Astronomy:61 Cygni

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Short description: Binary star system in the Cygnus constellation
61 Cygni
Cygnus constellation map.svg
Red circle.svg
Location of 61 Cygni (circled)
Observation data
{{#ifeq:J2000.0|J2000.0 (ICRS)|Epoch J2000.0      Equinox J2000.0 (ICRS)| Epoch J2000.0      [[Astronomy:Equinox (celestial coordinates)|Equinox J2000.0}}
Constellation Cygnus
61 Cygni A
Right ascension  21h 06m 53.9396s[1]
Declination +38° 44′ 57.902″[1]
Apparent magnitude (V) 5.21[2]
61 Cygni B
Right ascension  21h 06m 55.2638s[3]
Declination +38° 44′ 31.359″[3]
Apparent magnitude (V) 6.05[4]
Characteristics
61 Cyg A
Spectral type K5V[2]
U−B color index +1.155[5]
B−V color index +1.139[5]
Variable type BY Dra[6]
61 Cyg B
Spectral type K7V
U−B color index +1.242[5]
B−V color index +1.320[5]
Variable type Flare star[7]
Astrometry
61 Cygni A
Radial velocity (Rv)−65.97±0.12[1] km/s
Proper motion (μ) RA: 4,164.209[1] mas/yr
Dec.: 3,249.614[1] mas/yr
Parallax (π)285.9949 ± 0.0599[1] mas
Distance11.404 ± 0.002 ly
(3.4966 ± 0.0007 pc)
Absolute magnitude (MV)7.506[8]
61 Cygni B
Radial velocity (Rv)−64.59±0.12[3] km/s
Proper motion (μ) RA: 4,105.976 mas/yr
Dec.: 3,155.942 mas/yr
Parallax (π)286.0054 ± 0.0289[3] mas
Distance11.404 ± 0.001 ly
(3.4964 ± 0.0004 pc)
Absolute magnitude (MV)8.228[8]
Orbit[9]
Companion61 Cygni B
Period (P)678 ±34 yr
Semi-major axis (a)24.272 ±0.592″
Eccentricity (e)0.49 ±0.03
Inclination (i)51 ±2°
Longitude of the node (Ω)178 ±2°
Periastron epoch (T)1709 ±16
Argument of periastron (ω)
(secondary)		149 ±6°
Details
61 Cygni A
Mass0.70[10] M☉
Radius0.665 ±0.005[11] R☉
Luminosity0.153 ±0.01[11] L☉
Surface gravity (log g)4.40[12] cgs
Temperature4,526 ±66[13] K
Metallicity [Fe/H]–0.20[12] dex
Rotation35.54±0.47 d[14]
Age6.1 ±1[11] Gyr
61 Cygni B
Mass0.63[10] M☉
Radius0.595 ±0.008[11] R☉
Luminosity0.085 ±0.007[11] L☉
Surface gravity (log g)4.20[12] cgs
Temperature4,077 ±59[13] K
Metallicity [Fe/H]–0.27[12] dex
Rotation34.55±0.57 d[14]
Age6.1 ±1[11] Gyr
Other designations
GJ 820 A/B, Struve 2758, ADS 14636, V1803 Cygni, GCTP 5077.00[15]
61 Cygni A: V1803 Cygni, HD 201091, HIP 104214, HR 8085, BD+38°4343, LHS 62, SAO 70919[6]
61 Cygni B: HD 201092, HIP 104217, HR 8086, BD+38°4344, LHS 63[7]
Database references
SIMBADThe system
A
B

61 Cygni /ˈsɪɡni/ is a binary star system in the constellation Cygnus, consisting of a pair of K-type dwarf stars that orbit each other in a period of about 659 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 light-years, very close to the actual value of about 11.4 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 seventh-highest proper motion, and the highest among all visible stars or systems.[note 1][16][17]

Over the course of the twentieth century, several different astronomers reported evidence of a massive planet orbiting one of the two stars, but recent high-precision radial velocity observations have shown that all such claims were unfounded.[18] 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 western[19] or Chinese systems.[20]

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.[21][22] The star does not appear under that name in Flamsteed's Historia Coelestis Britannica,[23] although it has been stated by him that 61 Cygni actually corresponds to what he referred to as 85 Cygni in the 1712 edition.[24] It has also been called "Bessel's Star" or "Piazzi's Flying Star".[25][26] It features as "the Swan" (from its Latin origin, "cygnus") in Alastair Reynolds' science fiction novel Chasm City (London: Victor Gollancz, 2001).

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.[27]

GIF showing the proper motion of the stellar system, taken about in an interval of an year for the range from 2012 to 2020.
61 Cygni showing proper motion (movement from our vantage point) at some early 21st century one-year intervals.

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.[27] Piazzi's repeated measurements led to a definitive value of its motion, which he published in 1804.[28][29] It was in this record he christened the system as the "Flying Star".[30]

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.[29]

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 milliarcseconds (mas), and Christian Heinrich Friedrich Peters used Arago's data to calculate a value of 550 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 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.[27]

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 mas.[citation needed] 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 mas. All of these estimates, like earlier attempts by others, retained inaccuracies greater than the measurements.[27]

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[31][32] with a value of 369.0±19.1 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 units, or about 10.4 light-years. This was the first direct and reliable measurement of the distance to a star other than the Sun.[27][33] His measurement was published only shortly before similar parallax measurements of Vega by Friedrich Georg Wilhelm von Struve and Alpha Centauri by Thomas Henderson that same year.[34] 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 mas, made during observations in 1849.[27] This is close to the currently accepted value of 287.18 mas (yielding 11.36 light-years).[35]

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 seventh 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.[16]

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 gravitationally bound system or simply a juxtaposition of stars.[36] von Struve first argued for its status as a binary in 1830, but the matter remained open.[36]

However, by 1917 refined measured parallax differences demonstrated that the separation was significantly less.[37] The binary nature of this system was clear by 1934, and orbital elements were published.[38]

In 1911, Benjamin Boss published data indicating that the 61 Cygni system was a member of a comoving group of stars.[39] 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 km/s relative to the Sun.[40][41]

Observations taken by planet search programs show that both components have strong linear trends in the radial velocity measurements.[42]

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″).[43] So, under ideal viewing conditions, the binary system can be resolved by a telescope with a 7 mm aperture.[note 2] 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.[44]

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 K class (orange) main sequence stars, the brighter 61 Cygni A and fainter 61 Cygni B, which have apparent magnitudes of 5.2 and 6.1, respectively. Both appear to be old-disk stars,[45][46] 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.[10] This system will make its closest approach at about 20,000 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.[47] 61 Cygni A's long-term stability led to it being selected as an "anchor star" in the Morgan–Keenan (MK) classification system in 1943, serving as the K5 V "anchor point" since that time.[48] Starting in 1953, 61 Cygni B has been considered a K7 V standard star (Johnson & Morgan 1953,[49] Keenan & McNeil 1989[50]).

Diagram showing the size comparison between the two stars of the 61 Cygni binary system and the Sun.
A size comparison between the Sun (left), 61 Cygni A (bottom) and 61 Cygni B (upper right).

61 Cygni A is a typical BY Draconis variable star designated as V1803 Cyg while 61 Cygni B is a flare type variable star named HD 201092 with their magnitudes varying 5.21 V and 6.03, respectively.[51] The two stars orbit their common barycenter in a period of 659 years, with a mean separation of about 84 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 AU at periapsis and 124 AU at apoapsis.[note 3] 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.[11] 61 Cygni A has about 11% more mass than 61 Cygni B.[10]

The system has an 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 years.[52][53] 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.[54]

Diagram showing the trajectory of 61 Cygni B relative to A as seen from Earth and from above.
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.

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 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 astrosphere is likely due to the low mass outflow and the relatively high velocity through the local interstellar medium.[55]

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.[53] Both stars exhibit stellar flare activity, but the chromosphere of B is 25% more active than for 61 Cygni A.[56] 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.[54]

There is some disagreement over the evolutionary age of this system. Kinematic data gives an age estimate of about 10 Gyr. Gyrochronology, or the age determination of a star based on its rotation and color, results in an average age of 2.0 ±0.2 Gyr. The ages based on chromospheric activity for A and B are 2.36 Gyr and 3.75 Gyr, respectively. Finally the age estimates using the isochrone method, which involve fitting the stars to evolutionary models, yield upper limits of 0.44 Gyr and 0.68 Gyr.[57] However, a 2008 evolutionary model using the CESAM2k code from the Côte d'Azur Observatory gives an age estimate of 6.0 ±1.0 Gyr for the pair.[11]

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 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 perturbations suggested that a third body of about 16 Jupiter masses must be orbiting 61 Cygni A.[58] Reports of this third body served as inspiration for Hal Clement's 1953 science fiction novel Mission of Gravity.[59] 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 years, and a semi-major axis of 2.4 AU, where 1 AU is the average distance from the Earth to the Sun.[60] 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,[61] and one giant planet with seven Jupiter masses around 61 Cygni B.[62]

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.[63][64]

In 2018, analysis of the DR2 data gathered by the 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.[65]

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 AU. For 61 Cygni B, the habitable zone is 0.24–0.50 AU.[66]

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 Jupiter masses and average separations spanning between 0.05 and 5.2 AU.[67]

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.[68] This mission is potentially capable of detecting planets with as little as 3 times the mass of the Earth at an orbital distance of 2 AU from the star.

Measurements of this system appeared to have detected an excess of far infrared radiation, beyond what is emitted by the stars. Such an excess is sometimes associated with a 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.[69] A 2011 study using the Keck Interferometer Nuller failed to detect any exozodiacal dust around 61 Cygni A.[70]

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.[71]

See also

Notes

  1. ↑ By convention, limiting visual magnitude of 6.0
  2. ↑ Per the Rayleigh criterion: [math]\displaystyle{ \begin{smallmatrix}\alpha_R\ =\ \frac{138}{D}\end{smallmatrix} }[/math] mm.
  3. ↑ At periapsis: [math]\displaystyle{ \begin{smallmatrix}r_{per}\ =\ (1\ -\ e)\cdot a\ \approx\ 44\end{smallmatrix} }[/math] AU
    At apoapsis: [math]\displaystyle{ \begin{smallmatrix}r_{ap}\ =\ (1\ +\ e)\cdot a\ \approx\ 124\end{smallmatrix} }[/math] AU

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  2. ↑ 2.0 2.1 Fischer, Mark (9 February 2019). "61 Cygni". Mark Fisher. http://www.glyphweb.com/esky/stars/61cygni.html. 
  3. ↑ 3.0 3.1 3.2 3.3 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  4. ↑ "61 Cygni". The Internet Stellar Database. 4 April 2011. http://www.stellar-database.com/Scripts/search_star.exe?Name=61+cygni. 
  5. ↑ 5.0 5.1 5.2 5.3 Blanco, C.; Marilli, E.; Catalano, S. (5 January 1979). "Photoelectric observations of stars with variable H and K emission components. III". Astronomy and Astrophysics Supplement Series 36: 297–306. Bibcode: 1979A&AS...36..297B. 
  6. ↑ 6.0 6.1 "SIMBAD Query Result: V* V1803 Cyg -- Variable of BY Dra type". Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=HD+201091.  (61 Cygni A)
  7. ↑ 7.0 7.1 "SIMBAD Query Result: NSV 13546 -- Flare Star". Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=HD+201092.  (61 Cygni B)
  8. ↑ 8.0 8.1 Kovtyukh, V. V.; Soubiran, C.; Belik, S. I.; Gorlova, N. I. (December 2003), "High precision effective temperatures for 181 F-K dwarfs from line-depth", Astronomy and Astrophysics 411 (3): 559–564, doi:10.1051/0004-6361:20031378, Bibcode: 2003A&A...411..559K  See Mv values in Table 1, p. 9.
  9. ↑ Hartkopf, W. I.; Mason, Brian D.. "Sixth Catalog of Orbits of Visual Binary Stars". U.S. Naval Observatory. http://ad.usno.navy.mil/wds/orb6.html. 
  10. ↑ 10.0 10.1 10.2 10.3 Staff (7 August 2007), RECONS Mission Statement, Research Consortium on Nearby Stars, Georgia State University, http://joy.chara.gsu.edu/RECONS/TOP100.posted.htm, retrieved 11 February 2019 
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 Kervella, P.; Mérand, A.; Pichon, B.; Thévenin, F.; Heiter, U.; Bigot, L.; Ten Brummelaar, T. A.; McAlister, H. A. et al. (September 2008), "The radii of the nearby K5V and K7V stars 61 Cygni A & B. CHARA/FLUOR interferometry and CESAM2k modeling", Astronomy and Astrophysics 488 (2): 667–674, doi:10.1051/0004-6361:200810080, Bibcode: 2008A&A...488..667K 
  12. ↑ 12.0 12.1 12.2 12.3 Luck, R. Earle; Heiter, Ulrike (2005), "Stars within 15 Parsecs: Abundances for a Northern Sample", The Astronomical Journal 129 (2): 1063–1083, doi:10.1086/427250, Bibcode: 2005AJ....129.1063L 
  13. ↑ 13.0 13.1 van Belle, Gerard T.; von Braun, Kaspar (2009), "Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars", The Astrophysical Journal 694 (2): 1085–109, doi:10.1088/0004-637X/694/2/1085, Bibcode: 2009ApJ...694.1085V 
  14. ↑ 14.0 14.1 Olspert, N. et al. (October 2018). "Estimating activity cycles with probabilistic methods. II. The Mount Wilson Ca H&K data". Astronomy & Astrophysics 619: A6. doi:10.1051/0004-6361/201732525. Bibcode: 2018A&A...619A...6O. 
  15. ↑ "SIMBAD Query Result: ADS 14636 AB -- Double or multiple star". Centre de Données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-id?Ident=61+cygni&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id.  (61 Cygni)
  16. ↑ 16.0 16.1 Staff (6 July 2007). "High Proper Motion Stars: Interesting Areas to View". ESA. http://www.cosmos.esa.int/web/hipparcos/high-proper-motion-stars-ii. 
  17. ↑ Hipparcos: Catalogues: The Millennium Star Atlas: The Top 20 High Proper Motion , European Space Agency, retrieved 2019-06-27
  18. ↑ Wittenmyer, R. A.; Endl, M.; Cochran, W.D.; Hatzes, A.; Walker, G. A. H.; Yang, S. L. S.; Paulson, D. B. (2006). "Detection limits from the McDonald Observatory planet search program". Astronomical Journal 132 (1): 177–188. doi:10.1086/504942. Bibcode: 2006AJ....132..177W. 
  19. ↑ Allen, Richard Hinckley (2003). Star Names and Their Meanings. Kessinger. p. 219. ISBN 978-0-7661-4028-8. 
  20. ↑ Sun, Xiaochun; Kistemaker, Jacob (1997). The Chinese Sky During the Han: Constellating Stars and Society. Brill. ISBN 978-90-04-10737-3. Bibcode: 1997csdh.book.....S. https://books.google.com/books?id=87lvBoFi8A0C. 
  21. ↑ "Naming Objects Outside the Solar System-Stars". IAU. http://www.iau.org/public/themes/naming/. 
  22. ↑ Kaler, Jim (8 July 2009). "61 Cygni". http://stars.astro.illinois.edu/sow/61cyg.html. 
  23. ↑ Flamsteed, John (1725). Historia Coelestis Britannica. Meere. p. 5. https://books.google.com/books?id=CWFDAAAAcAAJ&q=61. 
  24. ↑ Dibon-Smith, Richard (1998). The Flamsteed Collection. Clear Skies. p. xi. https://books.google.com/books?ei=UCdFVbivGe21sASToYDQCA. 
  25. ↑ "61 Cyg (Piazzi's Flying Star)". http://news.sky-map.org/starview?object_type=1&object_id=2282&object_name=HD+201091&locale=EN. 
  26. ↑ Covington, Michael (26 September 2002). Celestial Objects for Modern Telescopes: Practical Amateur Astronomy. Cambridge University Press. p. 209. ISBN 978-0-521-52419-3. https://archive.org/details/celestialobjects00covi. 
  27. ↑ 27.0 27.1 27.2 27.3 27.4 27.5 Hopkins, Mary Murray (1 November 1916). "The Parallax of 61 Cygni". Journal of the Royal Astronomical Society of Canada 10: 498–504. Bibcode: 1916JRASC..10..498H. 
  28. ↑ Piazzi, Giuseppe (1803). Præcipuarum stellarum inerrantium positiones mediae ineunte seculo XIX: ex observationibus habitis in specula Panormitana ab anno 1792 ad annum 1802. Typis regiis. p. 111. https://books.google.com/books?id=66lFAAAAcAAJ&pg=PR1. 
  29. ↑ 29.0 29.1 Fodera-Serio, G. (1990). "Giuseppe Piazzi and the Discovery of the Proper Motion of 61-Cygni" (in la). Journal for the History of Astronomy 21 (3): 275–282. doi:10.1177/002182869002100302. Bibcode: 1990JHA....21..275F. 
  30. ↑ Hirshfeld, Alan (2001). Parallax: The Race to Measure the Cosmos. Macmillan. ISBN 978-0-7167-3711-7. https://archive.org/details/parallax00alan. 
  31. ↑ Bessel, F. W. (1838). "On the parallax of 61 Cygni". Monthly Notices of the Royal Astronomical Society 4 (17): 152–161. doi:10.1093/mnras/4.17.152. Bibcode: 1838MNRAS...4..152B. 
  32. ↑ Bessel, F. W. (1838). "Bestimmung der Entfernung des 61sten Sterns des Schwans" (in de). Astronomische Nachrichten 16 (365–366): 65–96. doi:10.1002/asna.18390160502. Bibcode: 1838AN.....16...65B. https://zenodo.org/record/1424605. Retrieved 2 July 2019. 
  33. ↑ Frommert, Hartmut; Kronberg, Christine. "Friedrich Wilhelm Bessel". Students for the Exploration and Development of Space. http://messier.seds.org/xtra/Bios/bessel.html. 
  34. ↑ Hughes, Stefan (2012). Catchers of the Light. ArtDeCiel Publishing. p. 702. ISBN 978-1-62050-961-6. https://books.google.com/books?id=iZk5OOf7fVYC&pg=PA702. 
  35. ↑ Bessel, F. W. (1839). "Bestimmung der Entfernung des 61sten Sterns des Schwans. Von Herrn Geheimen – Rath und Ritter Bessel" (in de). Astronomische Nachrichten 16 (5–6): 65–96. doi:10.1002/asna.18390160502. Bibcode: 1838AN.....16...65B. https://zenodo.org/record/1424605. Retrieved 2 July 2019. "(page 92) Ich bin daher der Meinung, daß nur die jährliche Parallaxe = 0"3136 als das Resultat der bisherigen Beobachtungen zu betrachten ist". 
  36. ↑ 36.0 36.1 Davis, Merhan S. (1898). "Remarks regarding the parallaxes of 61 Cygni and the probable physical connection of these two stars". Astrophysical Journal 8: 246–247. doi:10.1086/140527. Bibcode: 1898ApJ.....8..246D. 
  37. ↑ Adams, W. S.; Joy, A. H. (1917). "The luminosities and parallaxes of five hundred stars". Astrophysical Journal 46: 313–339. doi:10.1086/142369. Bibcode: 1917ApJ....46..313A. —See Table I, page 326
  38. ↑ Baize, P. (1950). "Second catalogue d'orbites d'Etoiles Doubles visuelles" (in fr). Journal des Observateurs 33: 1–31. Bibcode: 1950JO.....33....1B. —on page 19, the authority is listed as Zagar (1934).
  39. ↑ Boss, Benjamin (1911). "Community of motion among several stars of large proper-motion". Astronomical Journal 27 (629): 33–37. doi:10.1086/103931. Bibcode: 1911AJ.....27...33B. 
  40. ↑ Eggen, O. J. (1959). "White dwarf members of the 61 Cygni group". The Observatory 79: 135–139. Bibcode: 1959Obs....79..135E.  – Gives space velocity components of U=+94, V=–53 and W=–7 for HD 201091/2.
  41. ↑ Sol Company. "System Summary Pi Mensae". Sol Company. http://www.solstation.com/stars2/pimensae.htm. 
  42. ↑ Howard, Andrew W.; Fulton, Benjamin J. (2016). "Limits on Planetary Companions from Doppler Surveys of Nearby Stars". Publications of the Astronomical Society of the Pacific 128 (969): 114401. doi:10.1088/1538-3873/128/969/114401. Bibcode: 2016PASP..128k4401H. 
  43. ↑ Espenak, Fred (25 July 1996). "Twelve Year Planetary Ephemeris: 1995–2006". NASA. http://eclipse.gsfc.nasa.gov/TYPE/TYPE.html. 
  44. ↑ Adler, Alan (26 July 2006). "More Pretty Double Stars". Sky & Telescope (Sky and Telescope). http://www.skyandtelescope.com/observing/more-pretty-double-stars/. 
  45. ↑ Gudel, M. (1992). "Radio and X-ray emission from main-sequence K stars". Astronomy and Astrophysics 264 (2): L31–L34. Bibcode: 1992A&A...264L..31G. 
  46. ↑ Eggen, Olin J. (October 1969), "Stellar Groups in the Old Disk Population", Publications of the Astronomical Society of the Pacific 81 (482): 553, doi:10.1086/128823, Bibcode: 1969PASP...81..553E 
  47. ↑ Nave, Rod. "61 Cygni". HyperPhysics. http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/cygni.html. 
  48. ↑ Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society 25: 1319, Bibcode: 1993AAS...183.1710G, http://www.astro.utoronto.ca/~garrison/mkstds.html, retrieved 4 February 2012 
  49. ↑ Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas". Astrophysical Journal 117: 313. doi:10.1086/145697. Bibcode: 1953ApJ...117..313J. 
  50. ↑ Keenan, P. C.; McNeil, R. C. (October 1989). "The Perkins Catalog of Revised MK Types for the Cooler Stars". Astrophysical Journal Supplement Series 71: 245–266. doi:10.1086/191373. Bibcode: 1989ApJS...71..245K. 
  51. ↑ "SIMBAD Query Result: HD 201092". Centre de Données astronomiques de Strasbourg. 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. 
  52. ↑ Frick, P.; Baliunas, S. L.; Galyagin, D.; Sokoloff, D.; Soon, W. (1997). "Wavelet Analysis of Chromospheric Activity". Astrophysical Journal 483 (1): 426–434. doi:10.1086/304206. Bibcode: 1997ApJ...483..426F. 
  53. ↑ 53.0 53.1 Hempelmann, A.; Schmitt, J. H. M. M.; Baliunas, S. L.; Donahue, R. A. (2003). "Evidence for coronal activity cycles on 61 Cygni A and B". Astronomy and Astrophysics 406 (2): L39–L42. doi:10.1051/0004-6361:20030882. Bibcode: 2003A&A...406L..39H. 
  54. ↑ 54.0 54.1 Böhm-Vitense, Erika (March 2007), "Chromospheric Activity in G and K Main-Sequence Stars, and What It Tells Us about Stellar Dynamos", The Astrophysical Journal 657 (1): 486–493, doi:10.1086/510482, Bibcode: 2007ApJ...657..486B 
  55. ↑ Wood, Brian E.; Müller, Hans-Reinhard; Zank, Gary P.; Linsky, Jeffrey L. (July 2002). "Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity". The Astrophysical Journal 574 (1): 412–425. doi:10.1086/340797. Bibcode: 2002ApJ...574..412W. 
  56. ↑ Hempelmann, A.; Robrade, J.; Schmitt, J. H. M. M.; Favata, F.; Baliunas, S. L.; Hall, J. C. (2006). "Coronal activity cycles in 61 Cygni". Astronomy and Astrophysics 460 (1): 261–267. doi:10.1051/0004-6361:20065459. Bibcode: 2006A&A...460..261H. 
  57. ↑ Barnes, Sydney A. (November 2007). "Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors". The Astrophysical Journal 669 (2): 1167–1189. doi:10.1086/519295. Bibcode: 2007ApJ...669.1167B. 
  58. ↑ Strand, K. Aa. (1943). "61 Cygni as a Triple System". Publications of the Astronomical Society of the Pacific 55 (322): 29–32. doi:10.1086/125484. Bibcode: 1943PASP...55...29S. 
  59. ↑ Darrell Schweitzer; Theodore Sturgeon; Alfred Bester (2009). Science Fiction Voices #1. Wildside Press LLC. p. 64. ISBN 978-1-4344-0784-9. https://books.google.com/books?id=v6DNXIN3UqEC&q=third+body+61+cygni+mission+of+gravity&pg=PA44. Retrieved 3 February 2019. 
  60. ↑ Strand, K. Aa. (1957). "The orbital motion of 61 Cygni". The Astronomical Journal 62: 35. doi:10.1086/107588. Bibcode: 1957AJ.....62Q..35S. 
  61. ↑ Cumming, A.; Marcy, G. W.; Butler, R. P. (1999). "The Lick planet search: detectability and mass thresholds". Astrophysical Journal 526 (2): 890–915. doi:10.1086/308020. Bibcode: 1999ApJ...526..890C. 
  62. ↑ Deich, A. N. (1977). "Invisible companions of the binary star 61 Cygni". Soviet Astronomy 21: 182–188. Bibcode: 1977SvA....21..182D. 
  63. ↑ Heintz, W. D. (1978). "Reexamination of suspected unresolved binaries". The Astrophysical Journal 220: 931–934. doi:10.1086/155982. Bibcode: 1978ApJ...220..931H. 
  64. ↑ Walker, G. A. H.; Walker, A. R.; Irwin, A. W.; Larson, A. M.; Yang, S. L. S.; Richardson, D. C. (1995). "A search for Jupiter-mass companions to nearby stars". Icarus 116 (2): 359–375. doi:10.1006/icar.1995.1130. Bibcode: 1995Icar..116..359W. 
  65. ↑ Kervella, Pierre; Arenou, Frédéric et al. (2019). "Stellar and substellar companions of nearby stars from Gaia DR2". Astronomy & Astrophysics 623: A72. doi:10.1051/0004-6361/201834371. ISSN 0004-6361. Bibcode: 2019A&A...623A..72K. ""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."". 
  66. ↑ Cantrell, Justin R.; Henry, Todd J.; White, Russel J. (October 2013), "The Solar Neighborhood XXIX: The Habitable Real Estate of Our Nearest Stellar Neighbors", The Astronomical Journal 146 (4): 99, doi:10.1088/0004-6256/146/4/99, Bibcode: 2013AJ....146...99C 
  67. ↑ Wittenmyer, R. A.; Endl, M.; Cochran, W.D.; Hatzes, A.; Walker, G. A. H.; Yang, S. L. S.; Paulson, D. B. (May 2006). "Detection Limits from the McDonald Observatory Planet Search Program". The Astronomical Journal 132 (1): 177–188. doi:10.1086/504942. Bibcode: 2006AJ....132..177W. 
  68. ↑ McCarthy, Christopher (2005). "SIM Planet Search Tier 1 Target Stars". San Francisco State University. http://tauceti.sfsu.edu/~chris/SIM/t1.html. 
  69. ↑ Kuchner, Marc J.; Brown, Michael E.; Koresko, Chris D. (1998). "An 11.6 Micron Keck Search for Exo-Zodiacal Dust". The Publications of the Astronomical Society of the Pacific 110 (753): 1336–1341. doi:10.1086/316267. Bibcode: 1998PASP..110.1336K. 
  70. ↑ Millan-Gabet, R.; Serabyn, E.; Mennesson, B.; Traub, W. A.; Barry, R. K.; Danchi, W. C.; Kuchner, M.; Stark, C. C. et al. (June 2011), "Exozodiacal Dust Levels for Nearby Main-sequence Stars: A Survey with the Keck Interferometer", The Astrophysical Journal 734 (1): 67, doi:10.1088/0004-637X/734/1/67, Bibcode: 2011ApJ...734...67M.  See Table 5, p. 58.
  71. ↑ Bill Steigerwald (2019-03-07). ""Goldilocks" Stars May Be "Just Right" for Finding Habitable Worlds". NASA. https://www.nasa.gov/feature/goddard/2019/k-star-advantage. "'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." 

External links

Coordinates: Sky map 21h 06m 53.9434s, +38° 44′ 57.898″



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