Sigma Octantis

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Template:Short description

Sigma Octantis
Template:Location mark
Location of σ Octantis (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Octans
Right ascension Template:RA[1]
Declination Template:DEC[1]
Apparent magnitude (V) 5.47[2]
Characteristics
Spectral type F0 IV[3]
U−B Template:Engvar +0.13[2]
B−V Template:Engvar +0.26[2]
Variable type δ Sct[4]
Astrometry
Radial velocity (Rv)+11.9[5] km/s
Proper motion (μ) RA: +26.323[1] mas/yr
Dec.: +4.721[1] mas/yr
Parallax (π)11.1005±0.0616 mas[1]
DistanceTemplate:Rnd ± Template:Rnd ly
(Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)Template:Val[6]
Details
Mass1.59[7] Template:Solar mass
Radius4.4[1] Template:Solar radius
Luminosity44[1] Template:Solar luminosity
Surface gravity (log g)3.71[7] cgs
TemperatureTemplate:Val[7] K
Metallicity [Fe/H]−0.5[1] dex
Rotational velocity (v sin i)145[4] km/s
Age912[7] Myr
Metallicity [Fe/H]{{{metal_fe2}}} dex
Other designations
Template:Odlist[8]
Database references
SIMBADdata

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Sigma Octantis is a solitary[9] star in the Octans constellation that forms the pole star of the Southern Hemisphere. Its name is also written as σ Octantis, abbreviated as Sigma Oct or σ Oct, and it is officially named Polaris Australis (Template:IPAc-en).[10] The star is positioned one degree away from the southern celestial pole of the Southern Hemisphere, lying in nearly opposite direction to the North Star on the celestial sphere.

Located approximately Template:Val from Earth, it is classified as a subgiant with a spectral type of F0 IV. Sigma Octantis has an apparent magnitude of 5.5, but is slightly variable and is classified as a Delta Scuti variable.

Nomenclature

File:SigOctLightCurve.png
A light curve for Sigma Octantis, plotted from TESS data[11]

σ Octantis (Latinised to Sigma Octantis) is the star's Bayer designation.

As the southern hemisphere's pole star it bore the name Polaris Australis, first applied in the 1700s.[12] In 2016, the IAU organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN approved the name Polaris Australis for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[10] It is the southernmost named star.

Properties

With a spectral class of F0IV,[3] Sigma Octantis appears to be a subgiant, although it has also been classified as F0III.[14] Evolutionary models place it at the very end of its main sequence life with an age of about 900 million years.[1][7] It has expanded somewhat to a size 4.4 that of the Sun and emits 44 times as much electromagnetic radiation from its photosphere at an effective temperature of Template:Val.

Sigma Octantis is a Delta Scuti variable, varying by about 0.03 magnitudes every 2.33 hours.[4] It is thought to pulsate only in the fundamental mode.[15]

Southern pole star

Sigma Octantis is the current 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 slightly more than a degree away from the true south pole, and the south celestial pole is moving away from it due to precession of the equinoxes.

File:Octans trapezoid.GIF
The 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.[16] Because of this, the constellation Crux is often preferred for determining the position of the South Celestial Pole.[17] Once Sigma Octantis' approximate position has been determined, either by the major stars in Octans or using the Southern Cross (Crux) 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.

In astrometrics

Sigma Octantis was used as a reference to measure the magnitudes of stars in the southern hemisphere for the 1908 Revised Harvard Photometry catalogue. The Pole Star and Lambda Ursae Minoris were used for the northern hemisphere. It was then noted that "Neither of these stars appears to vary perceptibly" but that, due to the procedures used "if they did, the variation would have no effect on the final measures."[18]

In culture

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.[19]

See also

References

Template:Reflist

Template:Pole star Template:Stars of Octans

Template:Authority control

  1. ↑ a b c d e f g h i Cite error: Invalid <ref> tag; no text was provided for refs named dr3
  2. ↑ a b c Cite error: Invalid <ref> tag; no text was provided for refs named Johnson1966
  3. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named houk1975
  4. ↑ a b c Cite error: Invalid <ref> tag; no text was provided for refs named Rodriguez2000
  5. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Wilson1953
  6. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Antonello1997
  7. ↑ a b c d e Cite error: Invalid <ref> tag; no text was provided for refs named David2015
  8. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named SIMBAD
  9. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Eggleton2008
  10. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named IAU-LSN
  11. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named MAST
  12. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Kaler2006
  13. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named WGSN
  14. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Malaroda1975
  15. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named ibvs
  16. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Jumk
  17. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named CSIRO
  18. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named rhpc
  19. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named FOTW
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