DEVELOPMENTS IN ASTRONOMY PLASKETT. 269 



roughly globular cluster about 15° in diameter. These stars are all 

 moving with a velocity of about 40 kilometers (25 miles) per second 

 toward a point in the northeastern part of Orion, about 35° from the 

 center of the group. Their parallax, computed as before, is 0.025", 

 or 130 light years distant. Boss has calculated that in 65,000,000 

 years they will form a globular cluster about 21' in diameter and of 

 magnitudes 9 to 12. 



The most recent discoveries in star streams were given by Prof. 

 Kapteyn, at the solar union meeting, on Mount Wilson, last August. 

 He has found, by selecting from Boss's Catalogue all the stars of the 

 Orion type characterized by the appearance of helium lines in their 

 spectrum and so called since most of the stars in the constellation of 

 Orion are of this class, that in a large region of the sky they are 

 moving in nearly the same direction and at nearly the same rate. 

 This region contains the constellation Scorpio and Centaurus, cover- 

 ing 4,500 square degrees and extending roughly from 12 h to 18 h E. A. 

 to and from Dec. 0° to —60°. In another region of 1,300 square 

 degrees in Perseus from 2 h 50 m to 4 h 30 m E. A. and from +15° to 

 + 55° in Dec, all the stars of the same type are moving in a different 

 direction. When motion of the sun among the stars is allowed for, 

 Prof. Kapteyn finds that these apparent motions are equivalent to 

 streams moving in exactly opposite directions and at equal rates. 

 He finds these stars are very distant from the sun — from about 125 

 to 500 light years. 



It is evident that the sidereal universe is a complex structure 

 and having complex drifts and motions of stars and systems of stare 

 in its part. We may be able to get a further idea of the magnitude 

 of the problem by considering some of the recent results obtained 

 for stellar distances. We all know, of course, that the nearest fixed 

 star, a Centauri, is slightly over 4 light years distant, about 275,000 

 times the distance of the earth from the sun, 25 millions of millions of 

 miles. There has been a very marked advance in recent years in the 

 determination of the distances of the stars, so that we now know with 

 reasonable accuracy by direct methods the parallax or distance of 

 about 200 stars. There are several indirect methods, one of which 

 has been mentioned in connection with star streams, which give us 

 what may be called mean or average parallaxes of groups of stars. 

 I have not time to go into these methods, but it may suffice to give 

 a couple of tables indicating in a general way the average distances 

 of stars of different magnitudes and of different types. If we take 

 the blue stars, those of the second magnitude are on the average 100; 

 of the fourth, 200; of the sixth, 400; of the eighth, 800; of the tenth, 

 1,600; and so on, light years away, doubling for a change of 2 mag- 

 nitudes, while if we consider stars of different types we have from a 



