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61 Cygni

61 Cygni is a star in the constellation of Cygnus (the swan).

Though it is among the least conspicuous of stars visible in the night sky to an observer without an optical instrument, 61 Cygni attracted the attention of astronomers due to its large proper motion. This is so great, relatively speaking, that its apparent position shifts by an amount equal to the width of the full moon in a mere 150 years.

The large proper motion, greater than that of any other star visible with the naked eye, made 61 Cygni a candidate for the determination of its distance by the method of parallax when the quality of astronomical observations first made this possible. The star therefore has the distinction of being the first star (excluding the Sun) to have its distance determined. This was accomplished by Friedrich Wilhelm Bessel in 1838 who arrived at a figure close to the currently accepted value of 11.4 light years. Subsequent observations showed that, in addition to the closeness of the star, the high proper motion was attributable to its actual velocity relative to the Sun being large and almost transverse to our line of sight to the star.

From the point of the view of the casual astronomical observer, 61 Cygni is not particularly spectacular.

It is a widely separated binary system, composed of two K class (orange) main sequence stars, 61 Cygni A and 61 Cygni B.

The brighter is of apparent magnitude 5.2, the fainter 6.1. The two orbit their common center of mass in a period of 653.2 years.

An observer using 7x50 binoculars can find 61 Cygni two binocular fields south-east of the bright star Deneb. An observer using larger binoculars, or a telescope, will be able to resolve the two components of the binary.

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