54 ILLINOIS ACADEMY OF SCIENCE. 



of the series. They may be called blue stars, which precede the 

 white stars in the sequence. Inasmuch as the helium found in 

 the earth is attributed to the radium and uranium which are in 

 a sense its parents, it might be regarded as a fair inference that 

 radium must also be responsible for the abundance of helium 

 in spectra of this type. I do not regard this as a necessary 

 assumption, however, inasmuch as helium is a gas in some 

 respects similar to hydrogen, and perhaps we are therefore not 

 obliged to account for its presence as due to some other element 

 any more than we are in the case of hydrogen. 



It should be pointed out here that if radium were present in 

 the sun or stars, and were giving out the alpha, beta, and gamma 

 rays, it is unlikely that they would be able to penetrate either the 

 atmosphere of the sun or star, or the atmosphere of the earth. 

 As I understand it, the alpha rays penetrate but a few inches in 

 air at atmospheric pressure ; and the beta rays are soon absorbed. 

 The gamma rays may readily be passed through a block of iron 

 one foot thick, but the thickness of the reversing layer in the sun, 

 while regarded as a very narrow stratum, will be for iron vapor 

 probably at least two hundred miles, and for the gaseous elements 

 many times this, up to five thousand miles or more. Hence it is 

 clear that none of these rays would be expected to penetrate even 

 the thinnest stratum in the atmosphere of sun or star. 



It is an interesting fact that helium is found with hydrogen in 

 the spectrum of the temporary stars, or novae, which flash out 

 occasionally in the sky. We now know that they are not very 

 uncommon, and perhaps, if we could observe them all, there 

 would be found at least one (or more) each year. For instance, 

 a nova was discovered in Laccrta on December 30th last. It has 

 been shown that it was previously a star as faint as the thirteenth 

 or fourteenth magnitude. It suddenly increased probably a hun- 

 dred times in brightness and was of the seventh magnitude when 

 it was first observed. On February 21, 1901, there suddenly 

 developed a brilliant nova in Perseus, which was for a few days 

 the brightest star in the northern sky. Various hypotheses of 

 collision have been advanced to account for the extraordinarily 

 sudden development of these stars, but it seems to me that we 

 ought to take into account the possibihty that some of the immense 

 amount of energy which the study of radio-activity has shown 

 to be present in the molecules of even the light gases has suddenly 

 been released. We do not dare to say offhand how vast the 



