CONSTITUTION OF THE STARS— EDDINGTON ]^33 



I want to leave time to speak of recent problems, so I will run over 

 rather briefly the older part of the subject. Let us suppose that by- 

 observation from outside we have ascertamed the mass M and the 

 radius R of a star — just those two data. Armed with this informa- 

 tion, what can we deduce (by laws of physics) about its interior? 



Tlie first difficulty is that, although we have ascertained the total 

 mass, we have not found how it is distributed — whether it is fairly 

 uniform throughout the volum.e of the star or strongly concentrated 

 to the center. I will not stop to explain how we have got over this 

 difficulty; but it is a side of the problem in which considerable progress 

 has been made in the last year or two. Although we cannot deter- 

 mine the concentration accurately, we can assign limits by purely 

 theoretical deduction. The central density is not less than 5 times 

 the mean density, and not more than 50 times the mean density — so 

 that we know roughly the degree of concentration that we are up 

 against. 



Knowing then how the mass is distributed in the structure we can 

 calculate the pressure at any depth. Any civil engineer will tell you 

 that that is possible ; so that we know the pressure as well as the den- 

 sity at each point in the interior. Now the density, pressure, and 

 temperature are connected by a relation called the equation of state 

 of the material; if any two of them are known we can find the third. 

 In this case we know the pressure and density and we can therefore 

 find the temperature — which is, of course, an extremely important 

 thing to find out, in order to realize the sort of conditions we have to 

 deal with. For all the stars except white dwarfs, the equation of 

 state, wliich connects the temperature with the pressure and density, 

 is the well-lmown equation of a perfect gas. For the extremely dense 

 matter in white dwarf stars the equation is more complicated; but 

 the theoretical physicist by his terrestrial studies has worked out for 

 us the required equation. (Incidentally he has worked it out wrong — 

 but that is another story, and I'll speak about the white dwarfs 

 later. For the present we will keep to the ordinary stars.) 



The internal temperatures determined in this way are of the order 

 10 to 20 million degrees Centigrade. Having ascertained this, we 

 begin to realize the state of things that we have to deal with. At 

 this temperature all the atoms will be highly ionized. Light elements 

 such as oxygen will be stripped bare to the nucleus, and heavy elements 

 such as iron and lead will retain only a few of the innermost satellite 

 electrons. The rest of the electrons will be free. We have therefore 

 to deal with a population consisting of free electrons, the shattered 

 remnants of atoms and photons or quanta of radiation. Planck's law 

 determines both the amount and kind of radiation present at a given 

 temperature. At 10 to 20 million degrees the radiation consists of 

 rather soft X-rays. 



