29 375 



™"31 is '"'263 giving double weiglit to the former. For the 5 Pleiades considered, 

 the mean value of /^ is — "196, and for the Hyades of magnitude 4"^'8(i5 it is thus 

 found to be —-196 + "263 = +-067. This value of /;. corresponds to m,,,, = TM in 

 the Pleiades. Hence, the magnitude difference between stars of the same colour in 

 the Pleiades and the Hyades is 7"''54 — 4'"'865 = 2"^-675. Supposing that stars of 

 the same colour have the same absolute magnitude in the two groups and adopting 

 a parallax of "-027 for the Hyades (A. N. 5000, 114) the parallax of the Pleiades is 

 calculated to be "-0079. 



For the 6 stars ß, y, ô, e, g Ursae majoris and « Coronae borealis of the 

 Ursa major system the mean photographic magnitude is 2"^'74, the mean value of 

 'Vt is r72, and the mean parallax ""043. The same colour is found for Pleiades of 

 the photographic magnitude 6'"'17 differing by 6"^'17 — 2™'74 = 3'"'43 from the mean 

 magnitude of the 6 stars of the Ursa major system. From these data the parallax 

 of the Pleiades is in the same way as above calculated to be "'0088. The two re- 

 sults "'0079 and "'0088 agree with the first estimate made of the parallax of the 

 Pleiades by a reasoning of this kind, using the spectral classification of Antonia C. 

 Maury, viz. "-0085 ±"-002 (m. e.) (Zeitschr. für wiss. Photographie Bd. 5, 106, foot- 

 note 37; 1907). 



Orbital motions in double stars physically belonging to the Pleiades are so 

 far too inaccurately known to allow of a reliable calculation of hypothetical dy- 

 namical parallaxes. 



Taking the parallax of the Pleiades to be "'009, a distance of one parsec in- 

 side the group corresponds to an angle of 57°'3 X '009 = °"52 or 31' as seen from 

 the earth. The number of Pleiades with a space distance from Alcyone of less than 

 one parsec may be estimated at about 50 or about 150 times larger than the star 

 density in the neighbourhood of our sun (comp. B. A. N. 5, 21; 1922). The corres- 

 ponding proportion of mass is probably a few times greater, while the total light 

 intensity of the stars per unit volume in the inner region of the Pleiades is about 

 4000 times larger than in the neighbourhood of our sun. It therefore appears that 

 the conspicuousness of the Pleiades as seen in the sky is due partly to great star 

 density inside the group and partly to great luminosity of its members and that 

 the importance of these two causes is of the same order of magnitude. 



15. The Colours of Stars not Physically belonging 



to the Pleiades. 



In Figure 3 a diagram is given containing the 421 stars, for which the weight 

 of /^ is at least 100 m"^. (Adding the colour indices of Shapley and Richmond this 

 number would be increased with 28 to 449). Stars with a proper motion approx- 

 imately equal to that of the group are indicated by crosses, the other stars by 

 dots. Probably not all the stars marked with a cross and fainter than the 12'^ 

 photographic magnitude are physically connected with the cluster. 



