Sigma Sagittarii
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 55m 15.92650s |
| Declination | −26° 17′ 48.2068″ |
| Apparent magnitude (V) | +2.05 |
| Characteristics | |
| Spectral type | B2.5 V |
| U−B color index | −0.761 |
| B−V color index | −0.204 |
| Astrometry | |
| Radial velocity (Rv) | −11.2 km/s |
| Proper motion (μ) | RA: +15.14 mas/yr Dec.: −53.43 mas/yr |
| Parallax (π) | 14.32±0.29 mas |
| Distance | 224.7 ± 9.8 ly (68.9±3.0 pc) |
| Absolute magnitude (MV) | −2.17 |
| Orbit | |
| Period (P) | 134.779±0.025 days |
| Semi-major axis (a) | (18.0±0.2)×10−3" (1.26±0.05 au) |
| Eccentricity (e) | 0.492±0.003 |
| Inclination (i) | 19.7±1.9° |
| Longitude of the node (Ω) | 45.9±8.0° |
| Periastron epoch (T) | 48,588.1±2.2 MJD |
| Argument of periastron (ω) (secondary) | 76.4±7.1° |
| Details | |
| Aa | |
| Mass | 6.5±0.2 M☉ |
| Radius | 4.1±0.5 R☉ |
| Surface gravity (log g) | 4.0 cgs |
| Temperature | 18,500 K |
| Rotational velocity (v sin i) | 165 km/s |
| Age | 31.4±0.4 Myr |
| Ab | |
| Mass | 6.3±0.2 M☉ |
| Radius | 3.9±0.5 R☉ |
| Surface gravity (log g) | 4.0 cgs |
| Temperature | 18,500 K |
| Rotational velocity (v sin i) | 160 km/s |
| Age | 30 Myr |
| Other designations | |
| Nunki, Sadira, σ Sgr, Sigma Sgr, 34 Sagittarii, CD−26 13595, CPD−27 5241, FK5 706, GC 25941, HD 175191, HIP 92855, HR 7121, SAO 187448, PPM 269078, CCDM J18552-2618A, WDS J18553-2618Aa,Ab | |
| Database references | |
| SIMBAD | data |
Sigma Sagittarii, Latinized from σ Sagittarii; formally named Nunki /ˈnʌŋki/, is the second-brightest star in the constellation of Sagittarius. It is a binary star system, viewed as a single star of combined apparent magnitude +2.05, about the same brightness as Saiph in Orion. The distance to this system, determined using a dynamical parallax measurement, is 225 light-years (68.9 parsecs).
It is 3.45 degrees south of the ecliptic, so it can be occulted by the Moon and rarely by planets. The last occultation by a planet took place on November 17, 1981, when it was occulted by Venus.