PROGKESS IN ASTROPHYSICS — ABBOT. 



191 



to which our sun belongs, an average velocity of 32 or more kilo- 

 meters per second, which is considerably greater than 19.5, which is 

 assigned to our sun. 



The Milky Way, composed as it is of a vast number of stars, has 

 long been a circle of reference in the heavens for the discussion of the 

 distribution of the stars. Not only are the individual stars crowded 

 more closely in the Milky Way than elsewhere, but the crowding is 

 different with different spectral types. Thus Prof. Pickering 

 pointed out in his discussion of Harvard Revised Photometry that the 

 stars of the early types, type B especially, were to be found prepon- 

 deratingly in the neighborhood of the Milky Way. This tendency 

 of the stars to distribute themselves differently with respect to the 

 Milky Way has been summarized by Prof. Boss in the following 

 tables, in which he gives the numbers of stars of different spectral 

 types to be found in zones at different distances from the center of 

 the Milky Way, and also the numbers of stars of the different types 

 which occur in equal areas in these zones, assuming for the zone -}-10 

 to —10° a number of 100. We see that the stars of the so-called 

 "later types" G K M are nearly uniformly distributed over the 

 heavens, but that the stars of the " early types," especially B, are 

 very unequally distributed, and crowd more and more toward the 

 Milky Way. 



ENUMERATION OF TYPES IN GALACTIC ZONES. 



RELATIVE AREAL DENSITIES IN PERCENTAGES. 



In several respects the stars of class B are very remarkable. Dr. 

 Campbell has stated that in a space concentric with the sun, which 

 must contain hundreds of stars of other spectral classes, there would 

 probably not be a single one of class B. Thus, B stars are, on the 

 whole, excessively remote. In the second place, they seem to be very 

 bright stars, for, as Prof. Pickering states, a count of the class B 



