Sigma Sagittarii
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 Nunki is the σ star in the constellation of Sagittarius. |
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| Observation data Epoch J2000 |
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|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 55m 15.9s |
| Declination | -26° 17′ 48″ |
| Apparent magnitude (V) | 2.05 |
| Spectral Characteristics | |
| Spectral type | B2.5 V |
| U-B color index | -0.75 |
| B-V color index | -0.22 |
| Variable type | ? |
| Astrometry | |
| Radial velocity (Rv) | -11 km/s |
| Proper motion (μ) | RA: 13.87 mas/yr Dec.: -52.65 mas/yr |
| Parallax (π) | 14.54 ± 0.88 mas |
| Distance | 220 ± 10 ly (69 ± 4 pc) |
| Absolute magnitude (MV) | -2.14 |
| Physical Characteristics | |
| Mass | 7 M☉ |
| Radius | 5 R☉ |
| Luminosity | 3,300 L☉ |
| Temperature | 20,000 K |
| Metallicity | ? |
| Rotation | 201 km/s. |
| Age | ? years |
| Other designations | |
Sigma Sagitarii (σ Sgr / σ Sagittarii) is the second brightest star system in the constellation Sagittarius. It also has the traditional name Nunki.
Nunki has an apparent magnitude of +2.1 and belongs to the spectral classification B3. The total luminosity of σ Sgr is 3300 times that of the Sun while it has a mass of roughly 7 solar masses. The radius of the star is equivalent to 5 Suns and it has a surface temperature of 20,000 kelvins. Sigma Sgr has a magnitude +9.5 companion, Sigma Sagitarii B, located 5.2 arcminutes away. It may also have a closer companion.
Because it is close to the ecliptic, Nunki can be occulted by the Moon and very rarely by planets. The last occultation of Nunki by a planet took place on November 17, 1981, when it was occulted by Venus.
Furthermore, Nunki is the brightest star that can be principially occulted by an exterior planet. However, only Mars can do this, and only rarely; the last time Mars occulted σ Sgr was on September 3rd, 423.
