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POPULAR SCIENCE MONTHLY. 



surface brilliancy as the Sun, would express the ratio of density of 

 the stars to that of the Sun. Actually, it gives the product: 



Density x (brilliancy). » 



Star's 

 Density. 



k Pegasi. . . 

 8, Equulei. . 

 £> Sagittarii 

 F9 Argus. . . 

 42 Cornae. . . 

 P Delphirii . 



0.15 

 0.31 

 0.04 

 0.90 

 0.14 

 0.51 



The numbers in the last column being all less than unity, it fol- 

 lows that either the stars are much less dense than the Sun or they 

 are of much less surface brilliancy. Moreover, they belong to a selected 

 list in which the numbers of the last column are larger than the average. 



To form some idea of the result of a selection from the general 

 average, we may assume that the average of all the measured distances 

 between the components of a number of binary systems is equal to the 

 average radius of their orbits, and that the observed annual motion is 

 equal to the mean motion of the companion in its orbit. Taking a 

 number of cases of this sort, I find that the number corresponding to 

 the last number of the preceding table would be little more than one 

 thousandth. 



A very remarkable case is that of £> Orionis. This star, in the belt 

 of Orion, is of the second magnitude. It has a minute companion at a 

 distance of 2 ".5. Were it a model of the Sun, a companion at this ap- 

 parent distance should perform its revolution in fourteen years. But, 

 as a matter of fact, the motion is so slow that even now, after fifty years 

 of observation, it cannot be determined with any precision. It is prob- 

 ably less than 0°.l in a year. The number expressing the comparison of 

 its density and surface brilliancy with those of the Sun is probably less 

 than .0001. 



The general conclusion to be drawn is obvious. The stars in general 

 are not models of our Sun, but have a much smaller mass in propor- 

 tion to the light they give than our Sun has. They must, therefore, 

 have either a less density or a greater surface brilliancy. 



We may now inquire whether such extreme differences of surface 

 brilliancy or of density are more likely. The brilliancy of a star de- 

 pends primarily not on its temperature throughout, but on that of some 

 region near or upon its surface. The temperature of this surface can- 

 not be kept up except by continual convection currents from the in- 

 terior to the surface. We are, therefore, to regard the amount of light 



