CHAPTERS ON THE STARS. 643 



Vogel, of Potsdam, in 1889, to set the question at rest. His method of 

 reasoning and proceeding was this: 



If the fading out which we see is really due to an eclipse by a dark 

 body, that body must be nearly or quite as large as the star itself, else 

 it could not cut off so much of its light. In this case, it is probably 

 nearly as massive as the star itself, and therefore would affect the mo- 

 tion of the star. Both bodies would, in fact, revolve around their 

 common center of gravity. Therefore, when after the dark body has 

 passed in front of the star, it has made one-fourth of a revolution, 

 which would require about seventeen hours, the star would be moving 

 towards us. Again, seventeen hours before the eclipse, it ought to be 

 moving away from us. 



The measurement of six photographs of the spectrum, of which 

 four were taken before the eclipses and two afterward, gives the follow- 

 ing results: 



Before eclipses: Velocity from the sun equals 39 km. per second. 

 After eclipses: "Velocity toward the sun equals 47 km. per second. 



These results show that the hypothesis in question is a true one, 

 and afforded the first conclusive evidence of a dark body revolving 

 around a distant star. A study of the law of diminution and recovery 

 of the light during the eclipse, combined with the preceding motions, 

 enabled Vogel to make an approximate estimate of the size of the orbit 

 and of the two bodies. The star itself is somewhat more than a million 

 of miles in diameter; the dark companion a little less. The latter is 

 about the size of our sun. Their distance apart is somewhat more 

 than three millions of miles; the respective masses are about one-half 

 and one-fourth that of the sun. These results, though numerically 

 rather uncertain, are probably near enough to the truth to show us 

 what an interesting system we here have to deal with. We can say 

 with entire certainty that the size and mass of the dark body exceed 

 those of any planet of our system, even Jupiter, several hundred fold. 



The period of the star is also subject to variations of a somewhat 

 singular character. These have been attributed by Chandler to a mo- 

 tion of the whole system around a third body, itself invisible. This 

 theory is, however, still to be proved. Quite likely the planet which 

 causes the eclipse is not the only one which revolves around this star. 

 The latter may be the center of a system like our solar system, and the 

 other planets may, by their action, cause changes in the motion of the 

 body that produces the eclipses. The most singular feature of the 

 change is that it seems to have taken place quite rapidly, about 1840. 

 The motion was nearly uniform up to near this date; then it changed, 

 and again remained nearly uniform until 1890. Since then no avail- 

 able observations have been published. 



