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Binary stars are two stars that are in orbit around one another under their
mutual gravitational attraction.
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Close pairings of stars in the sky have been recognised since antiquity,
but most people thought they were just chance superpositions along the
line of sight of two unrelated stars.
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In 1767 the English clergyman John Michell realised that there are too many
doubles to be explained by chance and that in many cases the two stars must
actually be physically associated.
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In 1803, William Herschel discovered the first binary star - Castor in
Gemini. By 1821, he had completed the first major catalogue of binaries,
which contained 848 stars. Today, of order a hundred thousand binaries
are known.
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In 1991 and 1992, observing teams led by Michel Mayor and Geoffrey Marcy
surveyed the stars in the solar neighbourhood and concluded that nearly
half of all stars are binary and most have highly eccentric orbits.
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