Sigma Sagittarii

Sigma Sagittarii, Latinized from σ Sagittarii; formally named Nunki /ˈnʌŋki/,^[12][13] is the second-brightest star in the constellation of Sagittarius. It is a binary star system, viewed as a single star of combined apparent magnitude +2.05, about the same brightness as Saiph in Orion. The distance to this system, determined using parallax measurements from the Hipparcos astrometry satellite, is 228 light-years (70 parsecs).

Sigma Sagittarii

Location of σ Sagittarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	18h 55m 15.92650s[1]
Declination	−26° 17′ 48.2068″[1]
Apparent magnitude (V)	+2.05[2]
Characteristics
Spectral type	B2.5 V[3]
U−B color index	−0.761[4]
B−V color index	−0.204[4]
Astrometry
Radial velocity (Rv)	−11.2[2] km/s
Proper motion (μ)	RA: +15.14 mas/yr[1] Dec.: −53.43 mas/yr[1]
Parallax (π)	14.32±0.29 mas[1]
Distance	228 ± 5 ly (70 ± 1 pc)
Absolute magnitude (MV)	−2.17[5]
Details[6]
Aa
Mass	6.5±0.2 M☉
Radius	4.1±0.5 R☉
Temperature	18,890[7] K
Rotational velocity (v sin i)	165[8] km/s
Age	31.4±0.4[9] Myr
Ab
Mass	6.3±0.2 M☉
Radius	3.9±0.5 R☉
Temperature	18,600 K
Other designations
Nunki, Sadira[10][11], σ Sgr, Sigma Sgr, 34 Sagittarii, CD−26 13595, 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

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.^[14]

Properties

Sigma Sagittarii has a spectrum matching a stellar classification of B2.5 V,^[3] which indicates its components are B-type main-sequence stars. X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.2 × 10²⁸ erg s⁻¹.^[15]

The two component stars have masses of 6.5 and 6.3 solar masses (M_☉) and radii of 4.1 and 3.9 solar radii (R_☉), respectively. The effective temperatures are 18,900 K and 18,600 K respectively, over three times the Sun's effective temperature of 5,772 K. They are separated by about 0.6 astronomical units and take 50 days to complete an orbit around each other.^[6] Estimates suggest an age of about 30 million years.^[9][6]

Nunki is the nearest star expected to explode in a core-collapse supernova. 20 million years in the future, the primary star will evolve to a red giant, fill its roche lobe, and start to transfer mass to the secondary star. This will result in the system merging into a single star of about 12 solar masses, a mass sufficient to explode as a supernova.^[6]

Sigma Sagittarii has a 10th magnitude optical companion located 5.2 arcminutes away,^[16] but this is an unrelated background star.^[17] Spectroscopic observations in 2007-2008 hinted at the presence of a yet unseen stellar companion, estimated to have a mass of 1.2±0.2 M_☉ and a temperature of 6,100 K,^[18] but follow-up observations in 2016 by the same authors did not confirm this.^[19] However, in 2025 interferometric observations uncovered that Nunki is indeed a binary star, made up of two nearly identical stars. The separation of the components is too small to resolve them with a telescope, requiring an interferometer to be used.^[6]

Nomenclature

σ Sagittarii (Latinised to Sigma 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.^[20]

It bore the traditional name of Nunki, which was an Assyrian or Babylonian name recovered by archaeologists and popularized by R. H. Allen,^[21] though it is now thought that this name originally referred to an asterism in the area of Vela.^[22] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[23] 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.^[13]

This star, together with:

• Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii, Zeta Sagittarii, Lambda Sagittarii, Tau Sagittarii and Phi Sagittarii, comprised the Teapot asterism.^[24]
• Phi Sagittarii, Zeta Sagittarii, Chi Sagittarii and Tau Sagittarii were the Arabic Al Naʽām al Ṣādirah (النعم السادرة), the Returning Ostriches.^[21]
• Zeta Sagittarii and Pi Sagittarii may have been the Akkadian Gu-shi-rab‑ba, the Yoke of the Sea.^[21]

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.^[25]

In Chinese, 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.)^[26]

See also

• Spica – Second-closest star that will explode in a core-collapse supernova.