106 BARNARD— SOME PECULIARITIES OF THE NOV^. 



for some cause the forces of equilibrium in the star become unstable 

 and there occurs a great outburst of light in the nature of an ex- 

 plosion. This does not actually destroy the star, for later on, in the 

 course of a few years, it returns to its original brightness and prob- 

 ably to its former physical condition. This is shown by the fact that 

 Nova Persei of 1901 was originally a small variable star. Having 

 now returned to its original brightness, it is again variable. 



Sometimes in their decline from maximum these stars present a 

 well-defined, measurable, planetary disc and finally become perfectly 

 stellar again. Always, at one period of their decline, they present a 

 beautiful, sharply defined, crimson image 9 mm. outside the normal 

 focus in such a telescope as the 40-inch refractor of the Yerkes 

 Observatory. This is due to the presence of the a Hydrogen line, 

 which is then strong in their spectra. This always occurs several 

 weeks after their first appearance. 



When they first appear they are very white. They soon turn red 

 and in the end become white or colorless again. In their decline they 

 pass through many very beautiful colors, such as an intense blue and 

 gold and crimson — colors that no ordinary star shows in such ex- 

 quisite purity. This is all due to the nature of their light, through 

 their changing physical condition. 



Yerkes Observatory, 



Williams Bay, Wisconsin, 

 April 15, 1922. 



