332 



KNOWLEDGE. 



September, 1912. 



criminatfly in every dirc-ction ; tliat is to say. 

 tlio investigation of tlie parallactic and proper 

 motions of the stars has hitherto proceeded on the 

 assumption that the projjcr motion of a star 

 is as likely to be in the direction of one of 

 three rectangular axes as in any other, and 

 consequently that, in a general average, the 

 components of motion arc equally distributed 

 between the three axes. If the assumption is 

 erroneous, the method fails, or at least requires 

 extensive modifications. It has now been definitelv 

 ascertained that the assumption is wrong and that 

 the stars arc not moving indiscriminately. In 1905, 

 Mr. F. W. Dyson announced tiie discovery of two 

 great star-drifts which have been traced in his sub- 

 sequent investigations through all parts of the sky. 

 The stars belonging to one of these drifts diverge 

 from R..\. = 270°, D= + 12°; and this drift has been 

 mainly influential in the derivation of the accepted 

 value of the solar apex. The other great drift 

 diverges from R..-V. = 83'\ D=:+60^. The pheno- 

 menon has the appearance of an interpenetration of 

 two great star streams coming from directions 110" 

 apart. 



As a consequence of this discoverv, we must con- 

 sider the possible alterations which it mav impose 

 upon the value derived for the solar motion. Stars 

 in a stream whose motion is opposed to the Sun's 

 movement, will open out more rapidly in the 

 direction of the Sun's advance than members of the 

 stream to which the Sun itself belongs. The latter 

 may be advancing together on nearly parallel lines 

 and with nearly equal velocities, and may thus 

 maintain relatively fixed distances from each other 

 and from the Sun, with an almost total absence of 

 projjCr motion, although relati\cly near to us. The 

 selection of stars having large proper motions as 

 suitable candidates for parallactic determination ma\' 

 have to be revised in the light of this new concep- 

 tion. If stars without sensible proper motion are 

 mvestigated for parallax with the same assiduit\- as 

 those of large proper motion, we may perhaps find 

 some near neighbours in this neglected group. 



The phenomenon to which Proctor gave the name 

 of "star-drift," has been known from a few isolated 

 groups. The conception must now be extended. 

 What we have done is to select a particular drift 

 among stars of large apparent motion, which seemed 

 to be somewhat general, namely, that in the direction 

 of the constellation Argo, and to assign its cause to 

 the motion of the Sun, and we call this the general 

 parallactic motion ; but there has been, until quite 

 recently, no way of deciding which of these move- 

 ments — that of the Sun in the direction of Lyra, or 

 that of the other stars in the direction of Argo — is 



the real one; and no matter whether the peculiarit) 

 is one of solar motion or of solar rest, it is the Sun's 

 own peculiar condition as compared w ith that of a 

 considerable body of stars. 



Now, seeing that other stars move in groups, is it 

 reasonable to suppose that our Sun has no com- 

 panions to share in a common drift ? There is a 

 great multitude of stars with scarcely any appreci- 

 able proper motion, including many stars which are 

 fairly bright. Is it necessary to suppose that these 

 are all at such a great distance that their proper 

 motions are insensible on this account ? May not a 

 great many of them fall into one common star-drift 

 with our Sun ? If so, it is the ".\rgonauts " that are 

 peculiar. Actually, the direction of stellar motion 

 away from the solar apex may not be the pre- 

 dominant one. We begin to have evidence of this 

 solar association. Professor Stroobant* has chosen 

 stars with moderate annual proper motions (0"-00 to 

 0"-08), namely, a Cassiopeiae, /3 Persei, a Persei, 

 a Scorpii, y Cygni, e Pegasi, a Pegasi and the Sun, 

 and by combining the parallax, the motion in the 

 line of sight and the proper motion, he gets a true 

 picture of their system in space and finds that these 

 eight bodies agree in the direction of motion within 

 a range of about 6°, while their velocities range 

 between 11-3 and 22 • 1 kilometres per second. 



Professor Lewis Boss finds evidence of a great 

 star drift towards R.A. = 95""-6, D=— 7°o + , to 

 which Mr. Benjamin Boss assigns a mean linear 

 velocity of ninety-five kilometres per second. I 

 understand that this investigation is merely a 

 preliminary one. Since it rests upon the supposition 

 that there is but one great drift, and that the 

 direction and amount of the solar motion is that 

 commonly assumed, the foundation becomes insecure, 

 as soon as divers other drifts are admitted to exist. 

 Mr. Benjamin Boss refers to the drift of "the 

 Taurus group in the general direction of the vertex 

 (inclined to it about 15°), and the Ursa Major group 

 inclined about 18" with the antivertex,"t but the 

 effect of these and other drifts upon the assigned 

 solar motion is not considered. 



Mr. H. C. Plummer gives preliminarv data for 

 several new star-drifts.ij He finds 



No. of Velocity 



Stars. km. /sec. R A Dec. 



Drift i. ... 22 ... 9-7 ... 65°-2 ... — 24''-4 



.. ii. ... 19 ... 37-9 ... S7°-0 ... + 7°-3 



„ iii. ... 16 ... 21-6 ... 106°-1 ... -52°-9 



„ iv. ... 13 ... 10-0 ... .il7 -2 ... -23°-6 



No. ii. agrees with .Mr. Boss's value for the 

 Taurus group which is shown to be " no mere 

 localized cluster, but contains members which 

 are scattered over the whole sky." No doubt 

 manv other drifts remain to be discovered. 



(To be continued. I 



* Bulletin Astronoiiiiquc, November 1910, Vol. XXVII, page 433. 



f Astronomical Journal, No. 629, page 33, November 20th, 1911. 



I Astronomical Journal, No. 629, page 33. 



§ Monthly Notices Royal Astronomical Society, Vol. LXXII, No. 3, page 170, January 1912. 



