Editing 61 Cygni (section)

== 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" />