T9I2.] 



AND OTHER CHARACTERISTICS OF STARS. 



575 



TABLE HI. 

 Stars Belonging to the Pleiades. 



Hypothetical 

 Parallax. 



o .0091 



0.0050 



0.0055 



0.0052 

 (0.0066) 



The fifth column in the table gives the mean absolute magni- 

 tudes previously found for stars, of similar spectral type in other 

 clusters (choosing the brighter half of those of class F, and a few 

 of the brightest stars of class G, since it is evident that the limita- 

 tion to stars above a given magnitude compels a similar choice in 

 the Pleiades). From the differences between the observed and 

 absolute magnitudes, we may compute the distances to which a 

 group of stars similar to those already studied must be removed in 

 order to appear equal in average brightness to the stars of the same 

 spectral class in the Pleiades. The hypothetical parallaxes so ob- 

 tained are given in the last column of the table. With the exception 

 of that derived from class B, they are in extraordinary agreement. 

 If they are treated as independent determinations of the parallax, 

 of equal weight, the resulting mean is O."oo63 ± o".ooo6, corre- 

 sponding to a distance of 500 light-years. 



This estimate of the distance of the Pleiades depends upon the 

 assumption that, when we find in this cluster the same relation be- 

 tween the relative brightness of the stars of different spectral classes 

 that exists elsewhere, wherever the real brightness of the stars can 

 be investigated, the absolute brightness for each spectral class is 

 also approximately the same as elsewhere. This assumption is made 

 decidedly probable by the fact that it undoubtedly holds true for the 

 stars of the four clusters whose distances are known, and for more 

 than 100 other stars not belonging to clusters, with no serious 

 exceptions. It should however be remembered that no account has 

 been taken of possible absorption of light in space, and that there 



