THE HISTORY OF A STAR. 



79 



with ; we are practically dealing with a cooling globe of which 

 the exterior absorbing layers consist of hydrogen, iron, magne- 

 sium, and sodium. And now perhaps it will be obvious why I 

 was anxious in this general statement to begin as near as I could at 

 the beginning of things. It is only by going back in that way that 

 it is possible to explain this enormous development of hydrogen 

 in the hottest stars. We saw that first one or perhaps two un- 

 known substances together with hydrogen, carbon, magnesium, 

 manganese, lead, and iron wrote their record in the spectrum, 

 and that finally hydrogen was present in excess in the hottest 

 stars. By the phenomena of comets it has been demonstrated 

 that the radiant energy of our sun, and therefore the radiant 

 energy of all other masses of equal temperature to our sun, drives, 

 in all probability, everything of the nature of a permanent gas, 

 like hydrogen or carbon compounds, away from the center of the 

 system. Thus we may possibly explain the absence of oxygen and 

 carbon from the sun ; but hydrogen is present. The unknown sub- 

 stance or substances are concerned in most of the actions which 

 take place in the hottest parts of the sun, and they are always as- 

 sociated with hydrogen. In the atmospheres of the hottest stars, 

 again, hydrogen is enormously developed. Now that hydrogen, we 

 have reason to believe, can not have passed the cordon to which I 

 referred. The only supposition is that it and the unknown sub- 

 stances have as such been produced by the dissociation of the 

 chemical elements of which the meteoritic particles which have 

 formed the star in the manner I have indicated are composed. 

 Here, then, we have a series of facts which add very great proba- 

 bility to the idea which has been arrived at on other grounds, that 

 the chemical elements themselves are forms of hydrogen, or have 



a common origin. 



On the right-hand part of the temperature curve the hottest 

 state of things is represented at the top and the coolest at the bot- 

 tom, and we pass through groups IV, V, and VI. As the temper- 

 ature runs down, the hydrogen gradually disappears ; as this hap- 

 pens in a mass of gas, the temperature of which is gradually but 

 constantly reduced, we can only suppose that it is used to form 

 something else. We get association due to reduced temperature 

 in the same way that we get dissociation due to increasing tem- 

 perature. The sun is a star just about half-way down the descend- 

 ing side of the curve ; we know on other grounds that the sun is 

 cooling. 



The next part of the story is this : with decreasing hydrogen 

 we get gradually associated an increasing quantity of the metallic 

 elements (group V), and subsequently of carbon ; but now the car- 

 bon vapors are absorbing, they are not radiating in other words, 

 the spectrum includes dark bands instead of bright ones, as they 



