Double stars are of two types: optical pairs (that is to say line-of-sight effects) and binaries (physically associated pairs). Binaries are much the more frequent. They range from contact pairs, where the components are almost or quite touching, to very distant pairs separated by at least a light-year. In a binary system the components move round their common centre of gravity. For visual binaries the shortest period is that of Wolf 630 Ophiuchi (1.725 years), but shorter periods are known: the record-holder is X-1820–303, an X-ray star in the globular cluster NGC 6623, distance 30 000 light-years. Its period is 685 s or 11 min. It was discovered in 1987 by the aptly named L. Stella and collaborators with the Exosat satellite. It is impossible to say which is the binary with the longest period, and all we can say is that very widely separated components share a common motion through space. Table 21.1 lists prominent double stars.

EARLY OBSERVATIONS

The term ‘double star’ was first used by Ptolemy, who wrote that η Sagittarii was ‘διπλυοζ’. There are of course several doubles which can be separated with the naked eye, so that presumably they have been known since antiquity; of these the most celebrated is Mizar (ζ Ursæ Majoris), which makes a naked-eye pair with Alcor (80 Ursæ Majoris). The Arabs described it – although they regarded Alcor as a rather difficult object. This is not true today, but it is most unlikely that there has been any real change.