314 POPULAR SCIENCE MONTHLY. 



the present time, there is, in my opinion, little doubt that we should be 

 prevented from continuing upon it indefinitely. In the course of our 

 travels we should be carried, sooner or later, quite close to some indi- 

 vidual star whose attraction would be vastly more powerful than that 

 of all the other stars combined. This would draw us from our present 

 curve and cause us to follow a different one. At a later date, our 

 travels would carry us into the sphere of attraction of some other great 

 sun which would send us away in a still different direction. Thus 

 our path should in time be made up of a succession of unrelated curves. 



Spectroscopic binary systems, as by-products of radial velocity meas- 

 urements, are of exceedingly great interest, from the light which they 

 cast upon the construction of other systems than ours. When we look 

 at the sky on a clear night, we may be sure that at least one star in 

 six or seven is attended by an invisible companion, comparable in mass 

 with the primary body, the two revolving around their common center 

 in periods varying from two or three days in many cases, up to three 

 or more years in others. For the triple system of Polaris the long 

 period perhaps exceeds fifteen or twenty years. As the shortest-period 

 visual binary now known, that of 8 Equulei, is only 5.8 years, the gap 

 between visual and spectroscopic binaries has been definitely closed. 



The companions of binaries discovered by means of the spectro- 

 graph have not been observed visually in our powerful telescopes, 

 although they have been carefully searched for. They may be so close 

 to the principal star that, viewed from our distance, the two images 

 can not be resolved. The separation of the components is probably 

 less than one hundredth of a second of arc for most of the binaries 

 thus far announced. Again, for very few of the systems are the spectra 

 of both components recorded. This does not establish that the com- 

 panion is a dark body, but only that it is at least one or two photo- 

 graphic magnitudes fainter than the primary. The fourth-magnitude 

 companion of a second-magnitude star would scarcely be able to im- 

 press its lines upon the primary's spectrum. The invisible com- 

 ponents in many spectroscopic binaries might be conspicuous stars, 

 if they stood alone. 



Only those systems have been detected whose periods are relatively 

 short, and for which the variations of radial speed are considerable. 

 The smallest observed variation is that of Polaris — six kilometers per 

 second. Had the variation for Polaris been only one kilometer, it 

 would no doubt have escaped detection. Such a variation could be 

 measured by present instruments and methods; but this range would 

 not have excited the observer's suspicion, and the discovery would have 

 remained for the future. It is probable that there are more systems 

 with variations of speed under six kilometers than there are with 

 larger ones; and all such are awaiting discovery. The velocity of 

 our sun through space varies slightly, because it is attended by com- 



