31 Cygni

This article is about ο^1/2 Cygni. For other star systems with this Bayer designation, see ο Cygni.

31 Cygni, also known as ο¹ Cygni, Omicron¹ Cygni, ο² Cygni or V695 Cygni, is a ternary star system about 750 light years away in the constellation Cygnus.

31 Cygni

Location of 31 Cygni (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cygnus
31 Cygni A
Right ascension	20h 13m 37.908s[1]
Declination	+46° 44′ 28.76″[1]
Apparent magnitude (V)	3.73 - 3.89[2]
HD 192579
Right ascension	20h 13m 39.199s[3]
Declination	+46° 42′ 42.70″[3]
Apparent magnitude (V)	6.99[4]
Characteristics
31 Cygni A
Spectral type	K3Ib + B2IV-V[5]
Variable type	Algol[2]
HD 192579
Evolutionary stage	main sequence[4][3]
Spectral type	B5V[4]
Astrometry
31 Cygni A
Radial velocity (Rv)	−7.41±0.08[6] km/s
Proper motion (μ)	RA: 1.744[1] mas/yr Dec.: 3.390[1] mas/yr
Parallax (π)	2.9167±0.1327 mas[1]
Distance	1,120 ± 50 ly (340 ± 20 pc)
HD 192579
Proper motion (μ)	RA: 3.906[3] mas/yr Dec.: 1.978[3] mas/yr
Parallax (π)	2.9160±0.0336 mas[3]
Distance	1,120 ± 10 ly (343 ± 4 pc)
Orbit[7]
Primary	31 Cygni Aa
Companion	31 Cygni Ab
Period (P)	10.301+0.030 −0.037 years
Eccentricity (e)	0.203+0.007 −0.008
Inclination (i)	101.007+28.772 −18.623°
Longitude of the node (Ω)	307.271+9.802 −11.253°
Periastron epoch (T)	1950.508+0.171 −0.129
Argument of periastron (ω) (secondary)	198.619+2.771 −1.963°
Semi-amplitude (K1) (primary)	13.94±0.04[8] km/s
Semi-amplitude (K2) (secondary)	18.0[9] km/s
Details
K supergiant
Mass	9.44+1.51 −1.39[7] M☉
Radius	127[10] R☉
Luminosity	2,512 - 2,559[10] L☉
Temperature	4,043±170[10] K
B dwarf
Mass	6.475[7][a] M☉
Radius	5.2±0.5[11] R☉
Temperature	16,500+1,000 −2,000[12] K
Age	39.8[13] Myr
HD 192579
Mass	4.1[3] M☉
Radius	3.3[3] R☉
Luminosity	373[3] L☉
Surface gravity (log g)	4.04[3] cgs
Temperature	13,949[3] K
Metallicity [Fe/H]	+0.68[3] dex
Other designations
ο1 Cyg, ο2 Cyg,[14] 31 Cyg, ADS 13554, WDS J20136+4644
31 Cygni A (HD 192577): V695 Cyg, HD 192577, BD+46°2882, HIP 99675, HR 7735, SAO 49337
HD 192579: HD 192579, BD+46°2883, HIP 99676, SAO 49338
Database references
SIMBAD	data
	HD 192579

31 Cygni is the close pair, with 30 Cygni towards top left. (north is to the left)

The Bayer designation ο (omicron) has been variously applied to two or three of the stars 30, 31, and 32 Cygni. 31 Cygni has been designated, variously, as ο¹ or ο² Cygni — therefore for clarity, it is preferred to use the Flamsteed designation 31 Cygni.^[14]

31 Cygni consists of a visible pair of stars 109″ apart as of 2016, and the brighter of the two is also a spectroscopic binary. 31 Cygni A is also designated HD 192577 and HR 7735, while its 7th-magnitude visual companion is designated HD 192579. Some multiple star catalogues designate a 13th-magnitude star 36″ from 31 Cygni A as 31 Cygni B, and HD 192579 as 31 Cygni C.^[15] The 13th-magnitude star is likely to be an unrelated background object.^[16]

The spectroscopic pair are an orange supergiant of spectral type K3Ib and a blue-white star likely to be evolving off the main sequence with a spectral type of B2IV-V. The visible companion is a 7th-magnitude B5 main sequence star.^[4][3]

An ultraviolet band light curve for the 1982 eclipse of V695 Cygni, adapted from Stencel et al. (1984)^[17]

31 Cygni A is an Algol-type eclipsing binary and ranges between magnitudes 3.73 and 3.89 over a period of ten years.^[8] The eclipsing system has been studied in attempts to determine an accurate direct mass for a red supergiant. The value 6.73 M_☉ is believed to be accurate to about 2%, but there are some discrepancies in the orbital fit.^[9]

30 Cygni is another naked eye star a tenth of a degree away, forming a bright triple.

32 Cygni is about a degree away to the north, also a detached eclipsing binary system. It comprises a large cool evolved star and a small hot main sequence or subgiant companion.

Notes

1. from primary mass and mass ratio