How much we can find out about binary systems depends mostly on the separation of the two stars. Very wide pairs with rotation periods of many thousands of years yield little direct information, whilst close pairs with short periods and an orbital plane in the line of sight, thus producing eclipses, will allow many of the individual physical characteristics of the stars, such as mass, size and brightness, to be measured.
The most common type is the visual binary but this is simply due to the fact that these systems are near enough to us that we can resolve them optically. It is quite likely that during the next 10 or 20 years as more sophisticated satellites such as GAIA are launched, the number of binary stars known is likely to increase tremendously. This is to be expected since near the Sun we know that more than half our stellar neighbours are members of binary or multiple systems and there is no reason to suppose that this is just a phenomenon peculiar to this region of the Galaxy. At the time of writing the WDS catalogue contains more than 99,000 systems.
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