438 



SCIENCE 



[N. S. Vol. XLII. No. 1083 



may therefore hope for a rapid increase of 

 our knowledge of the stars within our im- 

 mediate neighborhood. 



VELOCITIES IN THE LINE OF SIGHT 



The determination of radial velocities 

 was initiated by Huggins in the early 

 'sixties, but trustworthy results were not 

 obtained until photographic methods were 

 introduced by Vogel in 1890. Since that 

 time further increase in accuracy has been 

 made, and the velocity of a bright star with 

 sharp lines is determinable (apart from a 

 systematic error not whoUy explained) 

 with an accuracy of | kilometer per second. 

 As the average velocities of these stars are 

 between 10 and 20 kilometers a second, the 

 proportional accuracy is of a higher order 

 than can be generally obtained in parallax 

 determinations or in other data of sidereal 

 astronomy. A number of observatories in 

 the United States and Europe, as well as in 

 South America, the Cape, and Canada, are 

 engaged in this work. Especially at the 

 Lick Observatory under Professor Camp- 

 bell's direction, the combination of a large 

 telescope, a well-designed spectroscope and 

 excellent climatic conditions have been 

 utilized to carry out a bold program. At 

 that observatory, with an offshoot at Cerro 

 San Christobal in Chile, for the observa- 

 tion of stars in the southern hemisphere, the 

 velocities of 1,200 of the brightest stars in 

 the sky have been determined. Among the 

 results achieved is a determination of the 

 direction and amount of the solar motion. 

 The direction serves to confirm the results 

 from proper motions, but the velocity is 

 only obtainable accurately by this method. 

 This quantity, which enters as a funda- 

 mental constant in nearly all researches 

 dealing with proper motion, is given by 

 Campbell at 19.5 kilometers per second, or 

 4.1 times the distance of the earth from 

 the sun per annum, though there is some 



uncertainty arising from a systematic error 

 of unknown origin. 



The observations of radial velocities have 

 shown within what limits the velocities of 

 stars lie and have given a general idea of 

 their distribution. The most important re- 

 sult, and one of a somewhat surprising char- 

 acter, is that the mean velocities of stars, 

 the motion of the sun being abstracted, in- 

 crease with the type of spectrum. Thus 

 the stars of type B, the helium stars, the 

 stars of the highest temperature, have 

 average radial velocities of only 6.5 kilo- 

 meters per second; the hydrogen stars of 

 type A have average velocities of 11 kilo- 

 meters per second; the solar stars of 15 

 kilometers per second; while for red stars 

 of types K and M it has increased slightly 

 more to 17 kilometers per second. Further, 

 the few planetary nebulae — i. e., condensed 

 nebuliE with bright line spectra — have aver- 

 age velocities of 25 kilometers per second. 

 There can be no question of the substantial 

 accuracy of these results, as they are closely 

 confirmed by discussions of proper motions. 

 They are, however, very difficult to under- 

 stand. On the face of it, there does not 

 seem any reason why stars of a high tem- 

 perature should have specially high veloc- 

 ities. A suggestion has been thrown out by 

 Dr. Halm that as the helium stars have 

 greater masses, these results are in accord- 

 ance with an equi-partition of energy. But 

 the distances of stars apart is so great that 

 it seems impossible that this could be 

 brought about by their interaction. Pro- 

 fessor Eddington suggests that the veloc- 

 ities may be an indication of the part of 

 space at which the stars were formed (e. g., 

 stars of small mass in outlying portions), 

 and represents the kinetic energy they have 

 acquired in arriving at their present posi- 

 tions. 



The stars the radial velocities of which 

 have been determined are, generally speak^ 



