574 RuSSELL— RELATIONS BETWEEN SPECTRA [April 20, 



Here again the stars whose parallaxes have been directly meas- 

 ured have been selected on account of their apparent brightness, and 

 are probably brighter than the average of all the giant stars. Indi- 

 vidual stars are in some cases still brighter; for example, Antares, 

 which is clearly shown by its proper-motion and radial velocity to 

 belong to the moving cluster in Scorpius, with a parallax of about 

 o".oio, and hence must be fully 2,500 times as bright as the sun. 

 Canopus and Rigel, whose parallaxes are too small to measure, are 

 probably equally bright or brighter. Whether there are many more 

 stars of such enormous luminosity, and, in general, whether the 

 giant stars of a given spectral class resemble one another in bright- 

 ness as closely as the dwarf stars do, cannot be determined from 

 existing data, at least of the kind considered here. 



The giant and dwarf .stars are fully separated only among the 

 spectral classes which follow the solar type in the Harvard classi- 

 fication. For class A the two series are intermingled, and even for 

 class F, where the average brightness of the two dififers by four 

 magnitudes, it would be difficult to say whether a star of absolute 

 magnitude near i.o should be regarded as an unusually faint giant 

 star or an unusually bright dwarf. From class G onward, the 

 reality of the separation into two groups is unequivocally indicated 

 by the observational data. 



As a practical application of the principles just developed, we 

 may consider the question of the distance of the Pleiades, a prob- 

 lem so far practically imsolved. 



The spectra of the fainter stars which are known to belong to 

 the cluster have been determined at Harvard, through the kindness 

 of Professor Pickering and Miss Cannon. They exhibit a very con- 

 spicuous relation between apparent magnitude and spectral type, as 

 is shown in the first four columns of Table HI. 



These stars evidently belong to the series of dwarf stars. The 

 relative brightness of the different spectral classes is in good agree- 

 ment with that previously found, except that the stars of class B5 

 in the Pleiades appear to exceed those of class A in brightness as 

 much as those of class Bo to B3 do among the stars previously 

 studied. 



