Astronomy. — "Kvprnision of a cosmic i/'tssphere, the netn stars 

 (ind the Cepheids'. By Dr. A. Pannkkokk. (Communicated by 

 Prof. W. DE Sitter). 



(Communicated in the meeting of October 1918). 



I. 



The new stars wliicli are most fully known in their changes of 

 intensity are of two distinct types. The sudden quick flaming up is 

 common to l)0th : hut they differ in (heir further changes of light. 

 To the one class belong the two brightest Novae of this century, 

 Nova Persei 1901 and Nova Aquilae 1918, as well as Nova Coronae 

 1866. Immediately after attaining the greatest l)iilliance the light 

 begins to decrease quickly ; then the diminution becomes slower, 

 while a periodicity sets in. In the oilier class, of which Nova 

 Aurigae 1892 is the best known exauq^le — and to which 

 Tycho's star Nova Cassiopeiae 1572 belongs — the star retains 

 its brilliance for a long time, fluctuates irregularly, and finally 

 loses its brilliance rather i'a[)idly. These two types of change of 

 light show a certain correspondence with the (wo types of light- 

 maximum, long and short, which are observed, alternately in 

 the same star, in the Antalgol stars such as SSCygni. Whether this 

 analogy is more than an accidental correspondence, or that a real 

 relationship exists, cannot yet be ascertained. 



In connection with the appearance of Nova Aurigae Seeligkr has 

 given an explanation which fits the phenomena of this type very 

 well; when a star enters a nebulous mass, thereby being brought 

 to a high temperature, as long as it flies through denser and thinner 

 parts, its temperature will fluctuate up and down. This theory fits 

 the other type less well. Here there is obviously an enormous rise 

 of temperature, caused by a momentary event, of which all the 

 further processes are merely the consequences. 



The cause from which this sudden heating arises need not be 

 discussed here. We only put the question of what may be deduced 

 concerning (he further events, from simple hypotheses. A cosmic 

 body, suddenly brought to such a high temperature, will not be in 

 equilibrium. It will expand adiabatically, and as a consequence it 



