68 



tinguished with any telescope, but betraying their 

 existence by the effect of their motion. Usually 

 one member of the pair is too faint to give any 

 spectrum. The lines in the spectrum of the visible 

 star then swing back and forth, toward the violet 

 as the star approaches the earth, toward the red 

 as it recedes. When both stars are nearly equal 

 in brightness both spectra appear superposed and 

 the lines, which are single when the two stars are 

 moving across the line of sight, become double 

 when one is approaching and the other receding. 



Thus far 115 spectroscopic binaries have been 

 discovered on Mount Wilson, or about I in 4 ot 

 all the stars whose velocities have been measured. 

 In some interesting cases of rapid motion the rela- 

 tive orbital velocities of the components range from 

 104 to 367 kilometers per second. 



TEMPORARY STARS. 



Among the most remarkable phenomena ot the 

 heavens are the "new" or temporary stars, which 

 burst out into sudden brilliancy and gradually 

 fade into extreme faint ness. With the 6o-inch 

 reflector it has been possible to photograph the 

 spectra of some interesting 'Novae" which ap- 

 peared several years ago and are now very faint. 

 These include Nova Aurigae, which was discovered 

 in 1891 and is now of magnitude 13.5; Nova 

 Persei of 1901, magnitude 12.0; Nova Lacertae of 

 1910, magnitude 13; and Nova Geminorum No. 2 

 of 1912, magnitude 10. For such faint objects 



