The density of double-star systems 561 
The period of 8 Lyrae is relatively long, being 12°21" 47™ the 
orbit is sensibly eccentric, and the two spheroids are not so much 
elongated as was the case with RR Centauri. The mass of the system 
is enormous, one of the two stars being 10 times and the other 
21 times as heavy as our sun. 
Further illustrations of this subject might be given, but enough 
has been said to explain the nature of the conclusions which have 
been drawn from this class of observation. 
In my account of these remarkable systems the consideration of 
one very important conclusion has been purposely deferred. Since 
the light-curve is explicable by eclipses, it follows that the sizes of 
the two stars are determinable relatively to the distance between 
them. The period of their orbital motion is known, being identical 
with the complete period of the variability of their light, and an easy 
application of Kepler’s law of periodic times enables us to compute 
the sum of the masses of the two stars divided by the cube of the 
distance between their centres. Now the sizes of the bodies being 
known, the mean density of the whole system may be calculated. In 
every case that density has been found to be much less than the sun’s, 
and indeed the average of a number of mean densities which have 
been determined only amounts to one-eighth of that of the sun. 
In some cases the density is extremely small, and in no case is it 
quite so great as half the solar density. 
It would be absurd to suppose that these stars can be uniform in 
density throughout, and from all that is known of celestial bodies it 
is probable that they are gaseous in their external parts with great 
condensation towards their centres. This conclusion is confirmed by 
arguments drawn from the theory of rotating masses of liquid’. 
Although, as already explained, a good deal is known about the 
shapes and the stability of figures consisting of homogeneous incom- 
pressible liquid in rotation, yet comparatively little has hitherto been 
discovered about the equilibrium of rotating gaseous stars. The figures 
calculated for homogeneous liquid can obviously only be taken to 
afford a general indication of the kind of figure which we might 
expect to find in the stellar universe. Thus the dotted curve in 
Fig. 5, which exhibits one of the figures which I calculated, has 
some interest when placed alongside the figures of the stars in 
RR Centauri, as computed from the observations, but it must not be 
accepted as the calculated form of such a system. I have more- 
over proved more recently that such a figure of homogeneous liquid 
is unstable. Notwithstanding this instability it does not necessarily 
1 See J. H. Jeans, ‘‘On the density of Algol variables,” Astrophysical Journ. Vol. xxn. 
(1905), p. 97. 
D. 
36 
