Sigma Octantis

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Sigma Octantis
Observational Data
Designation σ Octantis
Polaris Australis
Right ascension 21h 08m 46.8434s[1]
Declination -88° 57′ 23.4117″[1]
Constellation Octans
Type of object Star
Magnitude Apparent Mag: +5.42[1]
Absolute Mag: +0.77[2]
Distance from Earth 270 ly[3]
Radial velocity 11.9±2 km/s[1]
Proper motion RA: 26.67 mas/yr[1]
Dec.: 5.61 mas/yr[1]
Parallax 11.09±0.14 mas[1]

Sigma Octantis (σ Octantis, Polaris Australis) is the closest bright star to the South celestial pole.[3] It is the counterpart to Polaris in the Northern hemisphere, though Sigma Octantis is 25 times fainter than Polaris making it too dim to be useful for navigational purposes. Located in the Southern constellation of Octans, it is in fact a Delta Scuti variable star.[4] This variability is caused by radial and non-radial pulsations of the star's surface. The star's brightness varies only slightly by 0.03 magnitudes over the course of its period of 2.3 hours.[3]

The star is a yellow-white giant star with spectral class F0III.[3] Sigma Octantis has a mass of 2 solar masses, twice that of the Sun with a radius 3.7 times greater.[4] The star rotates about its axis with a period of 1.5 days, meaning a point on its equator moves at around 128 km/s.[4] The metallicity of the star is also high, about 1.8 times more than the Sun. It is situated around 1 degree from the South celestial pole. The star is also a little hotter than the Sun at 7,322 kelvin.[2]

The name "Sigma Octantis" was assigned to the star by John Bayer in 1603.[2] It can just about be observed with the unaided eye.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Sigma Octantis from the SIMBAD Astronomical Database
  2. 2.0 2.1 2.2 Polaris Australis, Sigma Octantis, HD177482, HIP104382, HR7228. Retrieved on 2019-03-19.
  3. 3.0 3.1 3.2 3.3 Octans Constellation. Retrieved on 2019-03-19.
  4. 4.0 4.1 4.2 Sigma Octantis. Retrieved on 2019-03-19.