Optical double stars are simply formed due to line-of-sight coincidence. They are usually widely separated (> 5'' or so) and the proper motions, or the individual motions in right ascension and declination, of each component, across the sky, are significantly different. In addition, the stars are usually unequally bright, reflecting the difference in distances but this by itself is not a criterion. A good example is 5 Herculis where the two stars were separated by more than 34'' at discovery by the elder Herschel in 1779, they closed up to about 8'.'8 in 1964 and are now at 11'' and widening (Figure 1.2a). Such pairs are usually of no direct scientific interest to astronomers but can produce some fine sights in small telescopes. The stars in 5 Herculis are, for instance, pale yellow and blue in colour and the primary is about 24 parsecs distant. Little is known about the companion.
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