61 Cygni - Observation History

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 argued for its status as a binary in 1830. For many years thereafter, however, some uncertainty remained as to whether this pair was a mere juxtaposition of stars or a gravitationally bound system.

The system's 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 system therefore has the distinction of being the first star (excluding the Sun) to have its distance from Earth measured. This was accomplished in 1838 by Bessel, 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 to the unaided eye (although Groombridge 1830 at magnitude 6.4 can be seen with the naked eye under exceptionally dark skies). 61 Cygni 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 group was later expanded to include 26 potential members. Possible members include Beta Columbae, Pi Mensae, 14 Tauri 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 the 61 Cygni 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 demonstrated that the separation was significantly less. The binary nature of this system was clear by 1934, and orbital elements were published.

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 viewing 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.