THE BINARY STARS. 323 



second class, is, when taken by itself, altogether separate from its 

 companions, found to be a very interesting spectroscopic double, 

 for when the spectroscope was first applied to Mizar by Professor 

 Pickering, he found that the lines in its spectrum were sometimes 

 single and sometimes were all double. When double, one set of 

 lines had been moved bodily a very little way towards the red end, 

 whilst the other set had all shifted slightly towards the blue ; they 

 then closed up again, and then shifted an equal amount in the 

 opposite direction, thus showing that Mizar by itself, away from 

 its companion, must really be two stars revolving round a common 

 centre of gravity, so that at one point in their orbit (which must 

 be placed almost edgeways to our line of vision) one star would 

 be moving towards us while the other would be retreating from us, 

 while when at the opposite points in their orbit the movements of 

 each star towards or away from us would of course be reversed. 

 When crossing our line of sight at right angles, the spectroscope 

 would show no movement at all, and the double set of lines from 

 the two stars would close up into one set and appear single. As 

 each set of lines is moved about an equal distance in an opposite 

 direction, it appears that each of the two components is moving at 

 about the same speed, which the spectroscope declares to be 

 approximately fifty miles a second, and it is only necessary to note 

 the time taken for the lines to open out and close up again twice 

 to arrive at the length of time occupied in performing one revolu- 

 tion of the orbit. This is found to be one hundred and four days. 



As, therefore, we know the time taken, and also the speed, we 

 can easily calculate the length of the path travelled, or the cir- 

 cumference of the orbit round which both the stars are moving, 

 and its diameter will therefore represent the distance separating 

 the two stars. This distance between them is found in round 

 numbers to be about 150 million miles. 



The mathematician is now provided with all the necessary data 

 to enable him to weigh these two companions, and arrive at their 

 joint mass as compared to the mass of the sun. It is to be 

 observed that the spectroscope has provided him with all this 

 information on which to base his calculation, quite regardless of 

 what the star's distance may be from us. He knows that the 

 distance separating the components is 150 miUion miles, and he 



