61 Cygni

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61 Cygni, sometimes called Bessel's Star, is a binary star system in the constellation Cygnus. It consists of a pair of orange dwarf stars—called a visual binary—that orbit (or revolve around) each other with a period of about 659 years. 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.

61 Cygni first attracted the attention of astronomers due to its large proper motion. In 1838, it became the first star whose distance from Earth, about 11 light years, was estimated by Friedrich Wilhelm Bessel. More recently, regular variations in the motion of the brighter component have been detected, which may indicate the presence of an invisible companion with about 1 percent the mass of the Sun, or 10 times that of the planet Jupiter.

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

Observation history

The large proper motion of 61 Cygni was first demonstrated by Giuseppe Piazzi in 1804, who christened it the "Flying Star". Piazzi's result, however, received little attention at the time due to the relatively short time span of his observations—a mere 10 years. It would take a publication by Friedrich Wilhelm Bessel in 1812 to bring this star to the widespread attention of astronomers.

Friedrich Georg Wilhelm von Struve first discovered it was a binary in 1830. For many years thereafter, however, there remained some uncertainty as to whether this pair was a mere juxtaposition of stars or an actual binary system.

The large proper motion for this system, 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 system therefore has the distinction of being the first star (excluding the Sun) to have its distance from Earth measured. This was accomplished by Bessel in 1838 who arrived at a parallax of 313.6 mas, close to the currently accepted value of 287.18 mas (yielding 11.36 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 unaided eye (although Groombridge 1830 at magnitude 6.4 can be seen with the naked eye under exceptionally dark skies). It has the seventh highest proper motion of all stellar systems listed in the Hipparcos Catalogue.

By 1911 Bessel's parallax of 0.3136 had only slightly improved to 0.310, and observations at Yerkes Observatory had measured its radial velocity as 62 km/s which together with its proper motion—transverse to our line of sight—of around 79 km/s yielded a space velocity of about 100 km/s towards a point about 12 degrees west of Orion's belt.

In 1911, Benjamin Boss published data indicating that the 61 Cygni system was a member of a comoving group of stars. This was group later expanded to include 26 potential members. Possible members include Beta Columbae, Pi Mensae, 14 Taurus and 68 Virginis. The typical space velocities of this group of stars is 105–114 km/s relative to the Sun.

Because of their wide angular separation (and correspondingly slow orbital motion), it was initially unclear whether the two stars in this system were physically connected. The respective parallax measurements of 0.360″ and 0.288″ gave a separation of more than two light years. However, by 1917 refined measured parallax differences had reduced the separation significantly, and the binary nature of this system was clear by 1934 with orbital elements being published.

Properties

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 appear to be 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 one of the nearest star systems to the Earth. 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.—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 is the slightly more massive of the pair. 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. (An earlier estimate gave a period of 7.3 years.) The combination of starspot activity combined with rotation and chromospheric activity is characteristic of a BY Draconis variable.

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.

An observer using 7×50 binoculars can find 61 Cygni two binocular fields south-east of the bright star Deneb. The angular separation of the two stars is slightly greater than the angular size of Saturn (16–20″). So, under ideal seeing conditions, the binary system can be resolved by a telescope with a 6 mm aperture. This is well within the capability of a typical pair of binoculars.

Possible low-mass companions

On several occasions it has been claimed that 61 Cygni has unseen low-mass companions, planets or a brown dwarf. Kaj Strand made the first such claim in 1942 using observations to detect tiny but systematic variations in the orbital motions of 61 Cygni A and B. These perturbations suggested that a third body was orbiting 61 Cygni A. In 1957 additional data allowed him to narrow his uncertainties, claiming that the object appeared to have about eight times the mass of Jupiter. With a calculated orbital period of 4.8 years, the estimated semi-major axis of 2.4 A.U. 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—equivalent to about 60 times the mass of Jupiter.

Because of the proximity of this system to the Sun, it is a frequent target of interest for astronomers. Both stars were selected by NASA as "Tier 1" targets for the proposed optical Space Interferometry Mission. This mission is potentially capable of detecting planets with as little as 3 times the mass of the Earth at an orbital distance of 2 A.U. from the star. Measurements of this system have detected an excess of far infrared radiation, beyond what is emitted by the stars. Such an excess is sometimes associated with a disk of dust, but in this case it lies sufficiently close to one or both of the stars that it has not yet been resolved with a telescope.

Notes

External links

61 Cygni 2. SolStation. Retrieved on 2007-07-16..
Kaler, James B. 61 Cygni. University of Illinois at Urbana-Champaign. Retrieved on 2007-07-16..