61 Cygni 2

JPL, CalTech, NASA



Larger illustration of NASA's
Space Interferometry Mission (SIM).



Astronomers have identified 61 Cygni A and B
as prime targets for NASA's optical SIM mission,
now indefinitely delayed.

Due to 61 Cygni AB's proximity to Sol, the system has been an object of high interest among astronomers. Stars A and B have been selected as "Tier 1" target stars for NASA's optical Space Interferometry Mission (SIM). The mission will attempt to detect planets as small as three Earth-masses within two AUs of each star. Although some summary system information and images of Star A and Star B may still be available from the SIM Teams, the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding.

© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)




61 Cygni A and B are orange-red dwarf
stars, like Epsilon Eridani at left center
of meteor. See a 2MASS Survey image
of 61 Cygni AB from the NASA Star
and Exoplanet Database.)

AB Binary Star System

In 1830, Friedrich Georg Wilhelm von Struve (1793-1864) made the first measurements of the star as a binary system. It is now known that 61 Cygni A and B orbit each other at an "average" distance (semi-major axis) of 86.4 times the Earth-Sun distance, or about 86 astronomical units (AUs) -- which is more than twice Pluto's orbital distance in the Solar System. 61 Cygni was the first star system (other than the Sun) to have its distance to Earth successfully calculated by Friedrich Wilhelm Bessel (1784-1846) using trigonometric parallax and the diameter of the Earth's orbit around the Sun in 1838. Smaller and dimmer than Sol, the stars are barely visible with the naked eye.

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Solar)	Diameter (Solar)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
AB Mass Center	0.0	...	...	...	...	...	...	...	...
61 Cygni A	39.6	722	0.40	51.85	0.70	0.48-1.05	...	...	0.79
Inner H.Z. Edge A?	0.385	0.286	0	51.85	...	...	...	...	...
Outer H.Z. Edge A?	0.755	0.784	0	51.85	...	...	...	...	...
61 Cygni B	46.8	722	0.40	51.85	0.63	0.43-0.80	...	...	0.79
Inner H.Z. Edge B?	0.315	0.223	0	51.85	...	...	...	...	...
Outer H.Z. Edge B?	0.617	0.611	0	51.85	...	...	...	...	...

61 Cygni A

This star is a orange-red main sequence dwarf star of spectral and luminosity type K3.5-5.0 Ve. It probably has about 70 percent of Sol's mass (RECONS), 72 percent of its diameter (Johnson and Wright, 1983, page 699), and about 8.5 percent of its luminosity. The star system appears to be about 79 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron. Dust has been detected around in this binary system (Backman et al, 1986; and Kuchner et al, 1998 -- in pdf). Stars A and B may be older than Sol, because they do not rotate rapidly and the system exhibits moderately high U,V,W velocities in their galactic orbital motion (Ken Croswell, 1995, pp. 253-4).

61 Cygni A seems to be a variable star. It has been given the variable star designation V1083 Cygni as well as the New Suspected Variable designation of NSV 13543. The star and its stellar companion B have a highly elliptical orbit (e= 0.40) that swings them between 51.7 and 121.0 AUs apart in an orbit that lasts about 722 years (Frederick Jerrold or "Jerry" Josties, 1980; in the Fourth Catalog of Orbits of Visual Binaries, Worley and Heintz, 1983 -- see Alcyone Software's web page on 61 Cygni). While the new Sixth Catalog of Orbits of Visual Binaries seems to have an error in its citation, the older Fifth Catalog provided orbital elements of: a=24.4", 85.2 AUs at 11.4 ly distance; P=659 years; e=0.48; and i=126° (Kiselev et al, 1997). Useful star catalogue numbers for 61 Cygni A include: V1083 Cyg, HR 8085, Gl 820 A, Hip 104214, HD 201091, BD+38 4343, SAO 70919, FK5 773, LHS 62, Struve 2758 A, NSV 13543, and ADS 14636 A.

Habitable Zone around Star A

According to calculations performed for the NASA Star and Exoplanet Database, the inner edge of Star A's habitable zone could be located at around 0.385 AUs from the star, while the outer edge lies farther out at around 0.755 AUs. Accounting for infrared heating, the distance from 61 Cygni A where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.57 AU -- between the orbital distances of Mercury and Venus in the Solar System. At that distance from the star, such a planet would have an orbital period of about 188 days -- just over half an Earth year.

61 Cygni B

This star is a orange-red main sequence dwarf star of spectral and luminosity type K4.7-7.0 Ve. This star probably has about 63 percent of Sol's mass (RECONS), 67 percent of its diameter (Johnson and Wright, 1983, page 699), and about 3.9 percent of its luminosity. Dust has also been found around this star, which appears to be a variable and has been given the New Suspected Variable designation NSV 13546. Useful star catalogue numbers for 61 Cygni B include: HR 8086, Gl 820 B, Hip 104217, HD 201092, BD+38 4344, LHS 63, Struve 2758 B, and ADS 14636 B.

Habitable Zone around Star B

Hunt for Substellar Companions

In 1942, Kaj Aage Gunnar Strand (1907-2000; obit) -- who later became scientific director of the U.S. Naval Observatory -- used photographic observations from Potsdam, Lick, and Sproul observatories to detect small systematic variations in the way that 61 Cygni A and B orbit each other, which suggested that there was a third, unseen body orbiting 61 Cygni A (Strand, 1943). In an article for the February 1957 issue of the Astronomical Journal (Strand, 1957), Strand analyzed more data and reported that the object appeared to have a mass about eight times that of Jupiter. Its orbital period was calculated to be 4.8 years which suggested an "average" orbital distance (semi-major axis) from 61 Cygni A of about 2.4 AUs -- around the inner side of the main asteroid belt in the Solar System. Despite its low mass, and a calculated diameter of less than one-tenth of Sol's, this unconfirmed third member of the system was often referred to as "61 Cygni C."

In 1978, however, Wulff Dieter Heintz (1930-2006; obit) reported on his failure to find evidence that supports the existence of an object below six percent of Sol's mass -- about 60 Jupiter masses -- in the 61 Cygni system (Heintz, 1978). Heintz found that the strength of later data suggests that the earlier results were "spurious."

According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 8085, later astrometric measures suggested the existence of three invisible companions with orbital periods of six, seven, and 12 years. However, the suspected objects were not resolved with speckle interferometry. Hence, the suspected planetary companions were "not substantiated."

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