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and 4958, the characteristic radiations of a gaseous nebula, 

 have replaced those due to high-temperature iron at 5016 and 

 4924, which are very prominent in the first stages of the bright 

 band spectrum. Another nebular radiation at wave-length 

 4363 also becomes quite pronounced. 



Not only spectroscopically but visually the star has changed 

 into a planetary nebula, for it shows a distinct greenish disc 

 when observed with a telescope. But this is not the final 

 stage, for eventually the characteristic nebular bands yield 

 pride of place to those which are found in a class of bright 

 band stars, called Wolf-Rayet stars from their discoverers. 

 Nova Aurigae, Nova Persei, Nova Geminorum, though now 

 very faint stars, are not beyond the reach of the giant telescopes 

 of Mount Wilson. 1 They all show a continuous spectrum 

 marked by the bright bands of hydrogen, but lacking in the 

 chief nebular bands, the places of which have been taken by 

 bands found in Wolf-Rayet or Class O stars. Nova Aquilae, 

 in June last, was still in the planetary nebular stage, but the 

 diameter of the green disc had increased from o'6s", as observed 

 by Barnard in October 191 8, to 3*69", according to the measures 

 of Aitken. Some remarkable spectrograms of the star were 

 secured by Moore and Shane, and by Dr. Wright at the Lick 

 Observatory. These showed that the hydrogen was not 

 expanding to the same dimensions as the nebular radiations, or 

 in other words, that different portions of the disc into which 

 the star has developed give bright radiations displaced by 

 different amounts. Here again we have indications of an actual 

 expansion of the gases of the nova, and not of the illumination 

 of nebular gases rendered visible by the advance of a spherical 

 wave of light which originated in the nucleus. 



We have seen that new stars in our system are confined to 

 the Milky Way. So, too, are the Wolf-Rayet stars, and the 

 planetary nebulae, with but few exceptions. There are several 

 other characteristics which are shared by these three classes 

 of heavenly bodies. New stars are comparatively few amongst 

 the hosts of heaven. So, too, are the planetary nebulae and the 

 Wolf-Rayet stars. With regard to the number of the planetary 

 nebulae, Dr. H. W. Curtis states, in the truly wonderful and 

 fascinating volume of researches on the nebulae recently issued 

 from the Lick Observatory,' " On any reasonable and probable 

 basis of correlation between the planetary nebulae and the stars 

 of supposedly corresponding magnitudes, it would seem certain 

 that the relative proportion of the planetaries to the stars 

 must be of the order of one-thousandth of i per cent, or less. 



1 Adams and Pease, Proceedings National Academy of Science, i. 391. 



2 Publications of the Lick Observatory. Vol, xiii, " Studies of the 

 Nebulae," p. 72, 



