DSO of the Month
61 Cygni
AKA: STF 2758; Piazzi’s flying star. Position: 21 hr 06.9 min 38 44 58 Due south at 02:31 (BST) on 16 July 2020.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus) Used with permission
In the star of the month for June 2020, I discuss stars that move fast across the sky, or more precisely stars with a high proper motion. The fastest is Barnard’s star which is the star of the month for June 2020, but before the discovery of the high proper motion of Barnard’s star in 1916, 61 Cygni was one of the fastest stars known. The rapid motion of 61 Cygni was discovered by Giuseppe Piazzi of Palermo Observatory in 1804 and the German astronomer Friedrich Bessel measured its distance as 10.3 light years (it is actually 11.4 light years) in 1838. Thomas Henderson, His Majesty’s Astronomer at the Cape (Cape of Good Hope in South Africa), was concurrently measuring the parallax of Alpha Centauri but he resigned his position and returned home to Scotland on the grounds of ill-health in 1833, and did not publish his measurements until 1839. However our focus here is on 61 Cygni as a double star. James Bradley, the Astronomer Royal, observed 61 Cygni as a double star on 25 September 1753, making it one of the first relatively close double stars to be noted. It was also extensively studied by William Herschel, who hoped to use double stars to measure parallax on the assumption that they would nearly all be optical doubles, which soon turned out not to be the case.
61 Cygni is a binary and the two main stars are in fact part of an 18 star system, but most of the other members are too faint to be easily seen in a small telescope. It is also a member of a co-moving group of around 26 stars, which means it is similar to Mizar (double star of the month for May 2020). The primary and the brightest secondary are fairly dim but easily seen in a telescope (5.2 and 6.1), and the separation is a comfortable 31.8 arcseconds, which means it can be split by 15 x 70 binoculars as well as by a small telescope. The key visual aspect of this double is its colour. I tend to see stars as yellow rather than orange, but both stars in 61 Cygni are a gorgeous tangerine orange as they are both K type stars. It is roughly half way between Vega (Alpha Lyrae) and Scheat (Beta Pegasi, one of the corners of the Square of Pegasus) and forms a parallelogram with Deneb (Alpha Cygni), Sadr (Gamma Cygni) and Gienah (Epsilon Cygni). In binoculars or a finderscope it is easily spotted by its vivid orange colour.