362 
THE OUTSIDE OF A STAR 
Since H oc T 4 and k oc Pq\T' 2 
v oc g 
by (250-1). Hence at the reversing layer by (251-8) 
p G a: g$T * 
so that (251-91) becomes 
T$e-*№ T '/g$ = Tfe-V^/gJ (251-92), 
or approximately ^ 8 (log T) = 8 (log gr) (251-93). 
Using the values of g for the sun and Capella this gives T 1 fT 2 = 1-105. 
The effective temperatures are in the same ratio as the reversing layer 
temperatures, hence they are respectively: Sun 5740°, Capella 5200°. 
It is generally considered that the observed difference of temperature 
is of about this amount. In fact the temperature 5200° for Capella used 
throughout this book was an estimate from the observations, there being 
no theory available when the choice was made. 
The Chromosphere. 
252. We have seen that the thickness of the photosphere of the sun 
may be estimated at 15 km.; above this the density continues to diminish 
rapidly but there is no definite outer boundary to the distribution. In 
this region the conditions are practically isothermal, and the density 
therefore falls off exponentially with the height according to the well-known 
law for an isothermal atmosphere. The density decreases fourfold in the 
15 km. of photosphere, and it will continue to decrease at about this rate 
so that 150 km. higher it should be inappreciable. 
Now the flash spectrum observed at many eclipses shows that sufficient 
material to produce bright lines extends far above this limit. According 
to Evershed the H and K lines of calcium reach to a height of 8000 km.; 
Mitchell has traced them even to 14,000 km. Some of the hydrogen lines 
extend to 8500 km. above the limb. How is this material supported? 
It seems impossible that there can be any steady electric field, other 
than the weak field described in § 191 which prevents the electrons from 
diffusing apart from their ions. And indeed an electric field could not well 
support a material atmosphere which must necessarily contain almost 
identical numbers of positive and negative charges. The only possible 
explanation seems to be that this material is supported by radiation 
pressure. It consists of atoms on which the radiation pressure is exception 
ally strong, so that they are driven out from the photosphere and kept 
balanced at high level. The theory here described is due to E. A. Milne*. 
Why are certain elements selected to form this chromosphere 
* Monthly Notices, 84, p. 354; 85, p. 111.