Sigma Octantis

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Sigma Octantis
Octans 01.gif
Octans and its surroundings, with Sigma Octantis in the middle, marked in cyan.
(Click on the image to see it full size.)
Observation data
Epoch J2000      Equinox J2000
Constellation Octans
Right ascension 21h 08m 46.8s
Declination -88° 57′ 23″
Apparent magnitude (V) 5.42
Spectral type F0 III
U−B color index 0.13
B−V color index 0.27
Variable type Delta Scuti
Radial velocity (Rv) +11.9 km/s
Proper motion (μ) RA: 25.96 mas/yr
Dec.: 5.02 mas/yr
Parallax (π) 12.07 ± 0.48 mas
Distance 270 ly
(83 pc)
Absolute magnitude (MV) 0.83
Other designations
Polaris Australis, HR 7228, HD 177482, SAO 258857, FK5 923, HIP 104382.
Database references

Sigma Octantis (σ Oct, σ Octantis) is a magnitude 5.42 star in the constellation Octans, notable for being the current South Star. Sigma Octantis is approximately 270 light years from Earth, and is classified as a giant, with a spectral type of F0 III. It is a Delta Scuti variable, with magnitude varying by about 0.03 magnitudes over 2.3 hours.

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.[1]

Its position near the southern celestial pole makes it the southern hemisphere's pole star, hence it is occasionally called Polaris Australis.

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.[2] Because of this, the Crux constellation is often preferred for determining the position of the South Celestial Pole.[3] 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]


  1. ^ "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. 
  2. ^ "Sigma Octantis". Jumk.De. 26 July 2013. Retrieved 26 July 2013. 
  3. ^ "Finding South". CSIRO. 26 July 2013. Retrieved 26 July 2013.