130 Dr. Frank Watson Dyson [April 2-t, 



times as distant as the sun. At this great distance the sun would 

 appear as of magnitude 15-5, but these stars vary in magnitude from 

 9 • 5 to 1 1 • on the Potsdam scale, and are therefore intrinsically 

 from 250 to 6o times as bright as the sun. Now it happens that 

 among the stars nearest to the sun, whose distances have been actually 

 measured, there are several red stars, and these are all very much 

 fainter than the sun. It has been suggested by Prof. Russell and 

 Prof. Hertzsprung independently that the red stars are of two dis- 

 tinct classes, which they call the giants and the dwarfs, and that in 

 accordance with Sir Norman Lockyer's views the giant red stars are 

 in an early stage of evolution, and are increasing in temperature, 

 while the dwarf stars are at the other end of the series and are grow- 

 ing colder and darker. 



Leaving the red stars, it is seen that the stars whose colour 

 indexes lie between - 1 and + 4 are nearer to us than the groups on 

 either side of them. These stars are those whose spectra are of the 

 types F and G in the Harvard notation, and are the stars most like 

 the sun. The mean distances of these stars is only 130 parsecs for 

 the stars brighter than 9" '5, and 2:^0 parsecs for the stars fainter 

 than 9"" "5. At this distance the sun would be of magnitude 12-1. 

 It follows that these stars are on the average from eight times to twice 

 as bright as the sun. The A-F stars are a little, but not much 

 farther away, the stars fainter than ;)™'5 being at an average 

 distance of 263 parsecs. At this distance the sun would have a 

 magnitude 12 '5, and these stars are from sixteen to four times as 

 luminous as the sun. 



21. We have seen how the knowledge that the solar system is 

 moving in a known direction with a velocity of 19 '5 kil/sec. leads to 

 a determination of the distances of groups of stars whose angular 

 movements are known. The hypothesis we make is that in a number 

 like 100 or 200 stars, the irregular angular movements due to the 

 motions of the stars themselves, neutralize one another on th 

 average. But this is only the mean distance of the group, and somt 

 are much nearer and some much farther. The distribution of the 

 stars about this mean distance may be derived from the proper 

 motions, if we know how the linear velocities are distributed. I shall 

 apply this method to the group of stars which are like the sun in type 

 of spectrum, and, therefore, presumably like it in temperature and 

 physical constitution. 



Dividing them into three classes according to their magnitude, it 

 is found that their parallactic motion due to the sun's movement, and 

 their average motion in the perpendicular direction, due to their own 

 peculiar movements, are as given on next page (Table IV.). 



In the last column is given the ratio of the average cross motion 

 to the parallactic motion. The agreement of the numbers shows that 

 the bright stars and the faint stars have the same average velocity. 

 Taking the velocity of the sun as 19*5 kil/sec, it follows that the 



