Double stars

Mizar and Alcor
   The first double star ever recorded—Mizar in the Big Dipper— was discovered accidentally in 1650 by the Italian astronomer Giovanni Battista Riccioli. Subsequent discoveries by other astronomers were also accidental. By 1779, enough observations had been compiled to inspire the indefatigable William Herschel (1738-1822) to begin a systematic search for these stellar curiosities. Two years later, he had discovered more than 800 new double stars, assessing each pair with a filar micrometer, a device that allowed him to precisely measure the separation and orientation of the components. Later measurements of these stars by Herschel and others revealed that some of them were, in fact, orbiting each other.
   The American astronomer S, W. Burnham (1838-1921) kicked off a new age of double-star discovertes in 1873 when he published a list of 81 new pairs he had found with his 6 inch (150 mm) refractor. Over the next four decades, this tireless, sharp-eyed observer discovered an additional 1,340 double stars using telescopes of various sizes. In 1906, his observations were collected in the General Catalogue of Double Stars.