5 o 4 THE POPULAR SCIENCE MONTHLY, 



period of revolution of Mars is six hundred and eighty-seven days, 

 and would be less if the mass or weight of our sun was greater. 

 We can hence calculate how many times greater than the mass of 

 the sun must be the mass of the two stars of Mizar for the revo- 

 lution to be accomplished in one hundred and four days. The 

 result is forty times the mass of the sun. So this little point of 

 light which Mizar in the Great Bear appears to the eye is the 

 equivalent of forty of our suns. 



Before the news of this astonishing discovery made at Cam- 

 bridge had reached Europe, a similar investigation made at the 

 Potsdam Observatory was published. It was directed to the star 

 Algol in the head of Medusa. This star has been known for more 

 than two hundred years to be variable in brightness. It shines 

 for two days and a half with a steady white light, then loses 

 brightness for about four hours and a half, recovers during 

 about four hours and a half, and then continues steady again 

 for two days and a half. The changes go on with great regu- 

 larity, and it has been believed for the last hundred years that 

 Algol is attended by a double star revolving around it, by which 

 it is concealed from the earth at regular intervals, depending on 

 the period of its revolution. The periodical decrease of brill- 

 iancy is similar in its nature and cause to an eclipse of the sun, 

 when the dark moon is interposed between it and the earth. But 

 probable as this belief was, the fact had not been demonstrated. 

 A complete solution has been obtained by spectrum analysis. 

 Prof. Vogel, of the Astrophysical Observatory in Potsdam, and 

 his fellow- worker, Dr. Scheiner, have taken photographs of the 

 spectrum of Algol and carefully measured the dark lines. It has 

 thus been ascertained that these lines move toward the red before 

 the star appears at its weakest, toward the violet after that mo- 

 ment ; or, in other words, that Algol is receding from the sun in 

 the first half of its change, approaching it in the second half. This 

 would necessarily occur if the star was describing an orbit around 

 a dark body which should periodically conceal it for a time from 

 our view. The rate of motion of Algol is twenty-three English 

 miles in a second, and its period of revolution is two days, twenty 

 hours, and forty-nine minutes ; whence the circumference of its 

 orbit and the distance apart of the centers of the two stars may be 

 computed as was done in the case of Mizar. The latter is found to 

 be less than 3,000,000 English miles, a small enough distance for two 

 so large bodies. From the period of the light-changes and the ve- 

 locity of the motion we calculate the diameter of the principal star 

 to be 920,000 and of its dark companion 750,000 English miles. 

 The two bodies which form the Algol system are each nearly as 

 large as our sun, the diameter of the sun being taken at 750,000 

 miles, but their total mass is only about two thirds the mass of 



