Tau Cygni

Tau Cygni, Latinised from τ Cygni, is a binary star system in the constellation Cygnus, approximately 69 light years away from Earth.[4] This visual binary system has a period of 49.6 years.[2]

τ Cygni

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)	3.65 - 3.75[1] (3.80 / 6.69[2])
Characteristics
Spectral type	F2 IV + G0 V
U−B color index	+0.03 / +0.09
B−V color index	+0.38 / +0.60
Variable type	δ Sct[1]
Astrometry
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)
Orbit[3]
Period (P)	810 days
Semi-major axis (a)	0.796 mas
Eccentricity (e)	0.43
Inclination (i)	92.6°
Longitude of the node (Ω)	78.7°
Periastron epoch (T)	2453139.5
Argument of periastron (ω) (secondary)	90°
Details
τ Cyg A
Mass	1.65[2] M☉
Radius	2.48[2] R☉
Surface gravity (log g)	3.87[2] cgs
Temperature	6,600[2] K
Metallicity [Fe/H]	0.05[2] dex
τ Cyg B
Mass	1.03[2] M☉
Radius	0.93[2] R☉
Surface gravity (log g)	4.52[2] cgs
Temperature	5,670[2] K
Other designations
τ Cygni, 65 Cygni, BD+37°4240, HD 202444, HIP 104887, HR 8130, SAO 71121, GC 29723, CCDM J21148+3803AB.
Database references
SIMBAD	data
	A
	B

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.[5] 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.[6]

Tau Cygni is classified as a δ Scuti variable. The magnitude range is given as 3.65 to 3.75, which is the combined magnitude for both components, although the variable component is A.[1]