66 NATIONAL ACADEMY OF SCIENCES 



Notwithstanding this, and this is the second remarkable fact, 

 the fact that bears directly on the question in hand we find not 

 a single helium star, neither in the Hyades nor in the Ursa Major 

 group. The stars in this group show a gradual change of spec- 

 trum with the brightness, but they certainly begin with the 

 second stage of the star's life. In the Pleiades, this is somewhat 

 different. Here there are some helium stars. But they begin 

 abruptly with the middle stage of helium stars. There is not a 

 single star of the early helium type in the Pleiades. So that 

 the series begins, not at the beginning, but at some different point 

 in the evolution. 



As to our second stream, then, we have to explain that there 

 is no second stream, or no appreciable number of helium stars in 

 the second stream. Our second stream, then, in which we meet 

 with no helium stars is absolutely in the same condition as the 

 group of the Hyades and the Ursa Major group; and the ex- 

 planation of the two cases must, in all likelihood, be the same. 



How, therefore, does it come to pass that in such groups as 

 those of the Hyades and the Ursa Major, the helium stars are 

 absolutely wanting? 



For those who, as I did in this lecture, adopt the view of the 

 order of evolution as helium, first, second, third, there can be 

 no question but that the stars which we now see are first type 

 stars, which must in past ages have been helium stars. 



Therefore, such a group as the Hyades, which nowadays does 

 not contain any helium stars, but which contains first type stars, 

 must in past ages have contained the helium stars in great num- 

 bers; and going back to times somewhat further still, these 

 helium stars must have been evolved from some primordial 

 matter. This primordial matter is possibly some nebulous 

 matter, and therefore, in the still earlier ages, these groups of 

 the Hyades and Ursa Major must have been filed with nebulae. 



Now, then, in the present day, so far as we know, there is no 

 trace of nebulas in these groups. Our conclusion, therefore, is 

 that there must have been some time in which the nebula in those 

 groups was exhausted. It was probably taken up in the forma- 



