THE SEMI-CENTENNIAL ANNIVERSARY 6 1 



the determination of the radial velocity with the spectroscope the 

 faintness of the object is a great obstacle. The consequence is 

 that, as yet, we know the radial velocity of only fourteen of these 

 objects in all. Still, this limited number is decisive in showing 

 that there can be no question that the real motions of these objects 

 are not even approximately parallel to the motion of the helium 

 stars, or even parallel to any fixed direction whatever. Their 

 velocity, moreover, is exceedingly unequal. Therefore, must we 

 conclude that the nebula? are not the birthplace of the stars? It 

 may seem so. 



Meanwhile let us not go too far. There are nebulae and 

 nebulae, and it so happens — and there is ample practical reason 

 for it — that with one exception observation of radial velocity 

 has, up to the present time, been confined to what we call the 

 planetary nebulae — nebulae elliptical or round, which show an 

 appearance remotely like that of a planetary disc. Herschel saw 

 in them a likeness to what, according to Laplace's cosmogony, 

 must have been the primitive stage of our own planetary system; 

 and therefore he thought that these planetary nebulae must be the 

 birthplace of the stars. 



Now then, according to what precedes, this view seems now 

 untenable. If they were, they would have shown the parallel and 

 equal motion of practically all the helium stars. The motions on 

 the contrary, are extremely unparallel, and unequal, and we must 

 assign these objects a place at the end of the order of evolution 

 rather than at the beginning. 



We may, perhaps, see an independent confirmation of this 

 view in the stars called temporary stars, but time will not permit 

 me to pursue that argument further. 



Meanwhile, as I said just now, there is one exception in which 

 the radial velocity of the nebulae has been determined, which is 

 not planetary. This exception is the Orion nebula, the well 

 known Orion nebula, which is classified under the irregular 

 nebulae. Therefore, we might ask if its position with the irreg- 

 ular nebulae was the birthplace of the stars. 



