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61 Cygni, sometimes called Bessel's Star, is a star in the constellation Cygnus. 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. The star is in fact a double star system.

This system should not be confused with 16 Cygni, which includes a Sun-like star with a very eccentric planet.

61 Cygni's proper motion 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. It was christened the "Flying Star" by Giuseppe Piazzi in 1792. Friedrich Georg Wilhelm von Struve first discovered it was a binary in 1830.

This large proper motion, the largest known for any star at the time, 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.

Only a few years later, however, Groombridge 1830 was discovered to have a larger proper motion. 61 Cygni retains the distinction of having the largest proper motion of any star visible with the naked eye (although Groombridge 1830 at magnitude 6.4 can be seen with the naked eye under exceptionally dark skies).

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.

Although it appears to be a single star to the naked eye, 61 Cygni is in fact a widely separated binary system, composed of two K class (orange) main sequence stars, 61 Cygni A and 61 Cygni B. The brighter star 61 Cygni A is of apparent magnitude 5.2, the fainter 61 Cygni B is 6.1. The two orbit their common barycenter in a period of 653.2 years, with a mean separation of about 84 AU.

An observer using 7×50 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.

On several occasions is has been claimed that 61 Cygni has unseen low-mass companions, planets or a brown dwarf.

Kaj Aage Gunnar Strand made the first such claim in 1942 using observations from Potsdam, Lick, and Sproul observatories to detect small systematic variations in the way that 61 Cygni A and B orbit each other, which suggested that there was a third body orbiting 61 Cygni A^[1]. In 1957 additional data allowed him to narrow his uncertainties, claiming that the object appeared to have a mass about eight times that of Jupiter. Its orbital period was calculated to be 4.8 years which suggested an "average" orbital distance (semi-major axis) from 61 Cygni A of about 2.4 AUs – around the inner side of the main asteroid belt in the Solar System. Despite its low mass, and a calculated diameter less than one-tenth the solar value, this unconfirmed third member of the system was often referred to as "61 Cygni C."^[2] In 1978 Wulff Dieter Heintz suggested these claims were "spurious", having failed to detect any evidence of such motion down to six percent of the Sun's mass, about 60 Jupiter masses.^[3]

According to the Yale Bright Star Catalogue notes entry for HR 8085, later astrometric measures suggested the existence of three invisible companions with orbital periods of six, seven, and 12 years. However, the suspected objects were not resolved with speckle interferometry. Hence, the suspected planetary companions were "not substantiated."

Due to 61 Cygni's proximity to the Sun, the system has been an object of high interest among astronomers. Stars A and B have been selected as "Tier 1" target stars for NASA's optical Space Interferometry Mission. The mission will attempt to detect planets as small as three Earth-masses within two AUs of each star.

1. ^ 61 Cygni as a Triple System
2. ^ The orbital motion of 61 Cyg.
3. ^ Reexamination of suspected unresolved binaries

• List of nearest stars
• 61 Cygni in fiction

• HD 201091. SIMBAD. Retrieved on 14 April, 2006. (61 Cygni A)
• HD 201092. SIMBAD. Retrieved on 14 April, 2006. (61 Cygni B)
• 61 Cygni 2. SolStation. Retrieved on 14 April, 2006.
• Jim Kaler. 61 Cygni. Stars. Retrieved on 14 April, 2006.