184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



cosmic rays are produced by nebulae in the early stages of their 

 existence, but not in the later stages such as our galaxy is in now. 



There is another reason why the cosmic rays are unlikely to be 

 produced by the stars in ordinary nebulae. This is the fact that the 

 outside layers of nearly all stars are rather similar. These outside 

 layers consist of a gaseous envelope at a temperature of, say, between 

 5,000 and 20,000 degrees centigrade. Now since we know that the 

 sun does not produce cosmic rays, it is difficult to see why other stars, 

 which have rather similar surface conditions, should do so. There 

 are exceptions to this general similarity of the outside of the stars. 

 The White Dwarfs are very small but very dense stars, and have 

 surface conditions which are very different from that of our sun, and 

 so might conceivably be considered possible origins for the rays. It 

 must be remembered that cosmic rays, although very penetrating 

 from our terrestrial standpoint in that they can penetrate some thou- 

 sand meters of water, cannot be considered penetrating from the 

 point of view of a stellar atmosphere. Any cosmic ray which was 

 produced inside a star would never get out. It is thus clear that it 

 is only from the outer layers of a star that the rays could come, and 

 these outer layers are nearly all alike. 



Many other possible explanations of the origin of the rays have 

 been given, but none put forward hitherto seem very plausible. It 

 is possible, of course, that the rays have their origin in electric fields 

 in extragalactic space, but there is no real reason to believe that such 

 fields exist, and it is difficult, perhaps, to explain on this basis the 

 isotropy of the rays. Then, Swann has suggested that they have 

 their origin in sunspots — not sunspots in the sun, of course, but 

 postulated spots in giant stars. 



Milne has predicted the existence of rays of very great energy from 

 his cosmological theory. This theory requires also the existence of 

 uncharged particles as well as electrically charged particles. 



Zwicky and Baade have sought the origin of the rays in the super- 

 novae, that once every few hundred years appear in every nebula 

 and grow to huge intensity for a period of a few weeks. Another 

 class of suggested origins are the archeological hypotheses. The 

 arguments in favor of these views are roughly as follows. It is 

 very hard to find an origin here and now in the universe for the cosmic 

 rays, so perhaps they were formed at the very beginning of time when 

 the world was quite young and, supposedly, very different. It is, 

 of course, obvious that if they have come from a very great distance 

 they must have their origin in the distant past. Lemaitre is an 

 exponent of one of these theories. He supposes that the rays have 

 their origin in some kind of superradioactive process from a single 

 primeval nucleus, from which has developed the universe as we know 

 it now; but, as there does not seem to be any possibility as yet of 



