Less common in the telescope, but more spectacular and worth seeking out, are the multiple stars. Systems like P Mon, with its three pure-white gems within 7'', Z Cancri, of which more later, and i Cas (yellowish, bluish and bluish, according to Robert Burnham).
If multiple stars are to be stable over a long time scale then they need to follow a certain hierarchy. In the case of a single star orbiting a close pair, the ratio of the orbital periods of the outer star around AB to that of the inner orbit AB is usually at least 10:1. This appears to apply from periods of about ten days up to thousands of years.
Quadruple stars, of which the most famous is the "double-double", epsilon Lyrae can be ordered in two ways. Firstly, as in epsilon's case, there are two pairs each orbiting the common centre of gravity. Alternatively, a double star is orbited by a distant third star and then even more distantly a fourth star circles the whole group.
Systems of higher multiplicity are known - perhaps the most famous is the sextuple system Castor, which is described in more detail in Chapter 9. A recent catalogue of multiple stars3 lists 626 triples, 141 quadruples, 28 quintuples and ten sextuples. The existence of two systems thought to be septuple (v Scorpii and AR Cas) awaits confirmation of further suspected components.
The Trapezium, which to a small telescope user is four stars embedded in the Orion Nebula, is the prototype of another sort of multiple star. It is not strictly ordered like the quadruples such as epsilon Lyrae, but is more a loose aggregation and can be regarded more as a small star cluster than a multiple star as such. It is none the less beautiful for this and seen against the glowing green background of the nebula, on a cold winter's night in a good telescope, it is one of the sky's most spectacular sights.
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