See — Temperature of the Sun and Ages of Stars and Nebulae. 43 



will go on rising * till a small radius is attained, and finally 

 when the dense mass, intensely hot, becomes incapable of 

 further shrinkage, from increase of resistance in molecular 

 forces, a cooling and liquefaction will rapidly take place. A 

 condition of darkness thus follows close upon a period of in- 

 tense brilliancy ; and hence the darkness of such bodies as the 

 companions of Sirius, Procyon, and Algol. 



Here the smaller masses, as in the solar system, have de- 

 veloped most rapidly. The theory of the ages of the stars 

 here adopted enables us to explain the colors and relative 

 masses of the double stars. On this point Kitter has gone 

 astray, by concluding that in double stars the companion, 

 usually bluish in color, has a larger mass than the principal 

 star, which is usually of a reddish or orange tinge. This view 

 is positively contradicted by the relations of the masses de- 

 termined from actual measurement in the cases of rj Cassi- 

 opeiae, Sirius, Procyon and, a Centauri, the only systems in 

 which the relation of the masses has been investigated. As 

 each of these systems is a typical double star, the rule of 

 assigning the fainter star the smaller mass — a mere inference 

 of common sense — will undoubtedly hold good generally. 

 And since the spectra of the companions of double stars are 

 generally of the first class, while those of the principal stars 

 are of the second class, the result also conforms to the theory 

 of the ages of the stars adopted above. 



Certain obscure companions of double stars recently dis- 

 covered by the writer in the southern hemisphere, as well as 

 the historical examples of Sirius and Procyon, lead him to 



* Proff 8sor Perry, of the Royal College of Science, London, has pointed 

 out in a letter to Sir Norman Lockyer {Nature, July 13th, 1899; reply by the 

 author in Nature of September 28th), some reasons for thinking that our 

 sun has long since passed the period of maximum temperature. He thinks 

 that after the central density exceeded one -tenth that of water, the mass 

 could no longer be considered a perfect gas; but it seems to the author that 

 Professor Perry regards too lightly the effect of the tremendous temperature 

 of the sun, which necessarily increases the " perfection " of the gas, and 

 perhaps to an enormous extent. He concurs however in the author's views 

 that old stars have their radiating layers near their surfaces, and that they 

 radiate more rapidly than young stars. The other interesting points sug- 

 gested by Professor Perry may be reserved for future research. 



