Eta Cygni

Eta Cygni (η Cygni) is a star in the northern constellation of Cygnus. It is visible to the naked eye with an apparent visual magnitude of 3.889.^[2] The star lies along the main body of the constellation, about midway between Gamma Cygni and Albireo.^[10] Based upon an annual parallax shift of 23.55 mas,^[1] it is located 138.5 light years from the Sun.

η Cygni

Location of η Cygni (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Cygnus
Right ascension	19h 56m 18.37184s[1]
Declination	+35° 05′ 00.3224″[1]
Apparent magnitude (V)	3.889[2]
Characteristics
Evolutionary stage	red clump[3]
Spectral type	K0 III[4]
U−B color index	+0.881[2]
B−V color index	+1.035[2]
Astrometry
Radial velocity (Rv)	−26.57±0.34[1] km/s
Proper motion (μ)	RA: −34.165 mas/yr[1] Dec.: −28.393 mas/yr[1]
Parallax (π)	23.5482±0.1078 mas[1]
Distance	138.5 ± 0.6 ly (42.5 ± 0.2 pc)
Absolute magnitude (MV)	0.74[5]
Details
Mass	0.9±0.1[6] M☉
Radius	10.2±0.3[6] R☉
Luminosity	60[6] L☉
Surface gravity (log g)	2.36±0.08[6] cgs
Temperature	4,783±20[7] K
Metallicity [Fe/H]	−0.02[6] dex
Rotational velocity (v sin i)	2.2 [8] km/s
Other designations
η Cygni, 21 Cygni, BD+34 3798, HD 188947, HIP 98110, HR 7615, SAO 69116, TYC 2677-1816-1, IRAS 19544+3456[9]
Database references
SIMBAD	data

This is an evolved red clump^[3] giant star with a stellar classification of K0 III.^[4] It is presently on the horizontal branch and is generating energy through the nuclear fusion of helium at its core. The star has about 0.9 times the mass of the Sun and has expanded to 10 It radiates 60 times the solar luminosity from its outer atmosphere at an effective temperature of 4,780 K.^[6]

Eta Cygni has five visual companions,^[11] of which only component B appears to be physically associated. This magnitude 12.0 star lies at an angular separation of 7.80 arc seconds along a position angle of 206°, as of 2007.^[12]