AN EVOLVING UNIVERSE JEANS 235 



the picture. In two other spiral nebulae, M. 81 and M. 101, the evolu- 

 tion has proceeded still further, so much so that in the last of these 

 there is very little nucleus left, and by far the greater part of what 

 we see is what we believe to be ejected matter forming the spiral 

 arms. In these last nebulae, we can see that the spiral arms proceed 

 from two antipodal points, exactly as required by dynamical theory. 



Yet this does not quite end the story, since the arms spread further 

 into space than we should expect if rotation alone were responsible 

 for their spreading. There must be other factors at work, and these 

 we do not yet understand ; the s^^iral formation of the nebular arms 

 remains a mystery. It seems possible that the theory of relativity 

 may ex]Dlain it all to us in time, but it has not done so yet. 



Gas set free out of an ordinary nozzle into a vacuum would immedi- 

 ately spread into the whole of the space accessible to it. Why then does 

 not the jet of gas shot oflF from the equator of the nebula do the same? 



The explanation is to be found in the gigantic scale on which this 

 latter process takes place. As we increase the scale of the phenom- 

 enon, the mutual gravitational attraction of the particles of gas be- 

 comes of ever greater importance until finally, when we come to very 

 large-scale phenomena (but before nebular dimensions are reached), 

 gravitation overcomes the expansive influence of gas pressure and 

 holds the jet together as a compact stream. 



But dynamical theory predicts that when this happens, a further 

 phenomenon ought also to appear. The influence of gas-pressure is 

 in the direction of keeping the density spread out uniformly along 

 the filament, while that of gravitation is towards making the stream 

 condense into compact globules. When nebular dimensions are 

 reached the latter tendency prevails, so that the jet of ejected matter 

 breaks up into drops, much as a jet of water issuing from a nozzle 

 does, although for a very different physical reason. In the photo- 

 graphs reproduced of N. G. C. 891, M. 51, M. 101, and M. 81 we can 

 trace this process going on. 



The nebula shown in N. G. C. 891 exhibits a lumpy or granulated 

 appearance in its outer regions. In M. 51 this takes the form of 

 pronounced condensations, and in the outer regions of M. 101 and 

 M. 81 these condensations have further developed into detached and 

 almost starlike points of light; indeed many of these are known to 

 be stars or groups of stars. 



Dynamical theory not only predicts that these globules of gas must 

 form, but can also predict their sizes and masses. The calculation 

 of the masses leads to an extremely interesting and significant result ; 

 the calculated mass of a single condensation proves to be approxi- 

 mately equal to the mass of the average star. 



This provides an excellent confirmation of our theory, and gives. 

 I believe, the key to the evolutionary process we have been consider- 

 ing—we have been watching the creation of the stars. 



