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Sigma Sagittarii
Sagittarius constellation map.png
Nunki is the σ star in the constellation of Sagittarius.
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 18h 55m 15.93s [1]
Declination –26° 17′ 48.2″ [1]
Apparent magnitude (V) 2.05
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: 15.14 ± 0.29 [1] mas/yr
Dec.: -53.43 ± 0.19 [1] mas/yr
Parallax (π) 14.32 ± 0.29[1] mas
Distance 228 ± 5 ly
(70 ± 1 pc)
Absolute magnitude (MV) -2.14
Details
Mass 7 M
Radius 5 R
Luminosity 3,300 L
Temperature 20,000 K
Metallicity ?
Rotation 201 km/s.
Age ? years
Other designations
Nunki, Sadira, 34 Sagittarii, HR 7121, CD -26°13595, HD 175191, SAO 187448, CPD P-26 6590, FK5 706, CCDM 18552-2618, HIP 92855.

Sigma Sagitarii (σ Sgr / σ Sagittarii) is the second brightest star system in the constellation Sagittarius.

Its modern name Nunki is an Assyrian or Babylonian name recovered by archaeologists and made public by R.H.Allen.[2]

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 principally occulted by an exterior planet. However, only Mars can do this, and only rarely; the last time was on September 3rd, 423.

References

  1. ^ a b c d e van Leeuwen, F. (2007). "HIP 92855". Hipparcos, the New Reduction. http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-out.add=.&-source=I/311/hip2&recno=92544. Retrieved 2009-12-13.  
  2. ^ p.359, Richard Hinckley Allen, "Star Names, their lore and meaning".

External links

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