How far away are the stars? It's an extremely old question, pondered unquestionably by our ancestors for thousands of years but only during the first half of the 19th century was the true answer finally revealed. The German mathematician, astronomer Friedrich Wilhelm Bessel made the breakthrough calculation in 1838 and the star that went down in history was 61 Cygni.
Astronomer's attention was first attracted to 61 Cygni due to its large proper motion that was first demonstrated by Italian Catholic priest, mathematician and astronomer Giuseppe Piazzi in 1804. A stars proper motion is its angular change in position over time as seen from our solar system. Piazzi measured the proper motion of 61 Cygni over a period of 10 years, determining it to be the largest for any known star at the time, and subsequently christened it the "Flying Star". At the time Piazzi's measurements gained little attention and it was not until a publication by Friedrich Wilhelm Bessel in 1812 brought this star to widespread attention in astronomy circles.
The large proper motion made 61 Cygni a prime candidate for determination of its actual distance from Earth. The method that astronomers use to measure the distance of nearby stars is parallax. This involves using the Earth’s orbit as a baseline and then accurately measuring the position of a star at a certain time and repeating the measurement 6 months later. The distance to star is then determined by the angular distance the star has moved, the diameter of the Earth's orbit around the Sun (about 300 million km) and some basic trigonometry.
In the 1830s there was intense competition between astronomers to be the first to measure a stellar parallax accurately. In 1838 Bessel won the race, announcing that 61 Cygni had a parallax of 0.3136 arc seconds, which corresponds 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 later that same year. Besel's distance calculation was remarkably accurate and a testament to his observation skills. It is less than 10% different than the current modern day accepted parallax value of 0.28718 arc seconds, yielding a distance of 11.36 light-years.
With a visual mag. of only +4.8, 61 Cygni is not at all a bright star. However, it is visible to the naked eye under dark skies – the trick is knowing exactly where to look. As the name suggest, 61 Cygni is located in the constellation of Cygnus (the Swan), about 8 degrees to the SE of the brightest star in the constellation, first mag. Deneb. This makes it ideally placed for observation by Northern Hemisphere astronomers during the summer months of June, July and August when it appears high in the night sky.
To find 61 Cygni, first locate brilliant mag. 1.25 star Deneb (α Cygni) the tail of the Swan. Deneb forms the northernmost and faintest star of the famous Summer Triangle asterism with Vega and Altair being the other two. From Deneb move 5 degrees in a southeast direction to mag. 3.94, ν (Nu) Cygni. Continue on, but this time change to a slightly more easterly direction for another 4 degrees to arrive at two more 4th mag. stars, τ (tau) Cygni and σ (sigma) Cygni. These three stars form an isosceles triangle with 61 Cygni located just over half way and a little to the south of a line connecting ν to τ Cygni.
|HD||201091 / 201092|
|HIP||104214 / 104217|
|Alternative Names||Piazzi's Flying Star / Bessel's Star|
|RA (J2000)||21h 06m 55s|
|Dec (J2000)||+38d 44m 45s|
|Parallax (milliarcseconds/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|
Double / Variable star
61 Cygni is a beautiful double star, consisting of two remarkably similar orange-red stars separated by only 30.7 arc seconds and a PA of 150 degrees. The brighter component is of mag 5.2 and the fainter mag. 6.0. Both stars are orange red dwarf stars of spectral type K and are slightly smaller and less luminous than our Sun. They are wonderful sight when viewed through a telescope, with prominent deep orange colors set against a velvet black sky surrounded by a number of fainter white stars. They are easily split in a 80mm (3.1-inch) telescope or even with good 10x50 binoculars under nights of excellent seeing. Both components appear to be slightly variable.
As of today only 6 stars have been measured with larger proper motions larger than the "Flying Star", and all of them are visually much fainter. This famous beautiful double star opened the door to stellar distance measurements and provided confirmation that the distances to even the nearest stars are extremely enormous.