ASTRONOMY: H. SHAPLEY 
461 
In the accompanying figure is given a comparative diagrammatic 
representation of some of these giant eclipsing double stars and also of 
the two systems of highest known density. The key to the diagram 
gives the data relative to densities and spectra. Making the reasonable 
assumption that the mass of the sun is sensibly equivalent to the mass 
of one component of any of the double stars, then the sun can be drawn 
to scale, as at the bottom of the diagram. If its mass is but one eighth 
that of a single component, its diameter in the diagram should be one-half 
as great; if eight times as massive as assumed, its diameter should be 
doubled. The true dimensions of the stars relative to the sun are prob- 
ably well within these Hmits. It is to be noticed that the components 
of five of the six binaries are elongated in the line of centers. 
Eclipsing binaries of low and high 
density. The mass of each component 
is assumed equal to that of the sun. 
a. SX Cassiopeiae; period 36^572; 
spectrum G3; density 0.0003. Reduced 
to one-half. 
b. RX Cassiopeiae; period 32^316; 
spectrum Ko; density 0.0005. Reduced 
to one-half. 
c. RZ Ophiuchi; period 261 *?9; spec- 
trum F8; density 0.001. Reduced to 
one-fourth. 
d. W Crucis; period 198*^5; spec- 
trum Gp; density 0.000002. Reduced 
to one-eighth. 
e. W Ursae Majoris; period 0'?334; 
spectrum G; density 1.84. 
f. U Pegasi; period 0^375; spectrum 
F?; density 0.75. 
oo O OO 
e The Sun f 
Summary. Because of its bearing on the question of the order of 
stellar evolution, the density of stars of the second spectral type has been 
discussed from the standpoint of the dependability of the observational 
and theoretical work that is the basis of the derivation of occasional 
extremely low values. The evidence discriminating between the two 
principal theories seems very definite, but further observational work 
would materially strengthen the case. The light curves of the variable 
stars involved, the relevant tabular data, and the derivation of certain 
density relations will appear in an early number of the Astrophysical 
Journal. 
