September, 1911. 



KNOWLEDGE. 



345 



no means certain. Clusters and resoh'able nebulae 

 give spectra similar to those given by the stars 

 showing their starlike nature, whilst the irresolvable 

 nebulae, as we have just said, give either bright 

 lines or continuous characterless spectra. Thus a 

 real distinction of a physical nature may be drawn 

 between the different classes of these objects. (Dr. 

 Path, however, is of opinion that, notwithstanding 

 what has been just stated. '" no spiral nebula has a 

 realK' continuous spectrum." that the continuous 

 spectrum of the Andromeda nebula, or at least 

 that of its central portions, is given b}- " an 

 unresolved star cluster consisting of stars mainly 

 of the solar type.") Of late years photograph}- has 

 been successfullv applied to spectroscopic purposes 

 and spectrograms of many of the brighter stars and 

 clusters, and nebulae, have been obtained. .Amongst 

 remarkable clusters and groups of stars with com- 

 munity of proper motion, which seem to be con- 

 nected, in some as \et unknown wa'-, b\" a force 

 acting far more powerfully and at greater distances 

 than gravitation, in addition to the Pleiades which 

 we ha\e already discussed (see also a paper by the 

 present writer in " Knowledge" for July, 1907). 

 ma\" be mentioned the " tive stars " in Ursa Major, 

 which were pointed out by the late Mr. Proctor about 

 fort\- \"ears ago, as well as by Flammarion and others" 

 and also the Taurus cluster investigated by Professor 

 Lewis Boss. Drs. Ludendorff and Hertzsprung ha\e 

 shown that Sirius, the brightest star in our sky, is 

 a member of a famih' which includes (besides the 

 five stars, /3, y, o, e and <,' Ursae Majoris), also /3 

 Aurigae, 37 Ursae Majoris, o Leonis and a Coronae 

 Borealis. These ten stars appear to drift together, 

 Iving approximately in one plane and nearly in a 

 right line. In the Asfroiwniical Journal, Number 

 604, Professor Lewis Boss has discussed a moving 

 cluster in Taurus, consisting of man\- of the Hyades. 

 For more than twenty-five years he has been investi- 

 gating their motions and found these '" nearly 

 identical," at least fortv whose directions of motion 

 converge towards a common though \erv distant 

 point, " at which their apparent velocities will 

 enable them to arrive almost simultaneoush' after 

 the lapse of some sixty-five million years ! " At 

 present this " cluster " is spread over a total area 

 of 15° but is somewhat condensed towards the 

 centre. 



It has been estimated that the average velocit\' of 

 the entire cluster is about forty-six kilometres (sa\' 

 under thirty milesi per second, and the '"average" 

 parallax 0"-025 (Kiistner) giving a corresponding 

 distance from our system of eight hundred billions 

 of miles, thirty times that of the nearest star, .\lpha 

 Centauri. As a result of Professor Boss's work in 

 another direction, Mr. Eddington. of Greenwich, has 

 given an account of a " Moving Cluster of Stars of 

 the Orion Type in Perseus." Sixteen stars, Iving 

 between R.A."3" IP" and 5'' 15'" Dec. +42" to +58". 



share very nearly the same motion, both as regards 

 magnitude and direction, and thus form a group 

 similar in character to those above mentioned. Their 

 magnitudes range from 2-9 (e Persei) to 6-4, 

 and the mean " i)ro[)er motion " is about 4" per 

 centurw 



On clusters of stars in general, and especialh" on 

 the regular globular clusters it may not be without 

 interest to notice the remarks of the late Sir John 

 Herschel. who was the highest authorit}' on such 

 matters, and who had himself observed more of these 

 objects than any other astronomer of his day. " We 

 can hardl}- look upon such a group ... as not 

 forming a system of a peculiar and definite character. 

 Their round figure clearh' indicates the existence of 

 some general bond of union in the nature of an 

 attractive force : and in manv of them there is an 

 evident acceleration in the rate of condensation as w'e 

 approach the centre, which is not referable to a 

 merely uniform distribution of equi-distant stars 

 through a globular space, but marks an intrinsic 

 density in their state of aggregation, greater in the 

 centre than at the surface of the mass." The 

 stability of such a s\-stem " without a rotator\- 

 motion and centrifugal force," is scarcely con- 

 ceivable, but if we suppose a globular space 

 filled with equal, and very numerous stars, 

 attracting one another according to the gravi- 

 tation law, an}- one of these will be urged b\' 

 a resultant force directed towards the centre of the 

 sphere and proportional to its distance therefrom 

 (law of direct distance, Newton). Under such a 

 resultant force each individual star would describe a 

 jierfect ellipse about the centre of the s\-stem, 

 no matter in what plane it might lie. or in what 

 direction it might be moving. Thus it would not 

 be necessar}- that the cluster should rotate as 

 a mass around a single axis. Each ellipse 

 described by the separate members would remain 

 unchanged in form and dimensions, and all 

 would be described in one common period, so that 

 at the end of each such period every member of the 

 system would be in exacth' the same position with 

 regard to ever}' other, and would run the same round 

 for an indefinite succession of ages. Such a system, 

 whose members were sufficientl\- distant from one 

 another so that their orbits did not intersect, might 

 exist and realise a state of abstract and ideal harmony, 

 transcending even the harmonious and stable con- 

 ditions of our own Solar system, w ithout any pre- 

 ponderating "'Central Sun." However, it is pirobable 

 that such a condition of affairs is but rare in the 

 sidereal spaces ; though man}- almost perfecth- 

 globular clusters are known, others are of a more or 

 less oval outline, and }-et others of irregular figure. 

 These latter are also less definite in outline, so that 

 it is not always eas}' to sa}- w-here they terminate, 

 and the\" are not often condensed towards a 

 centre. B}- far the greater proportion are situated 



■■' This community of proper motion was named by Proctor " stardrift." The stars appear not only to be moving 

 together, but their chemical constitution as revealed by the spectroscope is similar. 



