How far away are the stars? This question, pondered by our ancestors for thousands of years, was only answered during the first half of the 19th century. In 1838, German mathematician and astronomer Friedrich Wilhelm Bessel made the breakthrough calculation. The star that went down in history was 61 Cygni.
61 Cygni was a prime candidate for distance measurement due to its large proper motion. Italian Catholic priest, mathematician and astronomer Giuseppe Piazzi first measured this, in 1804. He studied the star over a period of 10 years and realised it was moving faster than any known star at that time. Christened the "Flying Star", Piazzi's measurements initially gained little attention until Friedrich Wilhelm Bessel published an article in 1812.
The method used to measure the distance of nearby stars is parallax, which involves using the Earth's orbit as a baseline. A star's positioned is accurately measured twice, 6 months apart, and then it's distance can be calculated from the angular displacement using trigonometry.
In the 1830's, there was intense competition between astronomers to be the first to measure a stellar parallax. In 1838, Bessel won the race announcing 61 Cygni had a parallax of 0.3136 arc seconds, corresponding to a distance from Earth of 10.4 light-years. He narrowly beat Friedrich Georg Wilhelm Struve and Thomas Henderson, who measured the parallaxes of Vega and Alpha Centauri respectively. In addition, Besel's calculation was remarkably accurate and a testament to his observation skills. For comparison, modern measurements place the star at 11.36 light-years distant.
With an apparent magnitude of +4.8, 61 Cygni is not a bright star. However, it's visible to the naked eye. This star is located in the constellation of Cygnus, about 8 degrees to southeast of first magnitude Deneb (α Cyg - mag. +1.25). From northern locations, it's ideally placed for observation during the summer months of June, July and August.
To find 61 Cygni, first locate Deneb at the tail of the Swan. Deneb forms the northernmost and faintest star of the well-known Summer Triangle asterism. Move 5 degrees southeast of Deneb to nu Cygni (ν Cyg - mag. +3.94). Then continue past this star, on a slightly more easterly path, for another 4 degrees. You will then arrive at two more 4th magnitude stars, τ Cygni and σ Cygni. These stars, along with nu Cygni form an isosceles triangle. 61 Cygni is located just over half way and a little south of a line connecting ν with τ Cygni.
61 Cygni is a beautiful double star consisting of two remarkably similar orange-red stars, separated by 30.7 arc seconds with a PA of 150 degrees. The primary component shines at magnitude +5.2, with the secondary at magnitude +6.0. Both stars are dwarf stars of spectral type K and slightly smaller and less luminous than the Sun. They are a wonderful sight when viewed through any telescope. Both stars dazzle against a velvet black sky, surrounded by a number of fainter white stars. They are easily split in 80mm (3.1-inch) refractors or even good 10x50 binoculars, especially on nights of excellent seeing.
As of today, only 6 stars have been measured with proper motions larger than the "Flying Star". All of them are much fainter than 61 Cygni. This is a beautiful double star that opened the door to stellar distance measurements, and provided confirmation that even our nearest stellar neighbours are extremely far away.
61 Cygni Data Table
|HD||201091 / 201092|
|Alternative Names||Piazzi's Flying Star / Bessel's Star|
|RA (J2000)||21h 06m 55s|
|DEC (J2000)||+38d 44m 45s|
|Parallax (milli-arcseconds / year)||287.18 ± 1.51|
|Distance (light-years)||11.36 ± 0.06|
|Other Designations||GJ 820 A/B, Struve 2758 A/B, ADS 14636 A/B, V1803 Cyg A/B, GCTP 5077.00 A/B|