Fig. 405 shows a cluster in the _Centaur_, which, according to the same astronomer, is beyond comparison the richest and largest object of the kind in the heavens, the stars in it being literally innumerable. Fig. 406 shows a cluster in _Scorpio_, remarkable for the peculiar arrangement of its component stars. Star clusters are especially abundant in the region of the Milky-Way, the law of their distribution being the reverse of that of the nebulæ. Double and Multiple Stars. 347. _Double Stars._--The telescope shows that many stars which appear single to the naked eye are really _double_, or composed of a pair of stars lying side by side. There are several pairs of stars in the heavens which lie so near together that they almost seem to touch when seen with the naked eye. [Illustration: Fig. 407.] [Illustration: Fig. 408.] Pairs of stars are not considered double unless the components are so near together that they both appear in the field of view when examined with a telescope. In the majority of the pairs classed as double stars the distance between the components ranges from half a second to fifteen seconds. [Illustration: Fig. 409.] _Epsilon Lyræ_ is a good example of a pair of stars that can barely be separated with a good eye. Figs. 407 and 408 show this pair as it appears in telescopes magnifying respectively four and fifteen times; and Fig. 409 shows it as seen in a more powerful telescope, in which each of the two components of the pair is seen to be a truly double star. [Illustration: Fig. 410.] [Illustration: Fig. 411.] 348. _Multiple Stars._--When a star is resolved into more than two components by a telescope, it is called a _multiple_ star. Fig. 410 shows a _triple_ star in _Pegasus_. Fig. 411 shows a quadruple star in _Taurus_. Fig. 412 shows a _sextuple_ star, and Fig. 413 a _septuple_ star. Fig. 414 shows the celebrated septuple star in _Orion_, called _Theta Orionis_, or the _trapezium_ of Orion. 349. _Optically Double and Multiple Stars._--Two or more stars which are really very distant from each other, and which have no physical connection whatever, may appear to be near together, because they happen to lie in the same direction, one behind the other. Such accidental combinations are called _optically_ double or multiple stars. [Illustration: Fig. 412.] [Illustration: Fig. 413.] 350. _Physically Double and Multiple Stars._--In the majority of cases the components of double and multiple stars are in reality comparatively near together, and are bound together by gravity into a physical system. Such combinations are called _physically_ double and multiple stars. The components of these systems all revolve around their common centre of gravity. In many instances their orbits and periods of revolution have been ascertained by observation and calculation. Fig. 415 shows the orbit of one of the components of a double star in the constellation _Hercules_. [Illustration: Fig. 414.]