45'> MASSliS UF VISl'AL lUNARY STARS. 



of i" in one year. Actually we would not be able to see the Earth 

 because it would be too faint. But there are other systems ; let 

 us take a Centaurus. for example ; it is a double star with a 

 parallax of o".76 (radial or distance = i ■^2) with a mass ecjual 

 to 1.9 times the Sun's mass. The chief star of a Centaurus 

 appears to be an exact duplicate of the Sun. We can therefore 

 ask if replicas of this system are to be found elsewhere in the 

 heavens. Thus, if there is a similar system ten times as far away, 

 it would appear to us as a 5th magnitude pair with a period of 

 80 years and a mean distance or semiaxis major of i".8. Table 

 I will enable us to answer this question by inspection, and it 

 shows us that in the quarter of a million stars examined by 

 double-star investigators there are but two or three stars which 

 api^ear to be of the a Centaurus or Sun type ; of the remainining 

 millions of fainter stars (loth to T3th magnitude) it can be 

 safely inferred there is amongst them no double star of this type. 



Table I may be considered complete for all stars brighter 

 than the 8th magnitude, and Avhose periods are less than 90 

 years, and which are not closer than o".4. This being so, we 

 remark first that, excepting the double companion of fi Her- 

 cules, there is no star so faint as 8th magnitude in the list. Let 

 us consider what this means. If the Sun was removed to such 

 a distance that it was of the 8th magnitude, it would then be 40 

 radials away (it = o".02^), and a companion at an angular 

 distance of ©".50 would revolve around it in 90 years. 



We can safely say that there is no such double star in the 

 heavens. There are very many pairs of stars about o".5 apart, and 

 with a combined magniture of S.o. but their motion is so slow 

 that their periods must be not only greater, but very much 

 greater than 90 years. 



Hence the evidence of the known douljle stars is that stars 

 of the 8th magnitude have less gravitative power than the Sun, 

 probably without any exception. This line of argument applies 

 to all magnitudes. Thus, if the Sun was removed to such a 

 distance that its magnitude w^as 5.0, its radial would be 10 

 (corresponding to tt =: o". 10), and com])anions to it would have 

 semi-axes and periods as follows : — 



Distance or Period. 



Semiaxis. Year. 



0. 10 1 .0 



0.30 .5-2 



0.40 8.0 



O . 50 I I . 2 



0.60 14.7 



0.80 22.6 



1 . 00 31-6 



A comparison of this table with Tables I and H at once shows 

 that few of the double stars therein have gravitative power equal 



