September 23, 1898.] 



SCIENCE. 



■389 



the other towards the red. By comparing 

 any of the lines in the spectrum of a 

 heavenly body with the same lines in the 

 spectrum of a stationary object it is possible 

 to tell not only the direction of motion of 

 the moving object, but to determine ac- 

 curately the amount of this motion, for there 

 is a known relation between the amount of 

 displacement and the actual velocity, and 

 this is independent of the distance. 



This peculiarity, besides showing the 

 motions of the individual stars, has revealed 

 to us, through the aid of photography, an 

 entirely new class of bodies, the so-called 

 spectroscopic binaries. 



The visual spectroscopic work, long ago, 

 in the hands of Dr. Huggins, had shown 

 the displacement of the spectral lines as 

 the stars moved towards or from us. It 

 remained, however, for photography fur- 

 ther to extend this remarkable work by 

 showing that not only were the lines dis- 

 placed, but that in the case of certain of the 

 stars the lines were periodically doubled 

 at short intervals, thus indicating the pres- 

 ence of two bodies which must be rapidly 

 revolving about each other. The doubling 

 of the lines is due to the alternate ap- 

 proach of one and recession of the other 

 body, which thus causes a displacement of 

 the two sets of spectra, for when the 

 motion is at right angles to the line of 

 sight (and this must occur at two points in 

 the orbit) the two spectra will be exactly 

 superposed. It can readily be shown from 

 the known periods of these stars and their 

 enormous distances that no telescope is 

 likely to be made so powerful as to show 

 visually their independent components. The 

 visual double star having the shortest 

 period is one discovered by Burnham, and 

 known as Kappa Pegasi, which he found to 

 have a period of about eleven years. The 

 spectroscopic binaries seem to revolve in 

 extremely short periods— a few days — and 

 in at least one case in a few hours, showing 



that they must be extremely close to each 

 other. The explanation, to account for 

 the observed peculiarities of their spectra, 

 that these are actual double stars in rapid 

 orbital motion must be accepted until some 

 better explanation of the phenomenon be 

 forthcoming, which does not at present 

 seem likely to occur. 



Among the first of these spectroscopic 

 binaries discovered was Beta Aurigaj, which 

 was detected at the Harvard College Obser- 

 vatory by Miss Maury, through the doubling 

 of its spectral lines as shown on the various 

 photographs obtained of it at that Observa- 

 tory. This star has a period of four days, 

 the relative motion of the components about 

 each other being about 150 miles a second, 

 and the distance between them about six 

 millions of miles. In a similar manner, 

 Dr. Vogel has found that the star Algol, so 

 famous for its light variations, alternately 

 approaches us and recedes, in a manner 

 that can only be explained at present by 

 the revolution of that star about some other 

 body or about the center of gravity of the 

 two. The spectrum of this star does not 

 show any doubling of the lines, but a sim- 

 ple displacement from one side to the other 

 of their normal position occurs consistent 

 with the changes of the star's light. As 

 there is no doubling of the lines, the con- 

 clusion is that there is but one spectrum. 

 One of the stars is, therefore, a non-lumi- 

 nous body, and hence produces no spectrum. 

 The old explanation of two hundred years 

 ago, that the variation in the light of Algol 

 is due to a dark body revolving about it 

 and partially eclipsing it at intervals of a 

 little less than three days, is hence proved 

 by the spectroscope and photography to be 

 the correct one. The frequent discovery of 

 these spectroscopic binaries shows that they 

 are by no means uncommon, and that pos- 

 sibly a considerable percentage of the stars 

 consist of two or more bodies rapidly whirl- 

 ing about each other. 



