44 



GEOLOGY 



Fig. 12. — A very large, highly dispersed nebula, Messier 33, in the Triangle, in which 

 the knots are very pronounced and the nebulous haze is irregular and flocculent. 

 It is interpreted as young, because the arms are not greatly coiled and the nebu- 

 lous haze has not become generally distributed. A succession of expulsive im- 

 pulses seems to be indicated. Judged by the luminosity, the amount of matter 

 in the nucleus is relatively small, and so the difference in the motions of the outer 

 and inner parts is much less than in a system of the solar type, and the evo- 

 lution is very slow. The distance is unknown, and hence the dimensions are un- 

 known, but they are probably much greater than those of the solar system. There 

 is much uncertainty in judging of the mass by the luminosity, for some of the 

 matter may have little luminosity and some may not be luminous at all. The 

 central and denser portions are likely to be relatively more luminous than the 

 dispersed portions. (Photo, by Ritchey, Yerkes Observatory.) 



