ON STELLAR DISTRIBUTION AND MOVEMENTS. 249 



such a space would contain about eighteen stars. Further, we notice 

 that eight stars out of the eighteen are binaries. I daresay that that 

 proportion may be higher than the average; still, there is good evidence 

 that double stars form a large fraction, perhaps a third, of the whole 

 number of stars. 



The intrinsic luminosities of these stars vary from forty-eight times 

 to less than one-hundredth that of the sun. Naturally, a larger sample 

 would give a greater range; probably there exist (very rarely) stars 

 giving as much as ten thousand times the light of the sun. We see 

 further that there are five stars more luminous and twelve stars less 

 luminous than the sun, so that the sun stands well above the average; 

 this also is confirmed by more elaborate investigations. Turning to 

 the types of spectrum, it is at first rather astonishing to find that 

 three of the stars are of the Third Type (Type M), because in the 

 catalogues stars of this type are rather rare — less than a fifteenth of 

 the whole number of stars. All three of these M stars are very faintly 

 luminous, and this fact leads to an explanation. We infer that M stars 

 are really very abundant everywhere, but they are too faint to be seen, 

 . except when they are near to us, and consequently not many appear 

 in the catalogues. The Orion Type stars (Type B) are rather more 

 numerous than the M stars in the catalogues, but there is not a single 

 one in this list. The -same explanation holds. Orion stars are really 

 very rare in space, but they are exceedingly bright, and can be seen 

 a long distance away, so that they are recorded in numbers out of all 

 proportion to their actual frequency in space. 



This illustrates a rather important conclusion. T^he proportions 

 of the different types of stars, as seen in the sky or as recorded in 

 the catalogues, is utterly misleading as an indication of their relative 

 abundance in space. 



Parallax-measuring carries us only a very short way into the vast 

 system of stars surrounding us. Of course, the short table given does 

 not exhaust all that is to be learnt from measured parallaxes. There 

 are now some three hundred and sixty stars for which more or less 

 trustworthy determinations have been made, 4 but these include a great 

 many for which the result is practically zero. In parallaxes of a 

 tenth of a second and under the error of determination becomes rela- 

 tively important, and these smaller parallaxes do not give much certain 

 information about particular stars ; they are, however, very important 

 for statistical analysis, and much statistical work must ultimately rest 

 on the measured parallaxes as a basis. 



To go beyond the small fringe of stars which is sounded by our 

 parallax determinations we must examine statistics of magnitudes and 

 motions. Now the prominent fact that appears from counts of the 

 numbers of stars down to definite magnitudes is the great crowding to 

 the plane of the Milky Way; and this following Hersehel, we explain 

 as being due to the flattened form of our stellar system. The crowding 

 shows a continuous increase from the galactic poles to the equator, and 

 seems to be uniform and symmetrical. Along the galactic equator there 



4 Kapteyn and Weersma, Groningen Publications, No. 24, 



