31 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.
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Cygnus |
| 31 Cygni A | |
| Right ascension | 20h 13m 37.908s |
| Declination | +46° 44′ 28.76″ |
| Apparent magnitude (V) | 3.73 - 3.89 |
| HD 192379 | |
| Right ascension | 20h 13m 39.199s |
| Declination | +46° 42′ 42.70″ |
| Apparent magnitude (V) | 6.99 |
| Characteristics | |
| 31 Cygni A | |
| Spectral type | K3Ib + B2IV-V |
| Variable type | Algol |
| HD 192579 | |
| Evolutionary stage | main sequence |
| Spectral type | B5V |
| Astrometry | |
| 31 Cygni A | |
| Radial velocity (Rv) | −7.41±0.08 km/s |
| Proper motion (μ) | RA: 1.744 mas/yr Dec.: 3.390 mas/yr |
| Parallax (π) | 2.9167 ± 0.1327 mas |
| Distance | 1,120 ± 50 ly (340 ± 20 pc) |
| HD 192579 | |
| Proper motion (μ) | RA: 3.906 mas/yr Dec.: 1.978 mas/yr |
| Parallax (π) | 2.9160 ± 0.0336 mas |
| Distance | 1,120 ± 10 ly (343 ± 4 pc) |
| Orbit | |
| Primary | 31 Cygni Aa |
| Companion | 31 Cygni Ab |
| Period (P) | 3,784.3 d |
| Eccentricity (e) | 0.2084±0.0031 |
| Periastron epoch (T) | 2,452,345±9 |
| Argument of periastron (ω) (secondary) | 204.5±1.0° |
| Semi-amplitude (K1) (primary) | 13.94±0.04 km/s |
| Semi-amplitude (K2) (secondary) | 18.0 km/s |
| Details | |
| K supergiant | |
| Mass | 6.73 M☉ |
| Radius | 127 R☉ |
| Luminosity | 2,512 - 2,559 L☉ |
| Temperature | 4,043±170 K |
| B dwarf | |
| Mass | 5.22 M☉ |
| Radius | 5.2±0.5 R☉ |
| Temperature | 16,500+1,000 −2,000 K |
| Age | 39.8 Myr |
| HD 192579 | |
| Mass | 4.1 M☉ |
| Radius | 3.3 R☉ |
| Luminosity | 373 L☉ |
| Surface gravity (log g) | 4.04 cgs |
| Temperature | 13,949 K |
| Metallicity [Fe/H] | +0.68 dex |
| Other designations | |
| 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 | |
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.
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. The 13th-magnitude star is likely to be an unrelated background object.
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.
31 Cygni A is an Algol-type eclipsing binary and ranges between magnitudes 3.73 and 3.89 over a period of ten years. 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.
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.