Sigma Sagittarii
 Location of σ Sagittarii (circled) |
|
| 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 ± 0.29mas |
| Distance | 228 ± 5 ly (70 ± 1 pc) |
| Details | |
| Mass | 7.8 ± 0.2 M☉ |
| Radius | 4.5 R☉ |
| Luminosity | 3,300 L☉ |
| Temperature | 18,890 K |
| Rotational velocity (v sin i) | 165 km/s |
| Age | 31.4 ± 0.4 Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Sigma Sagittarii (σ Sagittarii, abbreviated Sigma Sgr, σ Sgr), also named Nunki, is the second brightest star in the constellation of Sagittarius. It has an apparent magnitude of +2.05, making it readily visible to the naked eye. The distance to this star, determined using parallax measurements from the Hipparcos astrometry satellite, yields an estimated value of 228 light-years (70 parsecs) from the Sun.
Sigma Sagittarii has a spectrum matching a stellar classification of B2.5 V, which indicates this is a B-type main sequence star. Its total luminosity is 3300 times that of the Sun while it has a surface temperature of 18,890 K. X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.2 × 1028 erg s−1.
It has a 10th magnitude optical companion located 5.2 arcminutes away.
Because it is close to the ecliptic, it can be occulted by the Moon and very rarely by planets. The last occultation by a planet took place on November 17, 1981, when it was occulted by Venus. Furthermore, this is the brightest star that can be principally occulted by an exterior planet between 5000 BC and 5000 AD. However, only Mars can do this, and only rarely; the last time was on September 3, 423.
σ Sagittarii (Latinised to Sigma Sagittarii) is the star's Bayer designation.
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