30 REPOET— 1891. 



of not permitting a direct comparison of the star's spectrum with ter- 

 restrial spectra. It is obviously unsuited to a variable star like Algol, 

 where one star only is bright, for in such a case there would be no 

 doubling of the Hnes, but only a small shift to and fro in the spectrum 

 of the lines of the bright star as it moved in its orbit alternately towards 

 and from our system, which would need for its detection the fiducial 

 positions of terrestrial lines compared directly with them. 



For such observations the Potsdam spectrograph was well adapted. 

 Professor Vogel found that the bright star of Algol did pulsate back- 

 wards and forwards in the visual direction in a period corresponding to 

 the known variation of its light. The explanation which had been 

 suggested for the star's variability, that it was partially eclipsed at 

 regular intervals of 68'8 hours by a dark companion large enough to cut 

 off nearly five-sixths of its light, was therefore the true one. The dark 

 companion, no longer able to hide itself by its obscureness, was brought 

 out into the light of direct observation by means of its gravitational 

 effects. 



Seventeen hours before minimum Algol is receding at the rate of 

 about 24^ miles a second, while seventeen hours after minimum it is 

 found to be approaching with a speed of about 28^^ miles. Prom these 

 data, together with those of the variation of its light, Vogel found, on 

 the assumption that both stars have the same density, that the companion, 

 nearly as large as the sun, but with about one-fourth his mass, revolves 

 with a velocity of about fifty-five miles a second. The bright star of 

 about twice the size and mass moves about the common centre of gravity 

 with the speed of about twenty-six miles a second. The system of the two 

 stars, which are about 3;^ millions of miles apart, considered as a whole, 

 is approaching us with a velocity of 2-4 miles a second. The great 

 difference in luminosity of the two stars, not less than fifty times, suggests 

 rather that they are in different stages of condensation, and dissimilar in 

 density. 



It is obvious that if the orbit of a star with an obscure companion is 

 sufficiently inclined to the line of sight, the companion will pass above or 

 below the bright star and produce no variation of its light. Such systems 

 may be numerous in the heavens. In Vogel's photographs, Spica, which is 

 not variable, by a small shifting of its lines reveals a backward and forward 

 periodical pulsation due to orbital motion. As the pair whirl round 

 their common centre of gravity, the bright star is sometimes advancing, 

 at others receding. They revolve in about four days, each star moving 

 with a velocity of about fifty-six miles a second in an orbit probably 

 nearly circular, and possess a combined mass of rather more than 2^ 

 times that of the sun. Taking the most probable value for the star's 

 parallax, the greatest angular separation of the stars would be far too 

 small to be detected with the most powerful telescopes. 



If in a close double star the fainter companion is of the white-star 



