planets have forward 

 rotations. All hypotheses of the planetesimal type appeared, 



Fig. 334. A spiral nebula in Canes Venatici, Messier 51. The exposure 

 was long and has given relative exaggeration to the fainter parts. The 

 nucleus is apparently dense and relatively massive; the coiling is pro- 

 nounced and rather symmetrical in the inner parts, but departs from 

 symmetry in the outer parts. A second nucleus is attached to the ex- 

 tremity of one arm. If this be interpreted as the representation of the 

 disturbing star, it should perhaps be regarded as made of colder, heavier 

 material, much less subject to expulsive elasticity, and hence less dis- 

 persed, and only slightly affected by rotating influences. A notable 

 feature is the comet-like streamers of some of the knots and denser 

 portions. If these are true streamers, curved by motion, they imply 

 an active rotation, and strengthen the similar inference drawn from the 

 coiled condition. The system is perhaps to be interpreted as young, 

 but as having advanced rapidly in its rotatory evolution because of its 

 massive nucleus. (Photo, by Ritchey, Yerkes Observatory.) 



therefore, to be shut out; but closer analysis showed that the 

 earlier conclusion was an error and thus a field, previously supposed 

 to be closed, was opened. It has also been shown by astronomic 

 photography that there are a multitude of nebulae, at least ten 

 times as many as were known a few years ago, and that in this 

 multitude, there is one dominant form, the spiral nebula (Fig. 334). 

 This fonn has a central nucleus, from which two arms or sets of arms 



