THE SEMI-CENTENNIAL ANNIVERSARY 65 



It must be confessed, however, that in this theory also there 

 remain some hard nuts to crack; and until we succeed in this 

 it seems unsafe to claim any great certainty for the theory, and 

 it seems preferable to put it forward as one hypothesis, which, 

 for the time being, best fits the observed facts. 



There remains to be considered the question how to explain 

 that the second stream or cloud hardly contains any helium stars. 

 There is something in the small local groups which we know in 

 this type which may help us. Everybody knows the group of 

 the Pleiades. 



There can be no doubt but that the bright stars and many of the 

 faint stars that we see in this part of the sky, are near together in 

 space and not merely near together in the same visual line the 

 one far behind the other. They undoubtedly form a physical 

 system, and must have had a common origin. At present we 

 know several of such local groups, among them the Hyades, the 

 Ursa Major group, and we may perhaps add the great Scorpius 

 Centaur group. 



Now, in these local groups, we find, amongst others, two very 

 remarkable facts. The first is that if the stars of such groups 

 are arranged in the order of their brightness, we find that they 

 are arranged, with a few, and it must be confessed, significant 

 exceptions, in the order of the spectral classes. As an instance, 

 take the Scorpius Centaur group. We find that the very 

 brightest stars are of the earliest helium type; the somewhat 

 fainter ones are of the next helium type; the next fainter ones 

 are of the next stage in the stellar life, or the first type; and if 

 we can not follow the series further on to the second and perhaps 

 the third type, this is probably due to our ignorance about it or 

 lack of knowledge of the fainter stars belonging to the group. 



In the Pleiades, where we have a somewhat more extensive 

 knowledge of the fainter stars, we can follow the series at least 

 as far as the middle of the second type stars. It follows from 

 this that in all these groups, what there are of helium stars 

 can not be overlooked, for they all are of the very brightest stars, 

 and our knowledge of the brightest stars is pretty complete. 



