464 MASSES OF VISUAL BINARY STARS. 



a Crux. — Magnitude 1.05 ( couipcvioits 1.58 a)id 2.0*9). 



Spectrum B\. 



These two bright stars forrii one of the most beautiful 

 double Stars in the whole sky. Their distance apart is 5". 

 Although they have been measiu-ed frequently since 1S26. it is 

 still quite uncertain if there is any change. Certainly the angle 

 is unchanged, but the distance may have decreased slightlv. If 

 we assume that a solar-type double star of equal size would 

 shine as a star of the 4th magnitude at the distance of a Crux, 

 then it would revolve at the rate of from i to 2 degrees a vear. 

 There is no such change in this pair. Let us liear what Xew- 

 comb says concerning the somewhat similar jjair t* 'rion. Mag- 

 nitude 1. 91 (components 2.05 and 4.21.) Spectrum Bo: — 



This star, in the belt of Orion, is of the 2nd magnitude. It has a 

 minute companion at a distance of 2". 5. Were it a model of the Sun, a 

 companion at this apparent distance should perform its revolutions in 14 

 years. But, as a matter of fact, the motion is so slow, that even now, 

 after 50 years of observation, it cannot ])e determined with any precision. 

 It is probably less than o°.t in a year. The number expressing the com- 

 parison of the density and surface brilliancy of this star with those i)f the 

 Sun is probably less than o.cooi. The general conclusion to Ijc drawn is 

 obvious. The stars in general are not models of our .Sun. but have a 

 much smaller mass in proportion to the light they give than has our Sun 

 ("The Stars: A Study of the Universe." t>. 200). 



Gravitational motion is, however, exhibited in some of the 

 earlier B type stars. Thus with a Orion, magnitude 3.94, 

 Spectrum Bo, the angle has changed 28° in 13 years, the distance 

 remaining constant at o''.25. If this star had been a model of 

 the Sim, the change would have l)een about 180° a year, or 

 2,300° instead of 28°. 



The spectra Bo and HI have intense helium lines ; there are no metallic 

 lines. 



SiRius. — Magnitude -1.58 (couipouoits -1.58 and 9.0). Spcc- 



truni ^i. 



Auwers found 2. 125 for the ratio of the mass* of the larger 

 and brighter component to that of the smaller. t Hence we have 

 for Sirius 2. 16 0, and for the companion i .01 0. 



At the distance represented by the parallax o".39, the Sun 

 Avoidd shine as a star of the 2.04 magnitude. Solar-type stars 

 of the masses 2.16 and i.oi woitld shine as stars of the 1.48 

 and 2.03 magnitudes. Thus, so far as concerns Sirius, we can 

 say that, mass for mass, it emits so much more light that it gains 

 3.06 magnitudes — that is. an A type star, mass for mass, is three 

 magnitudes brighter than a G type star. 



The spectrtmi of the companion has not been determined.! 



* Using " mass " as the equivalent of gravit^tive power. 



f.4st. Nach., 3085. 



t Since the above was written, a note dealing with the spectrum of 

 the companion of Sirius has been published by Dr. W. S. Adams, in the 

 December, 1915. issue of the Publications of the Astron. Soc. of the 

 Pacific, pp. 236-7. He finds that the spectrum of the companion is" iden- 

 tical with that of Sirius. but that there appears to be a slight tendency for 

 ilie c( ntinuous spectrum to fade off more rapidly in the violet region. 



