61 Cygni

61 Cygni, sometimes called Bessel’s Star or Piazzi’s Flying Star, is a binary star system in the constellation Cygnus. It consists of a pair of K-type dwarf stars that orbit each other in a period of about 659 years, forming a visual binary. At fifth and sixth apparent magnitudes, they are among the least conspicuous stars visible in the night sky to an observer without an optical instrument. It is around 11.4 light years from Earth.

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. Both are old-disk stars, with an estimated age that is older than the Sun. The system has a net space velocity of 108 km/s relative to the Sun, which results in the high proper motion across the sky. At a distance of just over 11 light years, it is the 15th nearest known star system to the Earth (not including the Sun). 61 Cygni A is the fourth nearest star that is visible to the naked eye for mid-latitude northern observers, after Sirius, Epsilon Eridani, and Procyon A. Since 1943, 61 Cygni A has served as a stable K5 V “anchor point” of the MK classification system. Starting in 1953, 61 Cygni B has been considered a K7 V standard star. This system will make its closest approach at about 20,000 CE, when the separation from the Sun will be about 9 light years.

The two orbit their common barycenter in a period of 659 years, with a mean separation of about 84 A.U. or 84 times the separation between the Earth and the Sun. The relatively large orbital eccentricity of 0.48 means that the two stars are separated by about 44 A.U. at periapsis and 124 A.U. at apoapsis.

Component A has about 11% more mass than component B. It has an activity cycle that is much more pronounced than the solar sunspot cycle. This is a complex activity cycle that varies with a period of about 7.5±1.7 years. The combination of starspot activity combined with rotation and chromospheric activity is characteristic of a BY Draconis variable. Because of differential rotation, this star’s surface rotation period varies by latitude from 27 to 45 days, with an average period of 35 days.

The outflow of the stellar wind from component A produces a bubble within the local interstellar cloud. Along the direction of the star’s motion within the Milky Way, this extends out to a distance of only 30 AU, or roughly the orbital distance of Neptune from the Sun. This is lower than the separation between the two components of 61 Cygni, and so the two most likely do not share a common atmosphere. The compactness of the astrosphere is likely due to the low mass outflow and the relatively high velocity through the local medium.

Component B displays a more chaotic pattern of variability than A, with significant short-term flares. There is an 11.7-year periodicity to the overall activity cycle of B. Both stars exhibit stellar flare activity, but the chromosphere of component B is 25% more active than for component A. As a result of differential rotation, the period of rotation varies by latitude from 32 to 47 days, with an average period of 38 days.

While no planets exist in this system a large disk of dust surrounds the stars that make up the core of the system.

61 Cygni

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