6 SEE— DETERMINATION OF THE [Januarys. 



of ullerculis, \vc have used the mean magnitude, because the type 

 of variable does not appear to be as yet well established. 



Here then we have 225 helium stars at an average distance of 

 about 540 light-years. For in Lick Observatory Bulletin No. 195, 

 p. 121, Campbell finds the 180 class B, or helium, stars to have an 

 average distance of 543 light-years, while in Publications of the 

 Astronomical Society of the Pacific for June-August, 191 1, p. 159, 

 Professor Curtis gives 534 light-years as the average distance of 

 312 helium stars. The former distance for 180 stars being greater 

 than the latter distance for 312 stars, we may take 540 light-years 

 as the distance of the 225 helium stars here under discussion, the 

 average magnitude of which is 4.14. 



If the average magnitude were decreased to 21.14, by removal 

 to 2,512 times their present distance, which would reduce the average 

 brightness by 17 magnitudes, the distance of the stars would be 

 multiplied by 2,512, and become 1,356,480 light years. This is for 

 the helium stars as they are, without any hypothesis as to bright- 

 ness, or as to the extinction of light in space, which will be con- 

 sidered later. 



The question will naturally be asked whether helium stars really 

 exist at these great distances. We may unhesitatingly affirm that 

 they do, because of the well-known whiteness of the small stars of 

 the Milky Way. It is true that Pickering has investigated the dis- 

 tribution of the helium stars in the Harvard Annals, \o\. 56, No. 

 II., and Campbell quotes these data in Lick Observatory Bulletin 

 No. 195 as showing that the helium stars are all bright objects. 

 Pickering believed his tabulations to indicate " that of the bright 

 stars, one out of four belongs to this class (B), while of the stars 

 of the sixth magnitude there is only one out of twenty; and that 

 few if any would be found fainter than the seventh or eighth magni- 

 tude." The implication here is that no helium stars exist at very 

 great distances corresponding to small magnitudes; but of course 

 such a" view is untenable. 



It probably is true that the group of helium stars at a distance 

 of some 540 light-years from our sun, and thus comparatively near 



