systems. In Jonckheere’s “ Catalogue and Measures of Double Stars * ” 9’7 per 
cent, of the total of 3950 systems are either triple or still more complex. 
Roughly we may say that of every hundred stars in the sky, about 75 
are likely to be single stars, while the remaining 25 will be binary or multiple. 
And of these 25, some 2 or 3 are likely to be triple or multiple systems. 
26 . There is a continuous transition from the normal triple star, in which 
the periods are a few hundred years at most, through systems such as the triple 
system formed by a and Proxima Centauri, with periods running into millions 
of years, to systems such as the three well-known stars in Orion’s belt which 
are moving in one another’s company through space, although at such great 
distances from one another that their mutual gravitational attraction may 
almost be disregarded. We can pass still further and find groups of many 
more than three stars which are voyaging in company through space. Indeed 
the stars of Orion’s belt are only three members of a great party of such stars, 
which contains nearly all the bright stars in the constellation of Orion, with 
the conspicuous exception of the brightest of all, a Orionis (Betelgeux) which 
appears to be traversing space by itself. 
Most of the conspicuous groups of stars in the sky form parties of this 
kind which are travelling through space in company. The “ Great Bear ” in 
Ursa Major is perhaps the most obvious instance, although here again the 
brightest star of all, a Ursae Majoris is a solitary traveller and not a member 
of the party. As Hertzsprung and others have shewn, this group, or “moving 
cluster” to use the usual technical term, contains some twenty stars at least, 
Sirius almost certainly being a member. H. H. Turnerf found that the stars 
of this cluster form a much-flattened formation lying nearly in one plane. 
Another striking instance is provided by the Pleiades, the moving cluster 
containing all the stars which are visible to the naked eye, and many other 
fainter stars as well. The Hyades again form part of a huge moving cluster, 
the Taurus cluster. The principal stars in Perseus also belong to a clearly- 
defined cluster, and there are less clearly-defined clusters in Scorpio-Centaurus 
and Cygnus. Shapley’s “local system” (§15) may probably be regarded as 
forming a single huge moving cluster, and there is a further possibility that 
the great majority of stars in the neighbourhood of the sun belong either to 
this or to two inextricably intermingled moving clusters of enormous size and 
extent. Nearly all known clusters shew the characteristic flattened formation J. 
We shall consider the dynamics of moving clusters in a later chapter 
(Chap, xiv) in which we shall also discuss the general motions of the stars. 
* Memoirs of the Royal Astronomical Society, lxxi. (1917). 
t The Observatory , xxxiv. (1911), p. 246. 
+ N. H. Rasmuson, “A Research of Moving Clusters,” Lund Meddelanden, ii. No. 26 (1921).