134 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 5 



except for the special motions already mentioned, usually slight, 

 for which correction can be made.) All other stars — all that are not 

 in these four directions — both those farther from the center of the 

 s} 7 stem than we are and those closer to it, have more or less apparent 

 motion in the line of sight. Those having the greatest motion are 

 the stars halfway between the points of no motion. Therefore, at 

 45° along the Milky Way to each side of the center of the stellar 

 system and at 45° each side of the anticenter maximum radial motion 

 is indicated by the spectroscope. 



Hence, spectroscopic measurements of radial velocities of stars along 

 the Milky Way will show where the center is, around which the stars 

 revolve. It must be at one of the points of zero radial velocity, and 

 halfway between two points of maximum velocity. It is found to be 

 in the Milky Way in the direction of the great star clouds of Sagit- 

 tarius and Scorpio. The anticenter (180° from the center) is in the 

 Milky Way where the constellations of Taurus, Auriga, and Gemini 

 meet. A third point of interest is the one toward which the solar sys- 

 tem is moving. It is in the Milky Way also, and naturally is at right 

 angles to the line leading to the center. It is located in Cygnus, near 

 the star Deneb. 



The distance to the center, as well as the direction to it, can be found 

 from the star velocities given by the spectroscope. The principle upon 

 which this is done may be briefly stated. A star at the same distance 

 as the sun from their common center of revolution would, if fairly 

 near the sun, be almost in the tangent to the sun's orbit (that is, 90° 

 from their common center). But if the star is 6 or 8 thousand light- 

 years ahead of the sun, it would have turned in toward the center 

 several degrees from the tangent at the sun. At 1,000 light-years 

 distance it would be seen about 1° off the tangent, or 89° from the 

 center instead of 90°. Cepheid variables were measured out to a dis- 

 tance of about 8,000 light-years from the sun. The direction to a star 

 at that distance with no radial motion is not quite the same as the 

 direction to a closer star with no radial motion. There is a measurable 

 angle which gives the curvature of the sun's orbit. (See fig. 1.) 

 Joy's result with Cepheids and the results of others using different 

 classes of stars agree very well in giving about 33,000 light-years as 

 the distance of the solar system from the galactic center in Sagittarius. 



The diameter of the whole stellar system has been estimated at about 

 100,000 light-years; hence our place in the system is about two-thirds 

 of the way out from the center toward the edge of the Milky Way, 

 in the direction of the constellation Auriga and away from the con- 

 stellation Sagittarius. 



Another interesting value growing out of these spectroscopic meas- 

 urements is the velocity of sun and stars around the great central 

 nucleus of star clouds. 



