CHAPTERS ON THE STARS. 145 



"being from birth to old age. In its infancy a star is simply a nebulous 

 mass; it gradually condenses into a smaller volume, growing hotter, as 

 set forth in the last chapter, until a stage of maximum temperature is 

 reached, when it begins to cool off. Of the duration of its life we can- 

 not form an accurate estimate. We can only say that it is to be reck- 

 oned by millions, tens of millions or hundreds of millions of years. We 

 thus view in the heavens stars ranging through the whole series from 

 the earliest infancy to old age. How shall we distinguish the order of 

 development? Mainly by their colors and their spectra. In its first 

 stage the star is of a bluish white. It gradually passes through white 

 into yellow and red. Sir William gives the following series of stars 

 as an example of the successive orders of development: 



Sirius, a Lyrae. 

 a Ursse Ma j oris. 

 a Virginis. 

 a Aouilae. 

 BigeL 

 a Cygni. 



Capella — The Sun. 



A returns. 

 Aldebaran. 

 a Orionis. 



The length of the life of a star has no fixed limit; it depends en- 

 tirely on the mass. The larger the mass, the longer the life; hence a 

 small star may pass from infancy to old age many times more rapidly 

 than a large one. 



A remarkable confirmation of this order is found in the generally 

 yellow or red color of the companions of bright stars in binary systems. 

 The two stars of such a system naturally commenced their life history 

 at the same epoch, but the smaller one, going through its changes 

 more rapidly, is now found to be yellower than the other. Additional 

 confirmation is afforded by the very great mass of the companions of 

 Sirius and Procyon, notwithstanding the faintness of their light. 



At the same time, up to at least the yellow stage, the star continu- 

 ally grows hotter as it condenses. A difficulty may here suggest itself 

 in reconciling this order with a well-known physical fact. As a radiat- 

 ing body increases in temperature, its color changes from red through 

 yellow to white, and the average wave length of its light continually 

 diminishes. We see a familiar example of this in the case of iron, 

 which, when heated, is first red in color and then goes through the 

 changes we have mentioned. The ordinary incandescent electric light 

 is yellow; the arc light, the most intense that we can produce by 

 artificial means, is white. When the spectrum of a body thus increasing 

 in temperature is watched, the limit is found to pass gradually from the 



VOL. LVIII.— 10 



