164 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1916. 



we escape from the limitations imposed by the comparative meager- 

 ness of our knowledge of individual stellar distances. According 

 to Kapteyn's formulae, the vast majority of the stars are so far away 

 that it takes light thousands of years to come to the earth from them, 

 though light travels 186,000 miles per second. 



Eeturning now for a moment to the consideration of star motions, 

 w^e understand at once that, just as the mean proper motion of a 

 large group of stars corresponds to two-thirds of the average real 

 motion of these stars, so the mean radial motion of the group is 

 actually approximately two-thirds of the average real motion. 

 Director Campbell has in this way worked out the average real 

 motions of stars of different spectral classes,* and Prof. Boss also 

 has done the same, basing his result on the mean proper motions and 

 mean probable distances. Their results are in very close agreement. 

 Both find our sun to be moving a very little slower than the average 

 of all stars in their lists. 



When, however, the stellar motions are arranged by spectral 

 classes they find the B stars moving slower, other classes faster and 

 faster in a somewhat regular progression up to the M class stars. 

 Quite recently Adams has extended this investigation to fainter 

 stars. He finds these differences of speed between spectral classes 

 not so gi"eat as found by Boss and Campbell, and the average speed 

 of the fainter stars also less. It may be that the brighter stars, 

 being relatively near us, form a special group, not quite representa- 

 tive of all the stars in the universe. 



The greatest conception in regard to star grouping is that of 

 "star streaming," recently worked out by Kapteyn and by Edding- 

 ton, of the University of Cambridge, England. They find that 

 when the proper motions of the stars are cleared of the effects of 

 solar motion the remaining so-called " peculiar motions " of the in- 

 dividual stai's, while they go to some extent at random, plainly in- 

 dicate the governing influence of two great streams moving oppo- 

 sitely. If we could collect all the stars at one point and endow each 

 of them with its " peculiar motion " just as it has been observed, then 

 at the end of a century the stars would have stretched out, not into a 

 sphere but into an ellipsoid, owing to the influence of the two star- 

 streams. This grand phenomenon is attracting deep attention from 

 astronomers to-day, and will undoubtedly play a great part in future 

 studies. 



BRIGHTNESS AND VARIABILITY. 



Stars look hardly as bright as the fireflies of a summer night, but 

 in reality they glow like the sun, and seem faint only because far 

 away. Astronomers speak of " magnitudes " and of " absolute mag- 

 nitudes." The first gives the relative brightness of the stars as 



