Editing 61 Cygni

{{short description|Binary star system in the Cygnus constellation}}
{{Use dmy dates|date=March 2019}}
{{good article}}
{{Starbox begin
| name = 61 Cygni
}}
{{Starbox image
 | image=
{{Location mark
|image=Cygnus constellation map.svg|alt=|float=center|width=280
|label=|position=right
|mark=Red circle.svg|mark_width=10|mark_link=61 Cygni
|x=198|y=432
}}
|caption=Location of 61 Cygni (circled)
}}
{{Starbox observe 2s
| epoch = J2000.0
| constell = [[Cygnus (constellation)|Cygnus]]
| component1 = 61 Cygni A
| ra1 = {{RA|21|06|53.9396}}<ref name="vallenari-2023a" />
| dec1 = {{DEC|+38|44|57.902}}<ref name="vallenari-2023a" />
| appmag_v1 = 5.21<ref name="fischer-2019" />
| component2 = 61 Cygni B
| ra2 = {{RA|21|06|55.2638}}<ref name="vallenari-2023b" />
| dec2 = {{DEC|+38|44|31.359}}<ref name="vallenari-2023b" />
| appmag_v2 = 6.05<ref name="the internet stellar database-2011" />
}}
{{Starbox character
| component1 = 61 Cyg A
| class = K5V<ref name="fischer-2019" />
| b-v = +1.139<ref name="catalano s-1979" />
| u-b = +1.155<ref name="catalano s-1979" />
| variable = [[BY Draconis variable|BY Dra]]<ref name="simbad" />
| component2 = 61 Cyg B
| class2 = K7V
| b-v2 = +1.320<ref name="catalano s-1979" />
| u-b2 = +1.242<ref name="catalano s-1979" />
| variable2 = [[Flare star]]<ref name="simbad-2" />
}}
{{Starbox astrometry
| component1 = 61 Cygni A
| radial_v = {{val|-65.97|0.12}}<ref name="vallenari-2023a" />
| prop_mo_ra = {{val|4164.209|fmt=commas}}
| prop_mo_dec = {{val|3249.614|fmt=commas}}
| pm_footnote = <ref name="vallenari-2023a" />
| parallax = 285.9949
| p_error = 0.0599
| parallax_footnote = <ref name="vallenari-2023a" />
| absmag_v = 7.506<ref name="kovtyukh-2003" />
| component2 = 61 Cygni B
| radial_v2 = {{val|-64.59|0.12}}<ref name="vallenari-2023b" />
| prop_mo_ra2 = {{val|4105.976|fmt=commas}}
| prop_mo_dec2 = {{val|3155.942|fmt=commas}}
| pm_footnote2 = <ref name="vallenari-2023b" />
| parallax2 = 286.0054
| p_error2 = 0.0289
| parallax_footnote2 = <ref name="vallenari-2023b" />
| absmag_v2 = 8.228<ref name="kovtyukh-2003" />
}}
{{Starbox visbin
| reference = <ref name="hartkopf" />
| name = 61 Cygni B
| period = {{nowrap|678 ±34}}
| axis = {{nowrap|24.272 ±0.592}}
| eccentricity = {{nowrap|0.49 ±0.03}}
| inclination = {{nowrap|51 ±2}}
| node = {{nowrap|178 ±2}}
| periastron = {{nowrap|1709 ±16}}
| periarg = {{nowrap|149 ±6}}
}}
{{Starbox detail
| source = <!--[source url]-->
| component1 = 61 Cygni A
| mass = 0.70<ref name="staff-2007b" />
| radius = {{nowrap|0.665 ±0.005}}<ref name="kervella-2008" />
| gravity = 4.40<ref name="luck-2005" />
| luminosity = {{nowrap|0.153 ±0.01}}<ref name="kervella-2008" />
| temperature = {{nowrap|4,526 ±66}}<ref name="van belle-2009" />
| metal_fe = –0.20<ref name="luck-2005" />
| rotation = {{val|35.54|0.47|ul=d}}<ref name="olspert-2018" />
| age_gyr = {{nowrap|6.1 ±1}}<ref name="kervella-2008" />
| component2 = 61 Cygni B
| mass2 = 0.63<ref name="staff-2007b" />
| radius2 = {{nowrap|0.595 ±0.008}}<ref name="kervella-2008" />
| gravity2 = 4.20<ref name="luck-2005" />
| luminosity2 = {{nowrap|0.085 ±0.007}}<ref name="kervella-2008" />
| temperature2 = {{nowrap|4,077 ±59}}<ref name="van belle-2009" />
| metal2_fe = –0.27<ref name="luck-2005" />
| rotation2 = {{val|34.55|0.57|ul=d}}<ref name="olspert-2018" />
| age_gyr2 = {{nowrap|6.1 ±1}}<ref name="kervella-2008" />
}}
{{Starbox catalog
| names=[[Gliese-Jahreiss catalogue|GJ]] 820 A/B, Struve&nbsp;2758, ADS&nbsp;14636, V1803&nbsp;Cygni, [[General Catalogue of Trigonometric Parallaxes|GCTP]]&nbsp;5077.00<ref>{{Cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=61+cygni&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |title=SIMBAD Query Result: ADS 14636 AB -- Double or multiple star |website=SIMBAD |publisher=Centre de Données astronomiques de Strasbourg |access-date=3 February 2019 |archive-date=1 February 2014 |archive-url=https://web.archive.org/web/20140201202817/http://simbad.u-strasbg.fr/simbad/sim-id?Ident=61+Cygni&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id |url-status=live }} (61 Cygni)</ref>
| component1 = 61 Cygni A
| names1 = [[Variable star designation|V1803]]&nbsp;Cygni, [[Henry Draper catalogue|HD]]&nbsp;201091, [[Hipparcos Catalogue|HIP]]&nbsp;104214,
[[Harvard Revised catalogue|HR]]&nbsp;8085, [[Bonner Durchmusterung|BD]]+38°4343, [[Luyten Half-Second catalogue|LHS]]&nbsp;62, [[Smithsonian Astrophysical Observatory|SAO]]&nbsp;70919<ref name="simbad" />
| component2 = 61 Cygni B
| names2 = [[Henry Draper catalogue|HD]]&nbsp;201092, [[Hipparcos Catalogue|HIP]]&nbsp;104217, [[Harvard Revised catalogue|HR]]&nbsp;8086, [[Bonner Durchmusterung|BD]]+38°4344, [[Luyten Half-Second catalogue|LHS]]&nbsp;63<ref name="simbad-2" />
}}
{{Starbox reference
| Simbad = *+61+Cyg|sn=The system
| Simbad2 = TYC+3168-2800-1|sn2=A
| Simbad3 = TYC+3168-2798-1 |sn3=B
}}
{{Starbox end}}

'''61 Cygni''' {{IPAc-en|ˈ|s|ɪ|ɡ|n|i}} is a [[binary star]] system in the [[constellation]] [[Cygnus (constellation)|Cygnus]], consisting of a pair of [[K-type main-sequence star|K-type dwarf stars]] that orbit each other in a period of about 659&nbsp;years. Of [[apparent magnitude]] 5.20 and 6.05, respectively, they can be seen with binoculars in city skies or with the [[naked eye]] in rural areas without light pollution.

61 Cygni first attracted the attention of astronomers when its large [[proper motion]] was first demonstrated by [[Giuseppe Piazzi]] in 1804. In 1838, [[Friedrich Bessel]] measured its distance from [[Earth]] at about 10.4&nbsp;[[light-year]]s, very close to the actual value of about 11.4&nbsp;light-years; this was the first distance estimate for any star other than the [[Sun]], and first star to have its [[stellar parallax]] measured. Among all stars or stellar systems listed in the modern ''[[Hipparcos Catalogue]]'', 61 Cygni has the sixth-highest proper motion, and the highest among all visible stars or systems.<ref group="note">By convention, limiting visual magnitude of 6.0. As noted in a 2025 update to ESA's Top 20 High Proper Motion page, HIP 67593 had a spuriously high proper motion in Hipparcos, corrected by Gaia.</ref><ref name=":1">Hipparcos: Catalogues: The Millennium Star Atlas: [https://www.cosmos.esa.int/web/hipparcos/high-proper-motion The Top 20 High Proper Motion] {{Webarchive|url=https://web.archive.org/web/20250222233702/https://www.cosmos.esa.int/web/hipparcos/high-proper-motion|date=22 Feb 2025}}, [[European Space Agency]], retrieved 2025-02-22</ref>

Over the course of the twentieth century, several different astronomers reported evidence of a massive [[exoplanet|planet]] orbiting one of the two stars, but recent high-precision [[radial velocity]] observations have shown that all such claims were unfounded.<ref>{{cite journal
 | author=Wittenmyer, R. A.
 | author2=Endl, M.
 | author3=Cochran, W.D.
 | author4=Hatzes, A.
 | author5=Walker, G. A. H.
 | author6=Yang, S. L. S.
 | author7=Paulson, D. B.
 | date=2006 | title=Detection limits from the McDonald Observatory planet search program
 | journal=Astronomical Journal | volume=132
 | issue=1
 | pages=177–188 | doi=10.1086/504942 | bibcode=2006AJ....132..177W|arxiv = astro-ph/0604171 | s2cid=16755455
 }}</ref> No planets have been confirmed in this stellar system to date.

== Name ==
61 Cygni is relatively dim, so it does not appear on ancient star maps, nor is it given a name in [[List of Arabic star names|western]]<ref>{{cite book|last=Allen|first=Richard Hinckley|title=Star Names and Their Meanings|page=219|isbn=978-0-7661-4028-8|date=2003|publisher=Kessinger}}</ref> or [[Traditional Chinese star names|Chinese systems]].<ref>{{cite book|last1=Sun|first1=Xiaochun |first2=Jacob |last2=Kistemaker |title= The Chinese Sky During the Han: Constellating Stars and Society |url=https://books.google.com/books?id=87lvBoFi8A0C |publisher= Brill |date=1997 |isbn= 978-90-04-10737-3|bibcode=1997csdh.book.....S }}</ref>

The name "61 Cygni" is part of the [[Flamsteed designation]] assigned to stars. According to this designation scheme, devised by [[John Flamsteed]] to catalog his observations, stars of a particular constellation are numbered in the order of their [[right ascension]], not in Greek letters as the [[Bayer designation]] does.<ref>{{Cite web |url=http://www.iau.org/public/themes/naming/ |title=Naming Objects Outside the Solar System-Stars |publisher=IAU |access-date=3 February 2019 |archive-date=25 July 2021 |archive-url=https://web.archive.org/web/20210725053113/https://www.iau.org/public/themes/naming/ |url-status=live }}</ref><ref>{{Cite web |url=http://stars.astro.illinois.edu/sow/61cyg.html |title=61 Cygni |last=Kaler |first=Jim |date=8 July 2009 |website=Stars |access-date=3 February 2019 |archive-date=21 January 2019 |archive-url=https://web.archive.org/web/20190121063441/http://stars.astro.illinois.edu/sow/61cyg.html |url-status=live }}</ref> The star does not appear under that name in Flamsteed's ''[[Historia Coelestis Britannica]]'',<ref>{{cite book |first=John |last=Flamsteed |title=Historia Coelestis Britannica |url=https://books.google.com/books?id=CWFDAAAAcAAJ&q=61 |publisher=Meere |date=1725 |page=5}}</ref> although it has been stated by him that 61 Cygni actually corresponds to what he referred to as 85 Cygni in the 1712 edition.<ref>{{cite book |first=Richard |last=Dibon-Smith |title= The Flamsteed Collection |url=https://books.google.com/books?ei=UCdFVbivGe21sASToYDQCA |publisher= Clear Skies |date=1998 |page=xi}}</ref> It has also been called "Bessel's Star" or "Piazzi's Flying Star".<ref>{{Cite web |url=http://news.sky-map.org/starview?object_type=1&object_id=2282&object_name=HD+201091&locale=EN |title=61 Cyg (Piazzi's Flying Star) |website=Science&Space News |archive-url=https://web.archive.org/web/20190204014358/http://news.sky-map.org/starview?object_type=1&object_id=2282&object_name=HD+201091&locale=EN |archive-date=4 February 2019 |access-date=20 February 2019}}</ref><ref>{{Cite book |url=https://archive.org/details/celestialobjects00covi |url-access=registration |page=[https://archive.org/details/celestialobjects00covi/page/209 209] |title=Celestial Objects for Modern Telescopes: Practical Amateur Astronomy |last=Covington |first=Michael |date=26 September 2002 |publisher=Cambridge University Press |isbn=978-0-521-52419-3}}</ref>

== Observation history ==

=== Early observations ===
The first well recorded observation of the star system using optical instruments was made by [[James Bradley]] on 25 September 1753, when he noticed that it was a double star. [[William Herschel]] began systematic observations of 61 Cygni as part of a wider study of binary stars. His observations led to the conclusion that binary stars were separated enough that they would show different movements in [[parallax]] over the year, and hoped to use this as a way to measure the distance to the stars.<ref name="hopkins-1916">{{Cite journal |last=Hopkins |first=Mary Murray |date=1 November 1916 |title=The Parallax of 61 Cygni | bibcode=1916JRASC..10..498H |journal=Journal of the Royal Astronomical Society of Canada |volume=10 |pages=498–504}}</ref>

[[File:61 Cygni Proper Motion.gif|thumb|left|61 Cygni showing proper motion (movement from our vantage point) at some early 21st century one-year intervals.|alt=GIF showing the proper motion of the stellar system, taken about in an interval of an year for the range from 2012 to 2020.]]

In 1792, [[Giuseppe Piazzi]] noticed the high proper motion when he compared his own observations of 61 Cygni with those of Bradley, made 40 years earlier. This led to considerable interest in 61 Cygni by contemporary astronomers, and its continual observation since that date.<ref name="hopkins-1916" /> Piazzi's repeated measurements led to a definitive value of its motion, which he published in 1804.<ref>{{cite book
 | first=Giuseppe| last=Piazzi | date=1803
 | title=Præcipuarum stellarum inerrantium positiones mediae ineunte seculo XIX: ex observationibus habitis in specula Panormitana ab anno 1792 ad annum 1802
 | publisher=Typis regiis |url= https://books.google.com/books?id=66lFAAAAcAAJ&pg=PR1 | page=111}}</ref><ref name="foderaserio-1990">{{Cite journal |last=Fodera-Serio |first=G. |date=1990 |title=Giuseppe Piazzi and the Discovery of the Proper Motion of 61-Cygni |journal=Journal for the History of Astronomy |language=la |volume=21 |issue=3 |pages=275–282 |bibcode=1990JHA....21..275F|doi=10.1177/002182869002100302 |s2cid=117788717 }}</ref> It was in this record he christened the system as the "Flying Star".<ref>{{cite book | first=Alan | last=Hirshfeld | date=2001 | title=Parallax: The Race to Measure the Cosmos | publisher=Macmillan | isbn=978-0-7167-3711-7 | url-access=registration | url=https://archive.org/details/parallax00alan }}</ref>

Piazzi noted that this motion meant that it was probably one of the closest stars, and suggested it would be a prime candidate for an attempt to determine its distance through parallax measurements, along with two other possibilities, [[Delta Eridani]] and [[Mu Cassiopeiae]].<ref name="foderaserio-1990" />

=== Parallax measurement ===
A number of astronomers soon took up the task, including attempts by [[François Arago]] and [[Claude-Louis Mathieu]] in 1812, who recorded the parallax at 500&nbsp;[[minute of arc|milliarcseconds]] (mas), and [[Christian Heinrich Friedrich Peters]] used Arago's data to calculate a value of 550&nbsp;mas. Peters calculated a better value based on observations made by [[Bernhard von Lindenau]] at Seeburg between 1812 and 1814; he calculated it to be 470 ±510&nbsp;mas. Von Lindenau had already noted that he had seen no parallax, and as [[Friedrich Georg Wilhelm von Struve]] pointed out after his own test series between 1818 and 1821, all of these numbers are more accurate than the accuracy of the instrument used.<ref name="hopkins-1916" />

[[Friedrich Wilhelm Bessel]] made a notable contribution in 1812 when he used a different method to measure distance. Assuming the [[orbital period]] of the two stars in the binary to be 400 years, he estimated the distance between the two this would require, and then measured the [[angular distance]] between the stars. This led to a value of 460&nbsp;mas.{{citation needed|date=April 2019}} He then followed this up with direct parallax measurements in a series of observations between 1815 and 1816, comparing it with six other stars. The two sets of measurements produced values of 760 and 1320&nbsp;mas. All of these estimates, like earlier attempts by others, retained inaccuracies greater than the measurements.<ref name="hopkins-1916" />

When [[Joseph von Fraunhofer]] invented a new type of [[heliometer]], Bessel carried out another set of measurements using this device in 1837 and 1838 at [[Königsberg]]. He published his findings in 1838<ref>{{Cite journal| doi = 10.1093/mnras/4.17.152| last = Bessel | first = F. W. | author-link = Friedrich Bessel| title = On the parallax of 61 Cygni| journal = Monthly Notices of the Royal Astronomical Society| volume = 4| issue = 17| pages = 152–161| year = 1838| bibcode = 1838MNRAS...4..152B| doi-access = free}}</ref><ref>{{Cite journal | last = Bessel | first = F. W. | author-link = Friedrich Bessel | doi = 10.1002/asna.18390160502 | title = Bestimmung der Entfernung des 61sten Sterns des Schwans | language = de | trans-title = Determination of the distance to 61 Cygni | journal = Astronomische Nachrichten | volume = 16 | issue = 365–366 | pages = 65–96 | year = 1838 | bibcode = 1838AN.....16...65B | url = https://zenodo.org/record/1424605 | access-date = 2 July 2019 | archive-date = 22 January 2021 | archive-url = https://web.archive.org/web/20210122170129/https://zenodo.org/record/1424605 | url-status = live }}</ref> with a value of 369.0 ±19.1&nbsp;mas to ''A'' and 260.5 ±18.8 to ''B'', and estimated the center point to be at 313.6 ±13.6. This corresponds to a distance of about 600,000 [[astronomical unit]]s, or about 10.4 light-years. This was the first direct and reliable measurement of the distance to a star other than the Sun.<ref name="hopkins-1916" /><ref>{{cite web
 |author      = Frommert, Hartmut
 |author2     = Kronberg, Christine
 |title       = Friedrich Wilhelm Bessel
 |publisher   = Students for the Exploration and Development of Space
 |url         = http://messier.seds.org/xtra/Bios/bessel.html
 |access-date  = 3 April 2009
 |archive-url  = https://web.archive.org/web/20120204150631/http://messier.seds.org/xtra/Bios/bessel.html
 |archive-date = 4 February 2012
}}</ref> His measurement was published only shortly before similar parallax measurements of [[Vega]] by [[Friedrich Georg Wilhelm von Struve]] and [[Alpha Centauri]] by [[Thomas Henderson (astronomer)|Thomas Henderson]] that same year.<ref>{{cite book |first=Stefan |last=Hughes |title=Catchers of the Light |url=https://books.google.com/books?id=iZk5OOf7fVYC&pg=PA702 |page=702 |publisher=ArtDeCiel Publishing |date=2012 |isbn=978-1-62050-961-6}}</ref> Bessel continued to make additional measurements at Königsberg, publishing a total of four complete observational runs, the last in 1868. The best of these placed the center point at 360.2 ±12.1&nbsp;mas, made during observations in 1849.<ref name="hopkins-1916" /> This is close to the currently accepted value of 287.18&nbsp;mas (yielding 11.36&nbsp;light-years).<ref>{{cite journal
 | last=Bessel
 | first=F. W.
 | title=Bestimmung der Entfernung des 61sten Sterns des Schwans. Von Herrn Geheimen – Rath und Ritter Bessel
 | trans-title=Determining the distance of the 61st star of Cygnus. From Mr Geheimen, Rath and Ritter Bessel
 | journal=Astronomische Nachrichten
 | date=1839
 | volume=16
 | issue=5–6
 | pages=65–96
 | language=de
 | bibcode=1838AN.....16...65B
 | quote=(page 92) Ich bin daher der Meinung, daß nur die jährliche Parallaxe = 0"3136 als das Resultat der bisherigen Beobachtungen zu betrachten ist
 | doi=10.1002/asna.18390160502
 | url=https://zenodo.org/record/1424605
 | access-date=2 July 2019
 | archive-date=22 January 2021
 | archive-url=https://web.archive.org/web/20210122170129/https://zenodo.org/record/1424605
 | url-status=live
 }}</ref>

Only a few years after Bessel's measurement, in 1842 [[Friedrich Wilhelm Argelander]] noted that [[Groombridge 1830]] had an even larger proper motion, and 61 Cygni became the second highest known. It was later moved further down the list by [[Kapteyn's Star]] and [[Barnard's Star]]. 61 Cygni has the sixth highest proper motion of all stellar systems listed in the modern [[Hipparcos Catalogue]], but retains the title of highest proper motion among stars visible to the naked eye.<ref name=":1" />

=== Binary observations ===
Due to the wide angular separation between 61 Cygni A and B, and the correspondingly slow orbital motion, it was initially unclear whether the two stars in the 61 Cygni system were a  [[star system|gravitationally bound system]] or simply a [[optical binary|juxtaposition of stars]].<ref name="davis-1898">{{cite journal
 | last=Davis | first=Merhan S.
 | title=Remarks regarding the parallaxes of 61 Cygni and the probable physical connection of these two stars
 | journal=Astrophysical Journal
 | date=1898 | volume=8 | pages=246–247
 | bibcode=1898ApJ.....8..246D
 | doi=10.1086/140527 | doi-access=free
 }}</ref> von Struve first argued for its status as a binary in 1830, but the matter remained open.<ref name="davis-1898" />

However, by 1917 refined measured parallax differences demonstrated that the separation was significantly less.<ref>{{cite journal
 | author=Adams, W. S.
 | author2=Joy, A. H.
 | title=The luminosities and parallaxes of five hundred stars
 | journal=Astrophysical Journal
 | date=1917 | volume=46 | pages=313–339 | bibcode=1917ApJ....46..313A
 | doi = 10.1086/142369
}}—See Table I, page 326</ref> The binary nature of this system was clear by 1934, and [[orbital elements]] were published.<ref>{{cite journal
 | last=Baize | first=P.
 | title=Second catalogue d'orbites d'Etoiles Doubles visuelles
 | trans-title=Second catalog of orbits of visual double stars
 | journal=Journal des Observateurs
 | date=1950 | volume=33 | pages=1–31 | language=fr | bibcode=1950JO.....33....1B
}}—on page 19, the authority is listed as Zagar (1934).</ref>

In 1911, [[Benjamin Boss]] published data indicating that the 61 Cygni system was a member of a [[Comoving distance|comoving]] group of stars.<ref>{{cite journal
 | last=Boss | first=Benjamin
 | title=Community of motion among several stars of large proper-motion | journal=Astronomical Journal
 | date=1911 | volume=27 | issue=629 | pages=33–37
 | bibcode=1911AJ.....27...33B
 | doi=10.1086/103931 }}</ref> This group containing 61 Cygni was later expanded to include 26 potential members. Possible members include [[Beta Columbae]], [[Pi Mensae]], [[14 Tauri]] and [[68 Virginis]]. The space velocities of this group of stars range from 105 to 114&nbsp;km/s relative to the Sun.<ref>{{cite journal
 | last = Eggen | first = O. J.
 | title=White dwarf members of the 61 Cygni group
 | journal=The Observatory
 | date=1959 | volume=79 | pages=135–139
 | bibcode=1959Obs....79..135E }} – Gives space velocity components of U=+94, V=–53 and W=–7 for HD 201091/2.</ref><ref>{{cite web | url=http://www.solstation.com/stars2/pimensae.htm | title=System Summary Pi Mensae | publisher=Sol Company | access-date=1 May 2015 | author=Sol Company | archive-date=3 June 2012 | archive-url=https://web.archive.org/web/20120603142304/http://www.solstation.com/stars2/pimensae.htm | url-status=live }}</ref>

Observations taken by planet search programs show that both components have strong linear trends in the [[radial velocity]] measurements.<ref name="howard-2016" />

== Amateur observation ==
An observer using 7×50 [[binoculars]] can find 61 Cygni two binocular fields southeast of the bright star [[Deneb]]. The angular separation of the two stars is slightly greater than the [[angular size]] of [[Saturn]] (16–20″).<ref>{{Cite web |url=http://eclipse.gsfc.nasa.gov/TYPE/TYPE.html |title=Twelve Year Planetary Ephemeris: 1995–2006 |last=Espenak |first=Fred |date=25 July 1996 |publisher=NASA |access-date=3 February 2019 |archive-date=5 December 2012 |archive-url=https://archive.today/20121205061717/http://eclipse.gsfc.nasa.gov/TYPE/TYPE.html |url-status=live }}</ref> So, under ideal viewing conditions, the binary system can be resolved by a telescope with a 7&nbsp;mm aperture.<ref group="note">Per the [[Angular resolution|Rayleigh criterion]]:
<math>\begin{smallmatrix}\alpha_R\ =\ \frac{138}{D}\end{smallmatrix}</math>&nbsp;mm.</ref> This is
well within the capability for aperture of typical binoculars, though to resolve the binary these need a steady mount and some 10x magnification. With a separation of 28 arc-seconds between the component stars, 10× magnification would give an apparent separation of 280 arc-seconds, above the generally regarded eye resolution limit of 4 arc-minutes or 240 arc-seconds.<ref>{{Cite news |url=http://www.skyandtelescope.com/observing/more-pretty-double-stars/ |title=More Pretty Double Stars |last=Adler |first=Alan |date=26 July 2006 |newspaper=Sky & Telescope |publisher=Sky and Telescope |access-date=3 February 2019 |archive-date=4 February 2019 |archive-url=https://web.archive.org/web/20190204230953/https://www.skyandtelescope.com/observing/more-pretty-double-stars/ |url-status=live }}</ref>

== Properties ==
Although it appears to be a single star to the naked eye, 61 Cygni is a widely-separated binary star system, composed of two [[Stellar classification|K class (orange)]] [[main sequence]] stars, the brighter 61 Cygni A and fainter 61 Cygni B, which have [[apparent magnitude]]s of 5.2 and 6.1, respectively. Both appear to be [[old-disk star]]s,<ref>{{cite journal
 | last =Gudel | first = M.
 | title=Radio and X-ray emission from main-sequence K stars
 | journal=Astronomy and Astrophysics
 | date=1992 | volume=264 | issue=2 | pages=L31–L34
 | bibcode=1992A&A...264L..31G }}</ref><ref>{{citation
 | last1=Eggen | first1=Olin J.
 | title=Stellar Groups in the Old Disk Population
 | journal=Publications of the Astronomical Society of the Pacific
 | date=October 1969
 | volume=81
 | issue=482
 | page=553
 | bibcode=1969PASP...81..553E
 | doi=10.1086/128823
| doi-access=free
 }}</ref> with an estimated age that is older than the Sun. At a distance of just over 11 light-years, it is the 15th-nearest-known star system to the Earth (not including the Sun). 61 Cygni A is the fourth-nearest star that is visible to the naked eye for mid-latitude northern observers, after [[Sirius]], [[Epsilon Eridani]], and [[Procyon A]].<ref name="staff-2007b" /> This system will make its closest approach at about 20,000 [[Common Era|CE]], when the separation from the Sun will be about 9 light-years. Smaller and dimmer than the Sun, 61 Cygni A has about 70 percent of a [[solar mass]], 72 percent of its diameter and about 8.5 percent of its luminosity and 61 Cygni B has about 63 percent of a solar mass, 67 percent of its diameter, and 3.9 percent of its luminosity.<ref>{{Cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/cygni.html |title=61 Cygni |last=Nave |first=Rod |publisher=[[HyperPhysics]] |access-date=3 February 2019 |archive-date=29 March 2019 |archive-url=https://web.archive.org/web/20190329194621/http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/cygni.html |url-status=live }}</ref>
61 Cygni A's [[Stellar classification#Spectral peculiarities|long-term stability]] led to it being selected as an "anchor star" in the Morgan–Keenan (MK) classification system in 1943, serving as the  [[K-type main-sequence star|K5 V]] "anchor point" since that time.<ref name="garrison-1993" /> Starting in 1953, 61 Cygni B has been considered a [[K-type main-sequence star|K7 V]] standard star (Johnson & Morgan 1953,<ref>{{cite journal | bibcode=1953ApJ...117..313J  | title=Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas | first1=H. L. | last1=Johnson | first2=W. W. | last2=Morgan | date=1953 | journal=Astrophysical Journal | volume=117 | page=313 |doi = 10.1086/145697 }}</ref> Keenan & McNeil 1989<ref>{{cite journal | bibcode=1989ApJS...71..245K | title=The Perkins Catalog of Revised MK Types for the Cooler Stars | first1=P. C. | last1=Keenan | first2=R. C. | last2=McNeil | journal=Astrophysical Journal Supplement Series | volume=71 |date=October 1989 | pages=245–266 |doi = 10.1086/191373 | s2cid=123149047 }}</ref>).

[[File:Compare 61 cygni.png|left|thumb|upright=1|A size comparison between the Sun (left), 61 Cygni A (bottom) and 61 Cygni B (upper right).|alt=Diagram showing the size comparison between the two stars of the 61 Cygni binary system and the Sun.]]
61 Cygni A is a typical [[BY Draconis variable|BY Draconis]] [[variable star]] designated as V1803 Cyg while 61 Cygni B is a [[flare star|flare type]] variable star named HD 201092 with their magnitudes varying 5.21 V and 6.03, respectively.<ref>{{Cite web |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=HD+201092&NbIdent=query_hlinks&Coord=21+06+55.26395%2B38+44+31.4032&parents=1&submit=parents&siblings=1&hlinksdisplay=h_all |title=SIMBAD Query Result: HD 201092 |website=SIMBAD |publisher=Centre de Données astronomiques de Strasbourg |access-date=3 February 2019 |archive-date=4 February 2019 |archive-url=https://web.archive.org/web/20190204014345/http://simbad.u-strasbg.fr/simbad/sim-id?Ident=HD+201092&NbIdent=query_hlinks&Coord=21+06+55.26395%2B38+44+31.4032&parents=1&submit=parents&siblings=1&hlinksdisplay=h_all |url-status=live }}</ref> The two stars orbit their common [[Barycentric coordinates (astronomy)|barycenter]] in a period of 659 years, with a mean separation of about 84&nbsp;[[astronomical unit|AU]]—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&nbsp;AU at [[periapsis]] and 124 AU at [[apoapsis]].<ref group="note">At periapsis: <math>\begin{smallmatrix}r_{per}\ =\ (1\ -\ e)\cdot a\ \approx\ 44\end{smallmatrix}</math>&nbsp;AU<br />At apoapsis: <math>\begin{smallmatrix}r_{ap}\ =\ (1\ +\ e)\cdot a\ \approx\ 124\end{smallmatrix}</math>&nbsp;AU</ref> The leisurely orbit of the pair has made it difficult to pin down their respective masses, and the accuracy of these values remain somewhat controversial. In the future this issue may be resolved through the use of [[asteroseismology]].<ref name="kervella-2008" /> 61 Cygni A has about 11% more mass than 61 Cygni B.<ref name="staff-2007b" />

The system has an [[Stellar magnetic field|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&nbsp;years.<ref>{{cite journal
 | author=Frick, P.
 | author2=Baliunas, S. L.
 | author3=Galyagin, D.
 | author4=Sokoloff, D.
 | author5=Soon, W.
 | title=Wavelet Analysis of Chromospheric Activity
 | journal=Astrophysical Journal
 | date=1997 | volume=483
 | issue=1 | pages=426–434
 | bibcode=1997ApJ...483..426F | doi = 10.1086/304206
| doi-access=free
 }}</ref><ref name="donahue r a-2003">{{cite journal
 | author=Hempelmann, A.
 | author2=Schmitt, J. H. M. M.
 | author3=Baliunas, S. L.
 | author4=Donahue, R. A.
 | title=Evidence for coronal activity cycles on 61 Cygni A and B
 | journal=Astronomy and Astrophysics
 | date=2003 | volume=406
 | issue=2 | pages=L39–L42
 | bibcode=2003A&A...406L..39H | doi = 10.1051/0004-6361:20030882
| doi-access=free
 }}</ref> The starspot activity combined with rotation and chromospheric activity is a characteristic of a BY Draconis variable. Because of differential rotation, this star's surface rotation period varies by latitude from 27 to 45 days, with an average period of 35 days.<ref name="böhmvitense-2007" />

[[File:Orbit 61 Cygni arcsec.png|right|thumb|The orbital motion of component B relative to component A as seen from Earth as well as the true appearance from face-on view. The time steps are approximately 10 years.|alt=Diagram showing  the trajectory of 61 Cygni B relative to A as seen from Earth and from above.]]
The outflow of the stellar wind from component A produces a bubble within the local interstellar cloud. Along the direction of the star's motion within the Milky Way, this extends out to a distance of 30&nbsp;AU, or roughly the orbital distance of [[Neptune]] from the Sun. This is lower than the separation between the two components of 61 Cygni, and so the two most likely do not share a common atmosphere. The compactness of the [[Stellar-wind bubble|astrosphere]] is likely due to the low mass outflow and the relatively high velocity through the local interstellar medium.<ref>{{cite journal
 | author=Wood, Brian E.
 | author2=Müller, Hans-Reinhard
 | author3=Zank, Gary P.
 | author4=Linsky, Jeffrey L.
 | title=Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity
 | journal=The Astrophysical Journal | volume=574 | issue=1
 | pages=412–425 |date=July 2002
 | doi=10.1086/340797 | bibcode=2002ApJ...574..412W|arxiv = astro-ph/0203437 | s2cid=1500425
 }}</ref>

61 Cygni 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.<ref name="donahue r a-2003" /> Both stars exhibit stellar flare activity, but the [[chromosphere]] of B is 25% more active than for 61 Cygni A.<ref>{{cite journal
 | author=Hempelmann, A.
 | author2=Robrade, J.
 | author3=Schmitt, J. H. M. M.
 | author4=Favata, F.
 | author5=Baliunas, S. L.
 | author6=Hall, J. C.
 | title=Coronal activity cycles in 61 Cygni
 | journal=Astronomy and Astrophysics
 | date=2006 | volume=460 | issue=1 | pages=261–267
 | bibcode=2006A&A...460..261H | doi = 10.1051/0004-6361:20065459
| doi-access=free
 }}</ref> As a result of differential rotation, the period of rotation varies by latitude from 32 to 47 days, with an average period of 38 days.<ref name="böhmvitense-2007" />

There is some disagreement over the evolutionary age of this system. [[Stellar kinematics|Kinematic]] data gives an age estimate of about 10&nbsp;[[Byr|Gyr]]. [[Stellar rotation|Gyrochronology]], or the age determination of a star based on its rotation and color, results in an average age of {{nowrap|2.0 ±0.2 Gyr}}. The ages based on [[Stellar magnetic field|chromospheric activity]] for A and B are 2.36&nbsp;Gyr and 3.75&nbsp;Gyr, respectively. Finally the age estimates using the isochrone method, which involve fitting the stars to evolutionary models, yield upper limits of 0.44&nbsp;Gyr and 0.68&nbsp;Gyr.<ref>{{cite journal
 | last=Barnes | first=Sydney A.
 | title=Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors
 | journal=The Astrophysical Journal | volume=669 | issue=2
 | pages=1167–1189 |date=November 2007
 | doi=10.1086/519295 | bibcode=2007ApJ...669.1167B|arxiv = 0704.3068 | s2cid=14614725
 }}</ref> However, a 2008 evolutionary model using the CESAM2k code from the [[Côte d'Azur Observatory]] gives an age estimate of {{nowrap|6.0 ±1.0 Gyr}} for the pair.<ref name="kervella-2008" />

== Claims of a planetary system ==
On different occasions, it has been claimed that 61 Cygni might have unseen low-mass companions, planets or a [[brown dwarf]]. [[Kaj Aage Gunnar Strand|Kaj Strand]] of the Sproul Observatory, under the direction of [[Peter van de Kamp]], 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 [[Perturbation (astronomy)|perturbations]] suggested that a third body of about 16 Jupiter masses must be orbiting 61 Cygni A.<ref>{{cite journal
 | last=Strand | first=K. Aa.
 | title=61 Cygni as a Triple System
 | journal=Publications of the Astronomical Society of the Pacific
 | date=1943 | volume=55 | issue=322 | pages=29–32
 | bibcode=1943PASP...55...29S
 | doi=10.1086/125484 | doi-access=free}}</ref> Reports of this third body served as inspiration for [[Hal Clement]]'s 1953 science fiction novel ''[[Mission of Gravity]]''.<ref>{{Cite book |url=https://books.google.com/books?id=v6DNXIN3UqEC&q=third+body+61+cygni+mission+of+gravity&pg=PA44 |title=Science Fiction Voices #1 |last1=Darrell Schweitzer |last2=Theodore Sturgeon |last3=Alfred Bester |date=2009 |publisher=Wildside Press LLC |isbn=978-1-4344-0784-9 |page=64 |access-date=3 February 2019}}</ref> In 1957, van de Kamp narrowed his uncertainties, claiming that the object had a mass of eight times that of Jupiter, a calculated orbital period of 4.8&nbsp;years, and a semi-major axis of 2.4&nbsp;AU, where 1&nbsp;AU is the average distance from the Earth to the Sun.<ref>{{cite journal
 | last=Strand | first=K. Aa.
 | title=The orbital motion of 61 Cygni
 | journal=The Astronomical Journal
 | date=1957 | volume=62 | page=35
 | bibcode=1957AJ.....62Q..35S
 | doi=10.1086/107588 | doi-access=free
 }}</ref> In 1977, Soviet astronomers at the [[Pulkovo Observatory]] near [[Saint Petersburg]] suggested that the system included three planets: two giant planets with six and twelve [[Jupiter masses]] around 61 Cyg A,<ref>{{cite journal
 | author=Cumming, A.
 | author2=Marcy, G. W.
 | author3=Butler, R. P.
 | date=1999 | title=The Lick planet search: detectability and mass thresholds | doi=10.1086/308020
 | journal=Astrophysical Journal | volume=526
 | issue=2
 | pages=890–915
| bibcode=1999ApJ...526..890C
|arxiv = astro-ph/9906466 | s2cid=12560512
 }}</ref> and one giant planet with seven Jupiter masses around 61 Cygni B.<ref>{{cite journal
 | last=Deich | first=A. N.
 | title=Invisible companions of the binary star 61 Cygni
 | journal= [[Soviet Astronomy]]
 | date=1977 | volume=21 | pages=182–188
 | bibcode=1977SvA....21..182D}}</ref>

In 1978, [[Wulff-Dieter Heintz]] of the [[Sproul Observatory]] proved that these claims were spurious, as they were unable 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]].<ref>{{cite journal
 | last=Heintz | first=W. D.
 | title=Reexamination of suspected unresolved binaries
 | journal=The Astrophysical Journal
 | date=1978 | volume=220 | pages=931–934
 | bibcode=1978ApJ...220..931H
 | doi=10.1086/155982 | doi-access=free
 }}</ref><ref>{{cite journal
 | author=Walker, G. A. H.
 | author2=Walker, A. R.
 | author3=Irwin, A. W.
 | author4=Larson, A. M.
 | author5=Yang, S. L. S.
 | author6=Richardson, D. C.
 | date=1995 | title=A search for Jupiter-mass companions to nearby stars | journal=Icarus | volume=116 | issue=2
 | pages=359–375 | doi=10.1006/icar.1995.1130 | bibcode=1995Icar..116..359W}}</ref>

In 2018, analysis of the DR2 data gathered by the [[Gaia (spacecraft)|Gaia space telescope]] revealed significant proper motion anomalies in the orbits of the binary stars around each other; the stars were not quite orbiting around their centre of mass with 61 Cygni B also orbiting too slowly for its assumed mass. These anomalies taken together are indicative of the possible presence of a perturbing third object in orbit around 61 Cygni B.<ref>{{cite journal|last1=Kervella|first1=Pierre|last2=Arenou|first2=Frédéric|last3=Mignard|first3=François|last4=Thévenin|first4=Frédéric|display-authors=2|title=Stellar and substellar companions of nearby stars from Gaia DR2|journal=Astronomy & Astrophysics|volume=623|year=2019|pages=A72|issn=0004-6361|arxiv=1811.08902 |bibcode=2019A&A...623A..72K |doi=10.1051/0004-6361/201834371|s2cid=119491061|quote="This PMa offset between 61 Cyg A and B points at the possible presence of a third body in the system, likely orbiting around 61 Cyg B."}}</ref>

The [[habitable zone]] for 61 Cygni A, defined as the locations where liquid water could be present on an Earth-like planet, is 0.26–0.58&nbsp;[[astronomical unit|AU]]. For 61 Cygni B, the habitable zone is 0.24–0.50 AU.<ref>{{Citation |last1=Cantrell |first1=Justin R. |title=The Solar Neighborhood XXIX: The Habitable Real Estate of Our Nearest Stellar Neighbors |date=October 2013 |journal=The Astronomical Journal |volume=146 |issue=4 |page=99 |arxiv=1307.7038 |bibcode=2013AJ....146...99C |doi=10.1088/0004-6256/146/4/99|last2=Henry |first2=Todd J. |last3=White |first3=Russel J.|s2cid=44208180 }}</ref>

=== Refining planetary boundaries ===
Since no certain planetary object has been detected around either star so far, [[McDonald Observatory]] team has set limits to the presence of one or more planets around 61 Cygni A and 61 Cygni B with masses between 0.07 and 2.1&nbsp;Jupiter masses and average separations spanning between 0.05 and 5.2&nbsp;AU.<ref>{{cite journal
  | author=Wittenmyer, R. A.
 | author2=Endl, M.
 | author3=Cochran, W.D.
 | author4=Hatzes, A.
 | author5=Walker, G. A. H.
 | author6=Yang, S. L. S.
 | author7=Paulson, D. B.
 | title=Detection Limits from the McDonald Observatory Planet Search Program | journal=[[The Astronomical Journal]]
 |date=May 2006 | volume=132 | issue=1
 | pages=177–188 | doi=10.1086/504942 | bibcode=2006AJ....132..177W
 | arxiv=astro-ph/0604171| s2cid=16755455
 }}</ref>

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]].<ref>{{cite web
 |url         = http://tauceti.sfsu.edu/~chris/SIM/t1.html
 |title       = SIM Planet Search Tier 1 Target Stars
 |first       = Christopher
 |last        = McCarthy
 |year        = 2005
 |publisher   = [[San Francisco State University]]
 |access-date  = 23 July 2007
 |archive-url  = https://web.archive.org/web/20070804210039/http://tauceti.sfsu.edu/~chris/SIM/t1.html
 |archive-date = 4 August 2007
 }}</ref> This mission is potentially capable of detecting planets with as little as 3&nbsp;times the mass of the Earth at an orbital distance of 2&nbsp;AU from the star.

Measurements of this system appeared to have detected an [[infrared excess|excess of far infrared radiation]], beyond what is emitted by the stars. Such an excess is sometimes associated with a [[debris disk|disk of dust]], but in this case it lies sufficiently close to one or both of the stars that it has not been resolved with a telescope.<ref>{{cite journal
 | author=Kuchner, Marc J.
 | author2=Brown, Michael E.
 | author3=Koresko, Chris D.
 | title=An 11.6 Micron Keck Search for Exo-Zodiacal Dust
 | journal=The Publications of the Astronomical Society of the Pacific
 | date=1998 | volume=110 | issue=753 | pages=1336–1341
 | bibcode=1998PASP..110.1336K
 | doi=10.1086/316267 |arxiv = astro-ph/0002040 | s2cid=119479494
 }}</ref> A 2011 study using the [[W. M. Keck Observatory|Keck Interferometer Nuller]] failed to detect any [[exozodiacal dust]] around 61 Cygni A.<ref>{{Citation |last1=Millan-Gabet, R. |title=Exozodiacal Dust Levels for Nearby Main-sequence Stars: A Survey with the Keck Interferometer |date=June 2011 |journal=The Astrophysical Journal |volume=734 |issue=1 |page=67 |postscript=. |arxiv=1104.1382 |bibcode=2011ApJ...734...67M |doi=10.1088/0004-637X/734/1/67 |last2=Serabyn, E. |last3=Mennesson, B. |last4=Traub, W. A. |last5=Barry, R. K. |last6=Danchi, W. C. |last7=Kuchner, M. |last8=Stark, C. C. |last9=Ragland, S. |last10=Hrynevych, M. |last11=Woillez, J. |last12=Stapelfeldt, K. |last13=Bryden, G. |last14=Colavita, M. M. |last15=Booth, A. J.|s2cid=118614703 }} See Table 5, p. 58.</ref>

=== Object for biosignature research ===
The two stars are among five (all nearby star) paradigms listed among those K-type stars of a type in a 'sweet spot' between Sun-analog stars and M stars for the likelihood of evolved life, per analysis of Giada Arney from NASA's Goddard Space Flight Center.<ref name="bill steigerwald-2019" />

== See also ==
* [[List of nearest stars]]
* [[Barnard's Star]]

== Notes ==
{{reflist|group=note}}

== References ==
{{Reflist|refs=

<ref name="vallenari-2023a">{{Cite Gaia DR3|1872046609345556480}}</ref>
<ref name="vallenari-2023b">{{Cite Gaia DR3|1872046574983497216}}</ref>

<ref name="fischer-2019">{{Cite web |url=http://www.glyphweb.com/esky/stars/61cygni.html |title=61 Cygni |last=Fischer |first=Mark |date=9 February 2019 |publisher=Mark Fisher |access-date=9 February 2019 |archive-date=9 February 2019 |archive-url=https://web.archive.org/web/20190209232438/http://www.glyphweb.com/esky/stars/61cygni.html |url-status=live }}</ref>

<ref name="the internet stellar database-2011">{{Cite web |url=http://www.stellar-database.com/Scripts/search_star.exe?Name=61+cygni |title=61 Cygni |date=4 April 2011 |publisher=The Internet Stellar Database |access-date=3 February 2019 |archive-date=1 February 2020 |archive-url=https://web.archive.org/web/20200201152107/http://www.stellar-database.com/Scripts/search_star.exe?Name=61+cygni |url-status=live }}</ref>

<ref name="catalano s-1979">{{cite journal
 | author=Blanco, C.
 | author2=Marilli, E.
 | author3=Catalano, S.
 | title=Photoelectric observations of stars with variable H and K emission components. III
 | journal=[[Astronomy and Astrophysics Supplement Series]]
 | date=5 January 1979 | volume=36 | pages=297–306
 | bibcode=1979A&AS...36..297B }}</ref>

<ref name="simbad">{{Cite web |url=http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=HD+201091 |title=SIMBAD Query Result: V* V1803 Cyg -- Variable of BY Dra type |website=[[SIMBAD]] |publisher=Centre de Données astronomiques de Strasbourg |access-date=3 February 2019 |archive-date=20 January 2021 |archive-url=https://web.archive.org/web/20210120234526/http://simbad.u-strasbg.fr/simbad/sim-id?Ident=HD+201091 |url-status=live }} (61 Cygni A)</ref>

<ref name="simbad-2">{{Cite web |url=http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=HD+201092 |title=SIMBAD Query Result: NSV 13546 -- Flare Star |website=SIMBAD |publisher=Centre de Données astronomiques de Strasbourg |access-date=3 February 2019 |archive-date=21 January 2021 |archive-url=https://web.archive.org/web/20210121002454/http://simbad.u-strasbg.fr/simbad/sim-id?Ident=HD+201092 |url-status=live }} (61 Cygni B)</ref>

<ref name="kovtyukh-2003">{{citation
 | last1=Kovtyukh | first1=V. V.
 | last2=Soubiran |first2=C. |last3=Belik |first3=S. I.
 |last4=Gorlova |first4=N. I.
 | title=High precision effective temperatures for 181 F-K dwarfs from line-depth
 | journal=Astronomy and Astrophysics
 | date=December 2003
 | volume=411
 | issue=3
 | pages=559–564
 | bibcode=2003A&A...411..559K
 | doi=10.1051/0004-6361:20031378
 | arxiv=astro-ph/0308429
| s2cid=18478960
 }} See Mv values in Table 1, p. 9.</ref>

<ref name="bill steigerwald-2019">{{cite news
 |url          = https://www.nasa.gov/feature/goddard/2019/k-star-advantage
 |title="Goldilocks" Stars May Be "Just Right" for Finding Habitable Worlds
 |work         = [[NASA]]
 |author       = Bill Steigerwald
 |date         = 2019-03-07
 |access-date  = 2020-05-12
 |quote        = {{'}}I find that certain nearby K stars like 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 may be particularly good targets for future biosignature searches,{{'}} said Arney.
 |archive-date = 17 June 2019
 |archive-url  = https://web.archive.org/web/20190617062905/https://www.nasa.gov/feature/goddard/2019/k-star-advantage/
 |url-status   = live
}}</ref>

<ref name="staff-2007b">{{citation | author=Staff | date=7 August 2007 | url=http://joy.chara.gsu.edu/RECONS/TOP100.posted.htm | title=RECONS Mission Statement| publisher=Research Consortium on Nearby Stars, Georgia State University | access-date=11 February 2019 | archive-url= https://web.archive.org/web/20070701064630/http://joy.chara.gsu.edu/RECONS/| archive-date=1 July 2007 | url-status= live}}</ref>

<ref name="kervella-2008">{{citation | last1=Kervella | first1=P. | last2=Mérand | first2=A. | last3=Pichon | first3=B. | last4=Thévenin | first4=F. | last5=Heiter | first5=U. | last6=Bigot | first6=L. | last7=Ten Brummelaar | first7=T. A. | last8=McAlister | first8=H. A. | last9=Ridgway | first9=S. T. | last10=Turner | first10=N. | title=The radii of the nearby K5V and K7V stars 61 Cygni A & B. CHARA/FLUOR interferometry and CESAM2k modeling | journal=Astronomy and Astrophysics | volume=488 | issue=2 | pages=667–674 |date=September 2008 | doi=10.1051/0004-6361:200810080 | bibcode=2008A&A...488..667K|arxiv = 0806.4049 | s2cid=14830868 }}</ref>

<ref name="van belle-2009">{{citation | last1=van Belle | first1=Gerard T. | last2=von Braun | first2=Kaspar | title=Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars | journal=[[The Astrophysical Journal]] | volume=694 | issue=2 | pages=1085–109 | date=2009 | doi=10.1088/0004-637X/694/2/1085 | bibcode=2009ApJ...694.1085V|arxiv = 0901.1206 | s2cid=18370219 }}</ref>

<ref name="luck-2005">{{citation | last1=Luck | first1=R. Earle | last2=Heiter | first2=Ulrike | title=Stars within 15 Parsecs: Abundances for a Northern Sample | journal=[[The Astronomical Journal]] | date=2005 | volume=129 | issue=2 | pages=1063–1083 | bibcode=2005AJ....129.1063L | doi=10.1086/427250 | doi-access=free }}</ref>

<ref name="böhmvitense-2007">{{citation | last=Böhm-Vitense | first=Erika | title=Chromospheric Activity in G and K Main-Sequence Stars, and What It Tells Us about Stellar Dynamos | journal=The Astrophysical Journal | volume=657 | issue=1 | pages=486–493 |date=March 2007 | doi=10.1086/510482 | bibcode=2007ApJ...657..486B| doi-access=free }}</ref>

<ref name="hartkopf">{{cite web | last1=Hartkopf | first1=W. I. | first2=Brian D. | last2=Mason | url=http://ad.usno.navy.mil/wds/orb6.html | title=Sixth Catalog of Orbits of Visual Binary Stars | publisher=U.S. Naval Observatory | access-date=12 July 2008 | archive-url=https://web.archive.org/web/20090412084731/http://ad.usno.navy.mil/wds/orb6.html | archive-date=12 April 2009 }}</ref>

<ref name="garrison-1993">{{citation | last1=Garrison | first1=R. F. | title=Anchor Points for the MK System of Spectral Classification | journal=Bulletin of the American Astronomical Society | volume=25 | page=1319 | date=December 1993 | bibcode=1993AAS...183.1710G | url=http://www.astro.utoronto.ca/~garrison/mkstds.html | access-date=4 February 2012 | archive-date=25 June 2019 | archive-url=https://web.archive.org/web/20190625094716/http://www.astro.utoronto.ca/~garrison/mkstds.html }}</ref>

<ref name="howard-2016">{{cite journal | title=Limits on Planetary Companions from Doppler Surveys of Nearby Stars | last1=Howard | first1=Andrew W. | last2=Fulton | first2=Benjamin J. | journal=Publications of the Astronomical Society of the Pacific | volume=128 | issue=969 | at=114401 | year=2016 | arxiv=1606.03134 | bibcode=2016PASP..128k4401H | doi=10.1088/1538-3873/128/969/114401 | s2cid=118503912 }}</ref>

<ref name="olspert-2018">{{cite journal |last1=Olspert |first1=N. |last2=Lehtinen |first2=J. J. |display-authors=etal |date=October 2018 |title=Estimating activity cycles with probabilistic methods. II. The Mount Wilson Ca H&K data |journal=[[Astronomy & Astrophysics]] |volume=619 |issue= |pages=A6 |doi=10.1051/0004-6361/201732525 |arxiv=1712.08240 |bibcode=2018A&A...619A...6O}}</ref>

}}

== External links ==
{{Commons category}}
* {{cite web
 | title=61 Cygni 2
 | url = http://www.solstation.com/stars/61cygni2.htm
 | publisher = SolStation
 | access-date = 16 July 2007 | archive-url= https://web.archive.org/web/20070715025918/http://www.solstation.com/stars/61cygni2.htm| archive-date= 15 July 2007 | url-status= live}}
* {{cite web | last = Kaler | first = James B. | title = 61 Cygni | url = http://www.astro.uiuc.edu/~kaler/sow/61cyg.html | publisher = [[University of Illinois at Urbana–Champaign]] | access-date = 16 July 2007 | archive-url = https://web.archive.org/web/20070707012512/http://www.astro.uiuc.edu/~kaler/sow/61cyg.html | archive-date = 7 July 2007 }}

{{Sky|21|06|53.9434|+|38|44|57.898|11.36}}
{{nearest systems|2}}
{{Stars of Cygnus}}
{{Portal bar|Astronomy|Stars|Outer space}}

[[Category:Cygnus (constellation)]]
[[Category:Binary stars]]
[[Category:Durchmusterung objects|BD+38 4343 and BD+38 4344]]
[[Category:K-type main-sequence stars]]
[[Category:Flamsteed objects|Cygni, 61]]
[[Category:Gliese and GJ objects|0820]]
[[Category:Henry Draper Catalogue objects|201091 2]]
[[Category:Hipparcos objects|104214 and 104217]]
[[Category:Bright Star Catalogue objects|8085 and 8086]]
[[Category:Objects with variable star designations|Cygni, V1803]]
[[Category:BY Draconis variables]]
[[Category:Discoveries by Giuseppe Piazzi]]