CURTIS: MODERN THEORIES OF SPIRAL NEBULAE 221 



5. The space-velocities of the various classes of celestial ob- 

 jects are summarized in the following short table: 



TABLE 

 The Factors of Space- Velocity 



1 . The Diffuse Nebtdae. 



Velocities low. 



2 . The Stars. 



Velocities vary with spectral type. 

 Class B Stars: average speeds 8 miles per second. 

 Class A Stars: average speeds 14 miles per second. 

 Class F Stars: average speeds 18 miles per second. 

 Class G Stars: average speeds 19 miles per second. 

 Class K Stars: average speeds 21 miles per second. 

 Class M Stars : average speeds 2 1 miles per second. 



3 . The Star Clusters. 



Velocities unknown. 



4 . The Planetary Nebulae. 



Average speeds 48 miles per second. 

 5 . The Spiral Nebulae. 



Average speeds 480 miles per second. 



The peculiar variation of the space- velocity of the stars with 

 spectral type may ultimately prove to be a function of relative 

 mass. The radial velocities of but few spirals have been de- 

 termined to date; future work may change the value given, 

 but it seems certain that it will remain very high. 



It will be seen at once that, with regard to this iiriportant 

 criterion of space-velocity, the spiral nebulae are very distinctly 

 in a class apart. It seems impossible to place them at any point 

 in a coherent scheme of stellar evolution. We can not bridge 

 the gap involved in postulating bodies of such enormous space 

 velocities either as a point of stellar origin, or as a final evolu- 

 tion product. 



On the older theory that the spirals are a part of oiu: own 

 galaxy, it is impossible to harmonize certain features of the 

 data thus far presented. If this theory is true, their grouping 

 near the galactic poles, inasmuch as all evidence points to a 

 flattened or disk form for our galaxy, would indicate that they 

 are relatively close to us. In that event, we should inevitably 

 have detected in this class of objects proper motions of the same 

 order of magnitude as those found for the stars at correspond- 

 ing distances. Such proper motions are the more to be ex- 



