May 1, 1891.] 



KNO^VLEDGE 



91 



taken of the " spectroscopic binaries " discovered at 

 Harvard and Potsdam. There are very few misprints in 

 the work, but we must demur to the statement on page 9 

 that the resulting mean distance of the moon fi-om a 

 parallax of 57' 2" is 237,300 miles. As a matter of fact 

 the mean distance of the moon is about 238,840 miles, 

 the average distance is not the mean of the maximum and 

 minimum values of the distance, and a parallax of 57' 2" 

 would answer to a distance of about 238,900 miles. — H. S. 



THE PLEIADES CLUSTER, AND ITS PROBABLE 

 CONNECTION WITH THE MILKY WAY. 



By A. C. Ranyard. 



THE Pleiades lie a httle to the south of the MUky 

 Way, in a Ime with the Hyades — the three great 

 stars in the Belt of Orion, and Sirius, the 

 brightest star in the heavens. This striking 

 chain of jewels lies nearly parallel with the Milky 

 Way, just outside its southern border. It forms part of a 

 great belt or stream of bright stars first noticed by Sir John 

 Herschel,* and subsequently more closely studied by Dr. 

 B. A. Gould,+ which appears to gh-dlc the heavens very nearly 

 in a great circle that intersects the Milky Way at an angle of 

 about 20°, crossing it near the margin of the Southern Cross, 

 and in the northern hemisphere again crossing the Milky 

 Way in Cassiopeia. This stream of bright stars, like the 

 stream of milky light it crosses, is more striking in the 

 southern hemisphere than in the northern, and one can 

 hardly doubt its intimate connection with the stream of 

 smaller stars with which it appears to be associated. 



Of the twelve brightest stars in the heavens which rank 

 as of the first magnitude or brighter than the first magni- 

 tude, seven he in this brilliant girdle of stars, and three 

 are intimately associated with it, being situated only just 

 on the opposite border of the Milky Way. Taking the 

 stars Ln their order of brightness according to Professor 

 Pickering's photometric catalogue, they stand thus : — 

 1. Sirius, ranked as of the — 1'4 magnitude; 2. Arctuni,s, 

 O'O magnitude, that is, one magnitude brighter than the 

 first magnitude ; 3. Cdpella and Fi'//", both ranked as of 

 the 0-2 magnitude ; 5. yS Orionis, OS magnitude ; 6. Cuno- 



* Sir John Herschel says of it, in hla Kesu/ts of Astronomical Ob- 

 xervations made at the Cape of Good Hope, p. 385 : — " The medial 

 line of the Milky Way may be considered as crossed by that of the 

 zone of large otars which is marked out by the brilliant constellation 

 of Orion, the bright stars of Canis Major, and almost all the more 

 conspicuous stars oi Ar<jo, the Cross, the Centaur, Liipus and Scorpio. 

 A great circle passing through £ Orionis and a Cruris will mark out 

 the axis of the zone in question, whoso inclination to the galactic 

 circle is therefore about 20'^, and whose appearance would lead us to 

 suspect that our nearest neighbours in the sidereal system (if really 

 such) form part of a subordinate sheet or stratum deviating to that 

 extent from parallelism to the general mass which, seen projected on 

 the heavens, forms the Milky Way." 



t Prof. Gould says, in the branometria Arrjentina, p. 35o : — *• A 

 stream of especially conspicuous stars, which, beginning with Orion, 

 includes the brightest in Canis Major, Columlxi, Pnppis, Carina, 

 Crux, Centaurus, Lupus, and the head of Scorpius. In the northern 

 hemisphere its course is less distinctly marked, and it is especially 

 indistinct in Opiiiuchus and Hercules ; but its general direction is indi- 

 cated by the brightest stars in Taurus, Perseus, Cassiopeia, Cepheus, 

 Cytpius, and Lyra. This belt neems to bifurcate in a manner some- 

 what aiuilogous to the Milky Way. A well-marked branch diverges 

 from the main stream, not far from a Centauri, traversing the Galaxy, 

 which it spangles with the bright stars of Sagittarius and of the 

 tail of Scorpius, and passing through Atptila and Delphinus, reunites 

 with the principal belt in the north-preceding portion of Aiulromeila. 

 This bifurcation is likewise much less manifest in the northern hemi- 

 sphere thau in the southern, tho bright stars being more scattered, and 

 the course of the divided stream less distinctly traceable." 



jnis, Oi magnitude ; 7. Procyon, Q-o magnitude ; 8. 

 a Orionis, 09 magnitude ; 9. a Centauri, a double star, one 

 component of which is of the 1-0 magnitude and the other 

 3^ magnitude ; 10. Aldcbaran, a Aquilte, and a Eridani, 

 all three ranked as of the 1-0 magnitude. Of these stars 

 Aldehitran, a and /? Orionis, Siriux, Citnopus, a Centauri, 

 and Ve'/K he in the zone of brilhant stars alluded to above, 

 while i'apeUa, Pruci/nn, and a A'juilw lie, at no great dis- 

 tance, on the opposite border of the Mhky Way. Accord- 

 ing to Professor Gould, Capilla and a Acjuilm lie on a 

 well-marked branch of this great star-stream, which 

 bifurcates during half its course round the heavens simi- 

 larly to the Milky Way. 



At first sight, no doubt, it seems altogether improbable 

 that such large stars could be associated \rith very small 

 and apparently distant stars ; but the evidence afforded 

 by the Pleiades group, as well as by other clusters, shows 

 that one star difiers from another star in brightness, not 

 merely ten or a hundred tnues, but certainly as much as 

 a hundred thousand times, while the small streams and 

 closed curves of stars in the Milky Way contain stars 

 which differ several magnitudes in brightness, and yet fall 

 so accurately into lines and curves that we can hardly 

 doubt their physical association with one another. The 

 accompanying three small groups of stars have been cut 

 from an automatically-prepared block made from Mr. 



Barnard's jihotograph of the Saiiitturiu.s region. Such 

 closed curves of stars generally surround dark spaces, or 

 lie along the edges of dark channels. The great dark 

 arch shown in the photograph of the SiKjittariua region, 

 pubhshed in the July number of Knowledge last year, 

 seems to be a succession of such dark regions, each sur- 

 rounded by curves of stars. I only refer to them here as 

 showing that various magnitudes of stars seem to be 

 associated together in the brighter parts of the Milky 

 Way, and that we must not assume that there is not as 

 great a range of actual magnitudes in and about the Milky 

 Way as we shall see, there seems to be very httle doubt, 

 are associated together in the Pleiadej cluster. 



The Rev. John Miehell, in a paper published in the 

 Phil. Trans, for 17G7, seems to have been the first to 

 apply the doctrine of probabihty to the question whether 

 the Pleiades group corresponds to an actual cluster of 

 stars in space, all of which are relatively near together, 

 or whether it probably corresponds to a series of stars at 

 various distances from us, but all lying near to the same 

 line of sight. Considering the views then in vogue as to 

 the uniform distribution of stars with respect to the sun's 

 position in space, the paper is a very masterly one. He 

 concludes that there nuist be some physical connection 

 between the numerous double and triple stars which had 

 already been discovered by Sir William Herschel, but 

 which were not then known to be moving under the 

 influence of mutual gravity, and that such groups of 

 stars as the clusters in the sword-handle of Perseus and the 

 Pleiades must be actual clusters. Summing up, ho says : 

 " We may conclude with the higliest probability (the odds 

 against the contrary opinion being milUons to one) that 

 the stars are really collected in clusters in some places, 

 where they form akind of system, whilst in others there 

 are either few or none of them." 



The Pleiades group forms a very irregularly-shaped 



