264 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1911. 



distance by parallax measurements, that we can obtain them, and 

 that we can hope to increase our knowledge of stellar systems. 

 Another interesting variable is u Herculis, whose orbital elements 

 were determined by Schlesinger at Allegheny from radial velocity 

 measurements with the spectroscope. He finds that the brighter 

 star is about 5,000,000 miles in diameter — six times our sun — is 7.5 

 times as massive but only one twenty-seventh as dense as the sun. 

 The fainter star is 2.9 times as massive but only one-seventieth as 

 dense. The parallax of this star is not known, but if it is as luminous 

 as Algol the brighter star must give out about 8,000 times as much 

 light as the sun. 



There has been a very marked advance in recent years in stellar 

 spectroscopy, particularly in the line of the determination of the 

 radial velocities of the brighter stars, and several observatories are 

 now engaged in this work. Accurate radial velocity measures were 

 first obtained by Prof. Campbell at the Lick Observatory in 1896 or 

 1897, and for many years he was practically the only one doing that 

 work. Campbell's work has been the determination of the radial 

 velocity of all stars in the sky, containing spectra with well measur- 

 able lines, which are brighter than the fifth visual magnitude. This 

 work is now practically completed, and a preliminary value of the 

 direction and magnitude of the sun's motion in space, with numerous 

 other interesting and valuable deductions, are just being published. 



In his work and that of Frost, of the Yerkes Observatory, who 

 is measuring the radial velocities of Orion type stars, many spec- 

 troscopic binaries — stars whose radial motion varies, and which are 

 hence accompanied by invisible companions, as distinguished from 

 visual binaries where both stars are seen — have been discovered, 

 and it is believed that not fewer than one in three of all stars must 

 have a companion of approximately the same size, thus eliminating 

 in these cases all possibility of a planetary system like our own. 

 Great advances have been made in determining the orbits, the char- 

 acter of the motion around one another, of these binaries, and the 

 two institutions most active in this line of work are the Allegheny 

 and the Dominion Observatories. Of the 70 spectroscopic binary 

 orbits determined, our observatory has obtained 16, which, con- 

 sidering that the aperture of its telescope is only half or less that 

 of others engaged in the work and that it has been established only 

 a comparatively short time, is a creditable showing. The great 

 strides made in the determination of spectroscopic binary orbits 

 has led to no less than three summaries of the results, containing 

 deductions of important conclusions from them, by Gampbell, 

 Schlesinger, and Ludendorff. I have not time to enter into the 

 results deduced from these discussions except to say that it was 

 shown that most binary systems probably originate from a revolving 



