CHAPTERS ON THE STARS. 639 



the so-called 'new stars', which, at various periods in history, have 

 blazed out in the heavens, and then in a few weeks or months have 

 again faded away. It is a remarkable fact that no star of the latter 

 class has ever been known to blaze out more than once. This fact 

 distinguishes new stars from other irregularly variable ones. 



Periodic stars are those which go through a regular cycle of changes 

 in a definite interval of time, so that, after a certain number of days, 

 sometimes of hours, the star returns to the same brightness. But even 

 in the case of periodic stars, it is found that the period is more or less 

 variable, and, in special cases, the amount of the variation is such that 

 it cannot always be said whether we should call a star periodic or 

 irregular. 



The periodic stars show wide differences, both in the length of the 

 period and in the character of the changes they undergo. In most 

 cases they rapidly increase in brightness during a few days or weeks, 

 and then slowly fade away, to go through the same changes again at 

 the end of the period. In other cases they blaze up or fade out, from 

 time to time, like the revolving light of a lighthouse. Some stars are 

 distinguished more especially by their maximum, or period of greatest 

 brightness, while others are more sharply marked by minima, or periods 

 of least brightness. In some cases there are two unequal minima in 

 the course of a period. 



Chandler's third catalogue of variable stars gives the periods of 280 

 of these objects, which seem to have been fairly well made out. A 

 classification of these periods, as to their length, will be interesting. 

 There are, of periods: 



Less than 50 days 63 Stars. 



Between 50 and 100 days 6 



9 



18 



29 



40 



44 



44 



18 



6 



1 



It will be seen from this that, leaving out the cases of very short 

 period, the greater number of the periods fall between 300 and 400 

 days. From this value the number falls off in both directions. Only 

 three periods exceed 500 days, and of these the longest is 610 days. 

 We infer from this that there is something in the constitution of these 

 stars, or in the causes on which their variation depends, which limits 

 the period. This limitation establishes a well-marked distinction be- 



