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Sigma Sagittarii

Second brightest star in the constellation Sagittarius

Sigma Sagittarii

Location of σ Sagittarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	18h55m 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 mas[1][2]
Distance	228 ± 5 ly (70 ± 1 pc)
Absolute magnitude (MV)	−2.17[6]
Details
Mass	7.8±0.2[7] M☉
Radius	4.5[8] R☉
Luminosity	3,300[9] L☉
Temperature	18,890[9] K
Rotational velocity (v sin i)	165[10] km/s
Age	31.4±0.4[7] Myr
Other designations
Nunki, Sadira[11][12], σ Sgr, Sigma Sgr, 34 Sagittarii, 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/,^[13][14] is the second-brightest star in the constellationofSagittarius. It 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,^[15] yields a value of approximately 228 light-years (70parsecs) from the Sun.^[1][2]

Sigma Sagittarii has a spectrum matching a stellar classification of B2.5 V,^[4] which indicates this is a B-type main-sequence star. Its total luminosity is 3300^[16] times that of the Sun while it has a surface temperature of 18,890 K.^[9] X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.2 × 10²⁸ erg s⁻¹.^[17]

It has a 10th magnitude optical companion located 5.2 arcminutes away.^[18]

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.^[19] 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.^{[citation needed]}

σ Sagittarii (LatinisedtoSigma Sagittarii) is the star's Bayer designation. In his Uranometria star atlas, Johann Bayer placed this star in the fourth magnitude class, although it is a second-magnitude star by modern measurements.^{[citation needed]}

It bore the traditional name of Nunki, which was an AssyrianorBabylonian name recovered by archaeologists and made public by R. H. Allen.^[20] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[21] to catalogue and standardize proper names for stars. The WGSN approved the name Nunki for this star on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.^[14]

This star, together with :

• Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii, Zeta Sagittarii, Lambda Sagittarii, Tau Sagittarii and Phi Sagittarii, comprised the Teapot asterism.^[22]
• Phi Sagittarii, Zeta Sagittarii, Chi Sagittarii and Tau Sagittarii were the Arabic Al Naʽām al Ṣādirah (النعم السادرة), the Returning Ostriches.^[23]

Zeta Sagittarii and Pi Sagittarii may have been the Akkadian Gu-shi-rab‑ba, the Yoke of the Sea.^[23]

In the catalogue of stars in the CalendariumofAl Achsasi al Mouakket, this star was designated Thanih al Sadirah, which was translated into LatinasSecunda τού al Sadirah, meaning second returning ostrich.^[24]

InChinese, 斗 (Dǒu), meaning Dipper, refers to an asterism consisting of Sigma Sagittarii, Phi Sagittarii, Lambda Sagittarii, Mu Sagittarii, Tau Sagittarii and Zeta Sagittarii. Consequently, the Chinese name for Sigma Sagittarii itself is 斗宿四 (Dǒu Xiù sì, English: the Fourth Star of Dipper.)^[25]