VARIABLE STARS 
211 
for both effects when the light-curve is analysed and interpreted by the 
method of Russell and Shapley. 
For the moment we pass over the difficulty of translating a light-curve 
into a heat-curve, and suppose that the reflection effect can be determined 
in energy units. The theory is then very simple. A star necessarily re-emits 
the radiation incident on it. 
The solution for the interior of a star is determined by the differential 
equations together with a boundary condition; and we have seen (§93) 
that the latter is expressed with sufficient accuracy in the form “pq and 
p R both become less than 10 8 dynes per sq. cm.” The incident light from 
the companion star may contribute a radiation pressure of about 1 dyne 
per sq. cm. at the boundary. This gentle pat on the surface does not alter 
the boundary condition within the limits of accuracy required. Con 
sequently the solution for the interior is unaltered; in particular, the 
ordinary radiation L streams out from the interior unmodified. 
Accordingly, if we draw a sphere surrounding the star, there must be a 
net flow L outwards across the sphere. If a quantity L r from another star 
is flowing into the sphere, the gross outward flow must be L + L r . Thus 
the radiation from the star is increased by precisely the amount of the 
radiation incident on it. It is convenient to speak of the extra radiation 
as reflected, although the actual process is absorption with re-emission. 
In this sense a star is a perfect reflector of heat. 
It is probable that in these close binaries the components keep the 
same hemispheres turned towards each other. The state is then steady and 
the re-emission occurs from the hemisphere that is receiving the radiation. 
A relative rotation would introduce a lag between the incidence and the 
re-emission, time being needed for the newly-exposed surface to be heated 
up to the temperature required for the extra emission; but I do not think 
that the lag would be appreciable, since the changed conditions affect 
only a thin outer shell which has small heat capacity. 144 
144. The foregoing conclusion may be stated in the form that the heat- 
albedo of a star is 1. It is interesting to examine whether the observed 
reflection coefficients are in agreement with this result*. 
Let L x , L 2 be the ordinary heat emissions of the two stars S x , S 2 ; and 
let L r be the additional emission of the second star due to reflection. Let 
R x , R 2 be the radii and a the distance between the centres. The radiation 
of S x intercepted by S 2 is approximately 
if R 2 /a is not too large. Since this falls on one hemisphere only it has 
sometimes been assumed that (for albedo 1) S 2 is brightened at full phase 
7tR 2 2 . Lfl lira 2 
(144-1)