61 Cygni is a wide, easily-separated double star of two orange class K dwarfs, appearing in the south central part of Cygnus, the Swan. At only 11.4 light years distant, it is the twelfth nearest star system to our Solar System.
The stars are barely visible with the naked eye. Their color is probably best seen in binoculars, through which both stars are a stunning chrome orange-red. The two stars orbit each other with a period of 650 years, and will achieve maximum separation, 34", around 2100 A.D.
History
For its place in history, 61 Cygni is one of the most important stars in the sky. 61 Cygni was christened the "Flying Star" in 1792 by Giuseppe Piazzi for its unusually large proper motion. In 1830, Friedrich Georg Wilhelm von Struve made the first measurements of the star as a binary system. 61 Cygni was the first star system to have its distance successfully calculated using trigonometric parallax, by Friedrich Wilhelm Bessel in 1838.
Physical Properties
It is now known that 61 Cygni A and B orbit each other at an average distance of 86.4 AUs - more than twice Pluto's distance from the Sun - over a highly elliptical orbit (e=0.40) that brings them between 51.7 and 121.0 AU apart.
61 Cygni A is an orange-red main sequence star of spectral class K5 Ve. It contains about 70% of the Sun's mass, is 72% of its diameter, and about 8.5% of its luminosity. 61 Cygni A is a variable star with the designation V1083 Cygni. 61 Cygni B is an orange-red main sequence star of spectral class K7 Ve. This star has about 63% of the Sun's mass, 67% of its diameter, and 3.9% of its luminosity; it is also a suspected variable.
Dust has been detected around both stars in this binary system. As dimmer versions of sunlike stars, the two also have magnetic cycles similar to the Sun's - the brighter with a period of 8 years, and the fainter 11 years. As starspots move across the stars' surfaces, their brightnesses vary, yielding rotation periods of 35 days. Magnetically active, each star can also flare, causing a sudden brightening. The system contains only 79% of the Sun's abundance of elements heavier than hydrogen.
The system exhibits high-velocity galactic motion: 108 km/sec, over five times faster than the Sun moves relative to its neighbors, implying that 61 Cyg is a visitor to the solar neighborhood. All of this implies that the system is significantly older than our Sun.
Planets and Fiction
In 1942, Kaj Aage Gunnar Strand used photographic observations to detect small systematic variations in the orbit of 61 Cygni A and B, which suggested that there was a third, unseen body orbiting 61 Cygni A. In 1957, Strand reported 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 orbital distance of about 2.4 AU from 61 Cygni A. This unconfirmed third member of the system was often referred to as "61 Cygni C."
In 1978, however, Wulff Dieter Heintz reported his failure to find evidence that supports the existence of an object below about 60 Jupiter masses in the 61 Cygni system, suggesting that the earlier results were "spurious."
In Hal Clement's 1953 science fiction novel, "Mission of Gravity" is set on the planet Mesklin which revolves around 61 Cygni A. In the childrens' book "Danny Dunn and the Voice from Space" (1967), a modulated radio signal coming from 61 Cygni turns out to be a pictogram from aliens.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]