166 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1916. 



troscopic studies of motion in the line of sight. As the two stars re- 

 volve about their common center of gravity they alternately eclipse 

 each other as seen from the earth. Of course the eclipse may be 

 either total or partial, according to the relative sizes of the two stars 

 and the inclination of their orbit to our line of sight. By a careful 

 study of the variation of brightness of these objects it is possible to 

 fix the period of revolution, the relative size of the two stars, the in- 

 clination of their orbit, and other data. This branch of astronomy 

 has been much investigated at the Observatory of Princeton Univer- 

 sity under Director Russell, and a most interesting publication of the 

 results has just been made by his pupil Shapley, now at Mount Wil- 

 son Solar Observatory. 



MASS AND SIZE OF STARS. 



The spectroscope shows, by noting the periodic variability of ve- 

 locities of stars in the line of sight, that about one-fourth of all the 

 visible stars are really double or multiple, though apparently single to 

 the telescope. So, for instance, Campbell found that the polar star is 

 probably triple. In cases where the stars are so wide apart that the 

 telescope can perceive them as separated, not only can the distance 

 of the stars from each other and from the earth be determined, but 

 also the combined mass of the pair in terms of the mass of the sun. 

 When there is no visible separation, the mass can be determined for 

 some cases in which the plane of the orbit is known. For a Centauri, 

 the nearest star to the sun, there is visible separation of two compo- 

 nents, which revolve in 81 years. The total mass is twice that of the 

 sun, and the two components being nearly equal, each is of about the 

 sun's mass. The two are separated about 23.6 times as far as the 

 earth is from the sun. The periods of revolution of double stars thus 

 far determined spectroscopically range from 4| hours to 90 years. 



From the photometric study of eclipsing binary stars it has been 

 shown by Roberts and by Russell that the average densities of these 

 stars is small, no more than one-eighth of that of the sun. On this 

 and other grounds astronomers are of the opinion that stars are gen- 

 erally less dense than the sun, that is that they occupy a larger 

 volume when of equal mass. The sun is only 1.4 times as dense as 

 water, or half as dense as glass, while our earth is 5.5 times as dense 

 as water, or 4 times as dense as the sun. 



THE NUMBER OF THE STARS. 



Stars are divided according to brightness in classes called magni- 

 tudes. First magnitude stars like Aldebaran are rare. A good ex- 

 ample of the second magnitude is Polaris. Stars as faint as the fifth 



