Tau Cygni

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Tau Cygni AB[1][2][3]
Diagram showing star positions and boundaries of the Cygnus constellation and its surroundings
Cercle rouge 100%.svg

Location of τ Cygni (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cygnus
Right ascension 21h 14m 47.4916s
Declination +38° 02′ 43.141″
Apparent magnitude (V) 3.80 / 6.69[4]
Spectral type F2 IV + G0 V
U−B color index +0.03 / +0.09
B−V color index +0.38 / +0.60
Radial velocity (Rv) -21.1 km/s
Proper motion (μ) RA: 195.75 mas/yr
Dec.: 410.03 mas/yr
Parallax (π) 47.80 ± 0.61 mas
Distance 68.2 ± 0.9 ly
(20.9 ± 0.3 pc)
τ Cyg A
Mass 1.65[4] M
Radius 2.48[4] R
Surface gravity (log g) 3.87[4] cgs
Temperature 6,600[4] K
Metallicity [Fe/H] 0.05[4] dex
τ Cyg B
Mass 1.03[4] M
Radius 0.93[4] R
Surface gravity (log g) 4.52[4] cgs
Temperature 5,670[4] K
Other designations
Tau Cygni, 65 Cygni, BD+37°4240, HD 202444, HIP 104887, HR 8130, SAO 71121, GC 29723, CCDM J21148+3803AB.

Tau Cygni (τ Cyg) is a binary star system in the constellation Cygnus, approximately 69 light years away from Earth.[1] This visual binary system has a period of 49.6 years.[4]

The main star, 4th magnitude GJ 822.1 A, is a yellowish white subgiant star of the spectral type F2IV. It therefore has a surface temperature of 6,000 to 7,500 kelvins and is larger, hotter, and several times as bright as the Sun.[2] Its companion, 6th magnitude GJ 822.1 B, is a yellow main sequence star of the spectral type G0V. It is similar to the Sun in size, surface temperature, and luminosity.[3]


  1. ^ a b "Simbad Query Result". Simbad. Retrieved October 18, 2007. 
  2. ^ a b "Simbad Query Result". Simbad. Retrieved October 18, 2007. 
  3. ^ a b "Simbad Query Result". Simbad. Retrieved October 18, 2007. 
  4. ^ a b c d e f g h i j k Fuhrmann, Klaus (February 2008), "Nearby stars of the Galactic disc and halo - IV", Monthly Notices of the Royal Astronomical Society, 384 (1): 173–224, Bibcode:2008MNRAS.384..173F, doi:10.1111/j.1365-2966.2007.12671.x