1914] 



on The Stars around the North Pole 



120 



well shown in the photograph of the Southern Cross (Fig. 5), where 

 (^ and y have the same visual magnitudes, but y is a red star. Now 

 the visual magnitudes of most of these faint stars have been very 

 accurately determined at Potsdam by Messrs. Muller and Kron (and 

 have been kindly communicated to me in manuscript), and the 

 photographic magnitudes have been determined at Greenwich. The 

 differences have been taken between the photographic and visual 

 magnitudes, and serve to classify the stars according to their tempera- 

 ture. 



20. Separating the stars into two groups, those which are brighter 

 than 9™ • 5 on the Potsdam scale of magnitudes, and those which are 

 fainter than 9™ • 5, and dividing each group into four classes accord- 

 ing to the colour index, we may determine for each class the parallactic 

 motion, i.e. the mean angular movement per century, arising from 

 the motion of the sun through space. The results are exhibited in 

 the following table : — 



In this table the red stars are on the top line ; the third line con- 

 sists of stars which are in the same stage of development as the sun, 

 those in the second line are somewhat cooler and redder ; those in the 

 last line hotter and bluer. The last line includes a few, but only a 

 few, B stars, as there are not many in this part of the sky. The 

 quantities in the fourth and sixth columns of the table are a gauge of 

 the distance of the stars to which they refer. It is only necessary to 

 divide 337", which is the angle through which a star, in this part of 

 the sky, distant 1 parsec, would have been displaced by the solar 

 motion in 100 ^years, to obtain the distances in parsecs. Thus the 

 240 stars belonging to type A and F and brighter than 9"* 5 are at 

 an average distance of 337/1 ■ 97 or 170 parsecs. 



The first point to notice is that the quantities in the sixth column 

 are always considerably less than the corresponding quantities in the 

 fourth column. This is, of course, because the faint stars are on the 

 whole farther away. The average distance of stars of magnitude 

 10™- is approximately If times as great as for a star of 8™-0. Tiie 

 next point to notice is the very great distance of the red stars. The 

 269 faint red stars are very nearly 1000 parsecs away or 200 million 



Vol. XXI. (Xo. 108) k 



