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).