MASSES OF VISUAL IMNAR^- STARS. 47I 



very small density, they would no doubt react strongly to the 

 Sim's light-pressure. It is therefore not unreasonable to suppose 

 that when a double-star system is composed of two bodies of 

 great luminosity and small density (such as the double star a 

 Crux) light ]M-essure ])lays an im]iortant part — it may even neu- 

 tralise gravitative power, and o])pose by its disruptive force the 

 formation of close binary systems. Besides light pressure, it 

 will probably ultimately be found necessary to consider the 

 absorption or dispersion of atomic energies. 



Speculation is so easy that it should not be encouraged ; 

 what is really wanted is further research and more facts. 



APPENDIX. 



FoR^rUL.E, ETC. 



M := Gravitative power of the system, often for short, called 



the mass of the system, 

 R = Distance of the star from the Sun measured in radials, 

 one radial being the distance indicated by a parallax of 

 one second of arc; if tt is the parallax in seconds of 

 arc, then 



1 

 R = - 



TT 



N.B. — The parallax of a star is the angle sub- 

 tended by the Earth's mean distance from the Sun as 

 seen from the star. Hence one radial is equal to 

 206.265 times the mean distance of the Earth from the 

 vSun. 

 « = Semiaxis major of a binary system measured in units of 



the Earth's mean distance from the Sun. 

 a m Semiaxis major of a binary system measured in seconds 



a 

 of arc. Ta = — . Inn as the tangent is indistinguish- 

 R 



a 

 from the arc at verv small angles, we can w^ite a ^ — • 



R 

 111 = Magnitude of a star. 



;//.' =: Its magnitude, if brought to unit distance. 



;;/' =: ;;/ - 3 log R. 

 The Sun's magnitude at any given distance is equal to 5 log 

 R, so that at unit distance its magnitude is 0.0. This assumes 

 that the Sun's magnitude, as seen from the Earth, is -26.57, 

 and that the logarithm of the light ratio from one magnitude to 

 another is 0.4 (See Union Ohscn'olury Circular, No. 5). 

 Conversely, if we put 



;;/' = 0.0 = ;;; -5 log R, 

 we have the distance R at which a star of magnitude ;/; would 

 shine as a star of magnitude 0.0. that is. as brightly as the Sun 

 would at unit distance. 



