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definition - Sigma_Scorpii

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Sigma Scorpii

Sigma Scorpii
Diagram showing star positions and boundaries of the Scorpius constellation and its surroundings
Cercle rouge 100%.svg

Location of σ Scorpii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Scorpius
Right ascension 16h 21m 11.31571s[1]
Declination –25° 35′ 34.0515″[1]
Apparent magnitude (V) +2.88[2]
Spectral type B1 III[3] + B1 V[4]
U−B color index –0.70[2]
B−V color index +0.13[2]
Radial velocity (Rv) –0.4[5] km/s
Proper motion (μ) RA: –10.60[1] mas/yr
Dec.: –16.28[1] mas/yr
Distance 568+75
[4] pc)
Absolute magnitude (MV) –4.12 ± 0.34 / −3.32 ± 0.34[4]
Primary σ Sco A
Companion σ Sco B
Period (P) 33.010 ± 0.002 days
Semimajor axis (a) 3.62 ± 0.06 mas
Eccentricity (e) 0.3220 ± 0.0012
Inclination (i) 158.2 ± 2.3°
Longitude of the node (Ω) 104 ± 5°
Periastron epoch (T) 34889.0 ± 1.0 MJD
Argument of periastron (ω)
283 ± 5°
σ Sco A
Mass 18.4 ± 5.4[4] M
Radius 12.7 ± 1.8[4] R
Luminosity (2.9 ± 0.8) × 104[4] L
Surface gravity (log g) 3.85[4]
Temperature 26,150 ± 1,070[4] K
Metallicity [Fe/H] –0.20 ± 0.20[6] dex
Rotational velocity (v sin i) 25[7] km/s
Age 8.0 ± 0.2[8] Myr
σ Sco B
Mass 11.9 ± 3.1[4] M
Radius 11[4] R
Luminosity (1.6 ± 0.4) × 104[4] L
Other designations
Alniyat, Al Niyat, 20 Scorpii, ADS 10009, CD-25 11485, FK5 607, HD 147165, HIP 80112, HR 6084, SAO 184336.[9]

Sigma Scorpii (σ Sco, σ Scorpii) is a star system in the constellation Scorpius. It has the traditional name Al Niyat, although this name is sometimes also applied to Tau Scorpii, or to the asterism formed by both Sigma and Tau. This system has a combined apparent visual magnitude of +2.88,[2] making it one of the brighter members of the constellation, although greatly outshone by its neighbor Antares. Based upon parallax measurements made during the Hipparcos mission, the distance to Sigma Scorpii is roughly 696 light years (214 parsecs).[1] North et al. (2007) computed a more accurate estimate of 568+75
 light years (174+23

  The red at bottom right is light from Sigma Scorpii being reflected off the surrounding dust.

The brightest component of the system is a double-lined spectroscopic binary, which means that the pair has not been resolved using a telescope. Instead, their orbit is determined by changes in their combined spectrum caused by the Doppler shift. This indicates that the pair complete an orbit every 33.01 days and have an orbital eccentricity of 0.32.[4]

The primary component of the spectroscopic binary, σ Scorpii A, is an evolved giant star with a stellar classification of B1 III.[3] It has around 18 times the mass of the Sun and 12 times the Sun's radius.[4] This star is radiating about 29,000 times the luminosity of the Sun from its outer envelope at an effective temperature of 26,150 K.[4] This is a variable star of the Beta Cephei type, causing the apparent magnitude to vary between +2.86 and +2.94 with multiple periods of 0.2468429, 0.239671, and 8.2 days. During each pulsation cycle, the temperature of the star varies by 4,000 ± 2,000 K.[4]

The other member of the core pair, σ Scorpii B, is a main sequence star with a classification of B1 V.[4] Orbiting this binary at a separation half an arcsecond, or at least 120 Astronomical Units (AU), four times the SunNeptune distance, is the magnitude +5.2 σ Scorpii C, which has an orbital period of over a hundred years. Even farther out at 20 arcseconds, or more than 4500 AU, is σ Scorpii D with a magnitude of +8.7. It is classified as a B9 dwarf.

The Sigma Scorpii system is a likely member of the Gould Belt with an 83% probability. In particular, it is a candidate member of the Upper Scorpius group of the Scorpius-Centaurus Association (Sco OB2) of stars that share a common motion through space. Hence, these stars probably formed in the same molecular cloud at about the same time. The Upper Scorpius group has an estimated age of 8–10 million years based upon their evolutionary tracks,[10] which is a match for the age estimate of this star system.[4]


  1. ^ a b c d e van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. Bibcode 2007A&A...474..653V. DOI:10.1051/0004-6361:20078357. 
  2. ^ a b c d Johnson, H. L. et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory 4 (99). Bibcode 1966CoLPL...4...99J. 
  3. ^ a b Houk, Nancy (1978), Michigan catalogue of two-dimensional spectral types for the HD stars, 4, Ann Arbor: Dept. of Astronomy, University of Michigan, Bibcode 1988mcts.book.....H 
  4. ^ a b c d e f g h i j k l m n o p q r s North, J. R. et al. (September 2007), "Orbital solution and fundamental parameters of σ Scorpii", Monthly Notices of the Royal Astronomical Society 380 (3): 1276–1284, Bibcode 2007MNRAS.380.1276N, DOI:10.1111/j.1365-2966.2007.12188.x 
  5. ^ Wilson, R. E. (1953). General Catalogue of Stellar Radial Velocities. Carnegie Institute of Washington D.C.. Bibcode 1953GCRV..C......0W. 
  6. ^ Niemczura, E.; Daszyńska-Daszkiewicz, J. (April 2005), "Metallicities of the β Cephei stars from low-resolution ultraviolet spectra", Astronomy and Astrophysics 433 (2): 659–669, arXiv:astro-ph/0410440, Bibcode 2005A&A...433..659N, DOI:10.1051/0004-6361:20040396 . Note: value taken from [m/H].
  7. ^ Abt, Helmut A.; Levato, Hugo; Grosso, Monica (July 2002), "Rotational Velocities of B Stars", The Astrophysical Journal 573 (1): 359–365, Bibcode 2002ApJ...573..359A, DOI:10.1086/340590 
  8. ^ Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society 410 (1): 190–200, Bibcode 2011MNRAS.410..190T, DOI:10.1111/j.1365-2966.2010.17434.x 
  9. ^ "sig Sco -- Variable Star of beta Cep type", SIMBAD Astronomical Object Database (Centre de Données astronomiques de Strasbourg), http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Sigma+Scorpii, retrieved 2012-02-28 
  10. ^ Bobylev, V. V.; Bajkova, A. T. (September 2007), "Kinematics of the Scorpius-Centaurus OB association", Astronomy Letters 33 (9): 571–583, Bibcode 2007AstL...33..571B, DOI:10.1134/S1063773707090010 


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