Sigma Sagittarii
 Location of σ Sagittarii (circled) |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 55m 15.92650s[1][2] |
| Declination | –26° 17′ 48.2068″[1][2] |
| Apparent magnitude (V) | 2.05[3] |
| Characteristics | |
| Spectral type | B2.5 V[4] |
| U−B color index | –0.761[5] |
| B−V color index | –0.204[5] |
| Astrometry | |
| Radial velocity (Rv) | –11.2[3] km/s |
| Proper motion (μ) | RA: +15.14[1][2] mas/yr Dec.: –53.43[1][2] mas/yr |
| Parallax (π) | 14.32 ± 0.29[1][2] mas |
| Distance | 228 ± 5 ly (70 ± 1 pc) |
| Details | |
| Mass | 7.8 ± 0.2[6] M☉ |
| Radius | 4.5[7] R☉ |
| Luminosity | 3,300[8] L☉ |
| Temperature | 18,890[8] K |
| Rotational velocity (v sin i) | 165[9] km/s |
| Age | 31.4 ± 0.4[6] Myr |
| Other designations | |
Sigma Sagittarii (Sigma Sgr, σ Sagittarii, σ Sgr) is the second brightest star in the constellation Sagittarius. Its modern name Nunki is an Assyrian or Babylonian name recovered by archaeologists and made public by R. H. Allen.[10] Nunki has an apparent magnitude of +2.05,[3] making it readily visible to the naked eye. The distance to this star, determined using parallax measurements from the Hipparcos astrometry satellite,[11] yields an estimated value of 228 light-years (70 parsecs) from Earth.[1][2]
Nunki has a spectrum matching a stellar classification of B2.5 V,[4] which indicates this is a B-type main sequence star. The total luminosity of σ Sgr is 3300[12] times that of the Sun while it has a surface temperature of 18,890 K.[8] X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.2 × 1028 erg s−1.[13] Sigma Sgr has a magnitude +9.5 optical companion located 5.2 arcminutes away.[citation needed]
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 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.[citation needed]
Name and etymology
- This star, together with :
- In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Thanih al Sadirah, which was translated into Latin as Secunda τού al Sadirah, meaning second returning ostrich.[17]
- In Chinese, 斗 (Dǒu), meaning Dipper, refers to an asterism consisting of σ Sagittarii, φ Sagittarii, λ Sagittarii, μ Sagittarii, τ Sagittarii and ζ Sagittarii. Consequently, σ Sagittarii itself is known as 斗宿四 (Dǒu Sù sì, English: the Fourth Star of Dipper.)[18]
References
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- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 3.0 3.1 3.2 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 4.0 4.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 5.0 5.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 6.0 6.1 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 8.0 8.1 8.2 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 16.0 16.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
