Sigma Octantis

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Sigma Octantis
Octans IAU.svg
Red circle.svg

Location of σ Octantis (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Octans
Right ascension 21h 08m 46.83929s[1]
Declination −88° 57′ 23.3966″[1]
Apparent magnitude (V) 5.47[2]
Characteristics
Spectral type F0 III[3]
U−B color index +0.13[2]
B−V color index +0.26[2]
Variable type δ Sct[4]
Astrometry
Radial velocity (Rv) +11.9[5] km/s
Proper motion (μ) RA: +25.75[1] mas/yr
Dec.: +4.98[1] mas/yr
Parallax (π) 11.61 ± 0.31[1] mas
Distance 281 ± 8 ly
(86 ± 2 pc)
Absolute magnitude (MV) 0.86±0.09[6]
Details
Mass 1.59[7] M
Luminosity 38[8] L
Surface gravity (log g) 3.71[7] cgs
Temperature 7,415±252[7] K
Rotational velocity (v sin i) 145[4] km/s
Age 912[7] Myr
Other designations
Polaris Australis[9], σ Oct, CPD−89° 47, FK5 923, HD 177482, HIP 104382, HR 7228, SAO 258857[10]
Database references
SIMBAD data

Sigma Octantis (σ Octantis) is the current South Star. Its position near the southern celestial pole makes it the southern hemisphere's pole star; hence it is occasionally called Polaris Australis.[9] This is a solitary,[11] magnitude 5.47[2] star in the southern circumpolar constellation of Octans. Located approximately 281 light years from Earth, it is classified as a giant star with a spectral type of F0 III.[3] Sigma Octantis is a Delta Scuti variable, with its brightness varying by about 0.03 magnitudes every 2.33 hours.[4]

Sigma Octantis is the dimmest star to be represented on a national flag. It appears on the flag of Brazil, symbolising the Brazilian Federal District.[12]

Southern pole star[edit]

Sigma Octantis is the southern pole star, whose counterpart is Polaris, the current North Star. To an observer in the southern hemisphere, Sigma Octantis appears almost motionless and all the other stars in the Southern sky appear to rotate around it. It is part of a small "half hexagon" shape. It is over a degree away from the true south pole, and the south celestial pole is moving away from it due to precession of the equinoxes.

Locating Sigma Octantis[edit]

South Celestial Pole: the trapezoidal asterism at the centre includes Sigma Octantis

At magnitude +5.42, Sigma Octantis is barely visible to the naked eye, making it unusable for navigation, especially by comparison with the much brighter and more easily visible Polaris.[13] Because of this, the Crux constellation is often preferred for determining the position of the South Celestial Pole.[14] Once Sigma Octantis' approximate position has been determined, either by the major stars in Octans or using the Southern Cross method, it can be positively verified using an asterism: Sigma, Chi, Tau, and Upsilon Octantis are all stars of around magnitude 5.6, and form the distinctive shape of a trapezoid.

See also[edit]

References[edit]

  1. ^ a b c d e van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, 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 Malaroda, S. (August 1975), "Study of the F-type stars. I. MK spectral types", Astronomical Journal, 80: 637−641, Bibcode:1975AJ.....80..637M, doi:10.1086/111786. 
  4. ^ a b c Rodríguez, E.; et al. (June 2000), "A revised catalogue of δ Sct stars", Astronomy and Astrophysics Supplement, 144: 469–474, Bibcode:2000A&AS..144..469R, doi:10.1051/aas:2000221. 
  5. ^ Wilson, Ralph Elmer (1953), "General catalogue of stellar radial velocities", Washington, Carnegie Institution of Washington: 0, Bibcode:1953GCRV..C......0W. 
  6. ^ Antonello, E.; Mantegazza, L. (November 1997), "Luminosity and related parameters of δ Scuti stars from HIPPARCOS parallaxes. General properties of luminosity", Astronomy and Astrophysics, 327: 240−244, Bibcode:1997A&A...327..240A. 
  7. ^ a b c d David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv:1501.03154Freely accessible, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146. 
  8. ^ McDonald, I.; et al. (2012), "Fundamental parameters and infrared excesses of Hipparcos stars", Monthly Notices of the Royal Astronomical Society, 427: 343, arXiv:1208.2037Freely accessible, Bibcode:2012MNRAS.427..343M, doi:10.1111/j.1365-2966.2012.21873.x. 
  9. ^ a b Kaler, James B. (2006), The Hundred Greatest Stars, Springer Science & Business Media, p. 171, ISBN 0387216251. 
  10. ^ "sig Oct". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-08-10. 
  11. ^ Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878Freely accessible, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x. 
  12. ^ "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. 
  13. ^ "Sigma Octantis". Jumk.De. 26 July 2013. Retrieved 26 July 2013. 
  14. ^ "Finding South". CSIRO. 26 July 2013. Retrieved 26 July 2013.