io POPULAR SCIENCE MONTHLY. 



chapter, a circumstance which has made it one of the most interest- 

 ing objects to observers. Xo motion has yet been certainly detected 

 among the components. 



SPECTROSCOPIC BINAEY SYSTEMS. 



Among the many striking results of recent astronomical research 

 it would be difficult to name any more epoch-making than the discov- 

 ery that great numbers of the stars have invisible dark bodies revolving 

 round them of a mass comparable with their own. The existence of 

 these revolving bodies is made known not only by their eclipsing the 

 star, but by producing a periodic change in the radial motion of the 

 star. How their motion is determined by means of the spectroscope 

 has been briefly set forth in a former chapter. As a general rule 

 the motion is uniform in the case of each star. We have described in 

 a former chapter the periodic character of the radial motion of Algol, 

 discovered by Vogel. This was followed by the discovery that a 

 Virginis, though not variable, Avas affected by a similar inequality of 

 the radial motion, having a period of four days and nineteen minutes. 

 The velocity of the star in its apparent orbit is very great, about ninety- 

 one kilometers, or fifty-six English miles, per second. It follows that 

 the radius of the orbit is some three million miles. The mass of the 

 invisible companion must, therefore, be very great. 



£ MS 



Fig. 3. /a 



Fig. 3. Radial Motion of a Binary System. 



A now form of binary system was thus brought out which, from the 

 method of discovery, was called the spectroscopic binary system. But 

 there is really no line to be drawn between these and other binary 

 systems. "We have seen that as telescopic power is increased, closer and 

 closer binary systems are constantly being formed. We naturally infer 

 that there is no limit to the proximity of the pairs of stars of such sys- 

 tems and that innumerable stars may have satellites, planets or com- 

 panion stars so close or so faint as to elude our powers of observation. 

 Still, there is as yet a wide gap between the most rapidly moving visible 

 binary system and the slowest spectroscopic one, which, however, will 

 be filled by continued observation. 



The actual orbit of such a system cannot be determined with the 

 spectroscope, because only one component of the motion, that in the 

 direction of the earth, can be observed. In the case of an orbit of 

 which the plane was perpendicular to the line of sight from the earth 



