112 I 1 . W. Very — A Cosmic Cycle. 



stances from the depths of the star gives a continuous spectrum 

 of extraordinary brilliancy, rich in the shorter waves, which 

 rapidly passes into a spectrum of the Orion type with narrow 

 dark lines of hydrogen and helium. This is followed by an 

 enormous evolution of hot gases. The complex banded struc- 

 ture of the broad bright F line indicates radiation from a number 

 of streams moving with different velocities.* The great breadth 

 of the lines may mean that the gas is under great pressure, or in a 

 condition such as might be expected if intensely heated matter 

 from the depths of a star has been opened to view by disruption. 



This view of the case has been partly anticipated by Profes- 

 sor W. H. Pickering, although he has not predicated so pro- 

 found a disturbance as the actual disruption and dissipation of 

 a star. Assuming that the phenomenon is produced by gaseous 

 eruptions, he says : " Prominences upon an enormous scale 

 burst forth, spreading in every direction, and completely 

 enveloping the star upon all sides. When they first appear, 

 they present a spectrum of bright lines, but in a few hours the 

 gases first emitted have receded to a considerable distance 

 from the star, and have cooled down owing to the rapid 

 expansion involved in their recession." ' " This cold advancing 

 atmosphere produces a series of dark absorption lines." " The 



hot receding prominences, however, give out a light 



whose wave-length cannot be absorbed by the cold advancing 

 atmosphere. They therefore shine with their full brilliancy."f 



It is notable that every nova which has been examined spec- 

 troscopically at the proper stage, has exhibited this sequence : 



1. An intensely bright continuous spectrum rich in violet 

 light. 



2. Paired dark and bright lines of hydrogen, etc., the dark 

 lines on the violet side indicating approach, the bright on the 

 red side showing recession. The lines are very broad, denoting 

 either great pressure or a wide range of velocity in the line of 

 sight. 



3. Fading of the continuous spectrum, produced by cooling 

 of photospheric clouds" until these are entirely dissipated, 

 leaving : 



4. A purely gaseous spectrum. Pressure and differential 

 velocity diminishing until nothing is left but a planetary nebula 

 whose light may be due to collisions of meteorites, crystallized 

 out of the expanded and cold gases, perhaps aided by some 

 kind of electric or cathode discharge along filaments or sheets 

 made up of discrete particles. 



* M. H. Deslandres, Comptes Rendus, vol. cxxxii, p. 620, 1901, notes the com- 

 posite character of the bands in the spectrum of Nova Persei 1901, and assigns 

 velocities of approach of— 1200 km , — 1600 km , and — 1850 knl , for three strongly 

 marked divisions. 



f W. H. Pickering, Astronomy and Astrophysics, vol. xiii. p. 201, 1894. 



