# Sigma Octantis
**Sigma Octantis** (σ Octantis), officially named **Polaris Australis**, is a faint yellow-white giant star in the constellation [Octans](https://www.edgepedia.org/octans) and the closest naked-eye star to the south celestial pole, making it the southern counterpart of Polaris.[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/) At apparent magnitude 5.47 it sits at the very threshold of naked-eye visibility, so unlike Polaris in the north it is too dim to serve as a practical reference for stargazers and navigators.[2](https://www.skyatnightmagazine.com/advice/south-star)
| Key fact | Detail |
|---|---|
| Proper name | Polaris Australis, approved by the IAU Working Group on Star Names on September 5, 2017; the only named star in Octans and the southernmost star with a formal name[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/) |
| Position | Declination −88° 57′ 23″, about 1° from the south celestial pole (about 1.2° in 2026)[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/) • [3](https://www.sterngucker.de/en/stars/north-star-polaris/) |
| Brightness | Apparent magnitude 5.47, varying between 5.42 and 5.48; roughly 25 times fainter than Polaris at magnitude 1.97[4](https://nightbase.app/stars/polaris-australis) • [5](https://www.pretoria-astronomy.co.za/pdf/observing_targets.pdf) • [2](https://www.skyatnightmagazine.com/advice/south-star) |
| Stellar type | F0 III giant, about 294 ± 2 light-years away, with a mass of 1.59 solar masses and a luminosity of 43 solar luminosities[4](https://nightbase.app/stars/polaris-australis) • [1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/) |
| Variability | A Delta Scuti variable with a 0.03-magnitude variation over a period of 2.3 hours[5](https://www.pretoria-astronomy.co.za/pdf/observing_targets.pdf) |
| Practical use | Too faint for navigation; southern navigators instead locate the pole using the Southern Cross (Crux)[2](https://www.skyatnightmagazine.com/advice/south-star) |
| Pole-star status | Temporary: precession moves the pole in a circle over about 26,000 years, so Sigma Octantis has not always marked the south celestial pole and will not remain there[6](https://bimalkhatri.com.np/blogs/octans-constellation) |
## Name and designation
The [Bayer designation](https://www.edgepedia.org/bayer-designation) σ Octantis follows the genitive form of Octans, the constellation that contains the south celestial pole.[7](https://www.iau.org/IAU/IAU/Astronomy-FAQs/Constellations.aspx?hkey=bb9dc841-0618-41b5-ac70-149741062141) The proper name Polaris Australis is a contraction of *Stella Polaris Australis*, meaning "south polar star," a Latin name that first appeared in the 18th century.[8](https://www.rasc.ca/sites/default/files/World%20Asterisms%20Project%20Handbook%20V%202025.14.pdf) The name refers to the star's role as the southern pole star and has been in use since the 1700s; the International Astronomical Union's Working Group on Star Names approved it formally on September 5, 2017.[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/) Sigma Octantis is the only named star in Octans and the southernmost star carrying a formal name.[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/)
The star carries a long list of catalog designations, including HD 177482, HR 7228, HIP 104382, SAO 258857, and the variable-star identifier Sig Oct.[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/) • [9](https://simbad.u-strasbg.fr/simbad/sim-id?Ident=sigma+octantis)
## Physical properties
Sigma Octantis is classified as an F0 III giant, a yellow-white star that has left the main sequence.[4](https://nightbase.app/stars/polaris-australis) Measured values place it 294 ± 2 light-years from Earth (90.1 ± 0.5 parsecs, parallax 11.1005 ± 0.0616 mas), with a mass of 1.59 solar masses, a radius of 4.32 solar radii, a luminosity of 43 solar luminosities, and a surface temperature of 7,415 ± 252 K; its estimated age is 912 million years.[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/) Published distance figures vary between sources, with one popular astronomy reference giving 270 light-years.[2](https://www.skyatnightmagazine.com/advice/south-star)
The star is a fast rotator, with a projected rotational velocity listed at 145 km/s.[1](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/)
## Variability
Sigma Octantis is a [Delta Scuti variable](https://www.edgepedia.org/delta-scuti-variable), a class of pulsating stars whose brightness changes come from radial and non-radial pulsations of the surface. Its variation is small, 0.03 magnitudes over a period of 2.3 hours.[5](https://www.pretoria-astronomy.co.za/pdf/observing_targets.pdf) Spectroscopic and photometric studies have reported it as a hybrid [Delta Scuti](https://www.edgepedia.org/delta-scuti) / Gamma Doradus pulsator: one observing campaign using 350 high-resolution spectra from the University of Canterbury Mt John Observatory, photometry from four [Antarctic](https://www.edgepedia.org/antarctic) winters, and several TESS sectors identified 68 statistically significant frequencies, including 57 of Delta Scuti type and 8 of Gamma Doradus type, with no evidence of a radial pulsation mode.[10](https://www.iastro.pt/research/conferences/tasc8-kasc15/AbstractBooklet.pdf)
## The southern pole star problem
Solely by position, Sigma Octantis qualifies as the "South Star": it lies about 1° from the south celestial pole, a separation comparable to Polaris's 0.62° from the north celestial pole in 2026.[3](https://www.sterngucker.de/en/stars/north-star-polaris/) The difference is brightness. Polaris shines at magnitude 1.97 and is visible from cities; Sigma Octantis at 5.47 is invisible under urban skies and takes practice to spot even in the countryside.[2](https://www.skyatnightmagazine.com/advice/south-star) • [3](https://www.sterngucker.de/en/stars/north-star-polaris/) A single Polaris sight gives a navigator a latitude line because the star's altitude above the horizon approximately equals the observer's latitude; Sigma Octantis offers no comparable ease of use.[11](https://nauticalninja.com/sailing_guides/celestial_navigation/polaris-and-circumpolar-stars)
Southern navigators therefore locate the pole geometrically. The standard method draws a line through the stars Gamma (γ) and Alpha (α) Crucis in the Southern Cross and extends it 27 degrees toward the pole; an equivalent rule extends the Cross's long axis about 4.5 times.[2](https://www.skyatnightmagazine.com/advice/south-star) • [3](https://www.sterngucker.de/en/stars/north-star-polaris/) The faintness of the pole star had real historical consequences: southern celestial navigation developed techniques based on the Southern Cross and the Pointers because no usable pole star existed.[6](https://bimalkhatri.com.np/blogs/octans-constellation)
## Use in telescope alignment
Amateur astronomers in the [Southern Hemisphere](https://www.edgepedia.org/southern-hemisphere) do use Sigma Octantis for polar alignment of equatorial mounts and star trackers, and several mount and polar-scope manuals include it for that purpose; one polar-scope reticle uses the four-star pattern of Sigma, Tau, Chi, and Upsilon Octantis.[12](https://www.manualshelf.com/manual/vixen/sphinx-sxd/1-instruction-manual-english/page-56.html) • [13](https://gzhls.at/blob/ldb/1/3/9/e/058d73b151c8563a875b5560e394e3e7312d.pdf)
## Precession and the changing pole
Earth's axis precesses, tracing a circle over about 26,000 years, so the star nearest each celestial pole changes over time. Sigma Octantis has not always been the southern pole star and will not remain so.[6](https://bimalkhatri.com.np/blogs/octans-constellation) The same process displaced Polaris from the north celestial pole in the distant past, and the IAU notes that in the future the south will have a properly bright "South Star" even though no bright star lies near the pole today.[14](https://www.iau.org/static/archives/announcements/pdf/ann19029a.pdf)
## Octans and its history
Sigma Octantis lies in Octans, the southernmost of the 88 constellations, which contains the south celestial pole and is fully visible only from Earth's Southern Hemisphere.[15](https://h2g2.com/edited_entry/A41684457) The constellation was formed and published by the French astronomer Nicolas-Louis de La Caille in 1752, named in recognition of the octant, the navigation instrument invented in 1730 by John Hadley.[16](https://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Octans%2A.html) Early navigators remarked on the emptiness of the far-southern sky; [Peter Martyr](https://www.edgepedia.org/peter-martyr) wrote that sailors "knewe no starre there lyke unto this pole, that myght be decerned aboute the poynte."
[16](https://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Octans%2A.html)
## References
1. [South Star: Sigma Octantis (Polaris Australis), Star Facts](https://www.star-facts.com/south-star-sigma-octantis-polaris-australis/)
2. [Southern star and how to find south in the Southern Hemisphere, BBC Sky at Night Magazine](https://www.skyatnightmagazine.com/advice/south-star)
3. [How to Find the North Star (Polaris), sterngucker.de](https://www.sterngucker.de/en/stars/north-star-polaris/)
4. [Polaris Australis (Sigma Octantis), Nightbase](https://nightbase.app/stars/polaris-australis)
5. [The COLOUR of CREATION observing targets, Pretoria Astronomy](https://www.pretoria-astronomy.co.za/pdf/observing_targets.pdf)
6. [Octans Constellation: The Octant, and the Faint Star That Marks the South Pole](https://bimalkhatri.com.np/blogs/octans-constellation)
7. [The Constellations, IAU](https://www.iau.org/IAU/IAU/Astronomy-FAQs/Constellations.aspx?hkey=bb9dc841-0618-41b5-ac70-149741062141)
8. [World Asterisms Project Handbook, RASC](https://www.rasc.ca/sites/default/files/World%20Asterisms%20Project%20Handbook%20V%202025.14.pdf)
9. [SIMBAD: sigma octantis](https://simbad.u-strasbg.fr/simbad/sim-id?Ident=sigma+octantis)
10. [TASC8/KASC15 Abstract Booklet, University of Canterbury sigma Octantis study](https://www.iastro.pt/research/conferences/tasc8-kasc15/AbstractBooklet.pdf)
11. [Polaris and Circumpolar Stars, Nautical Ninja](https://nauticalninja.com/sailing_guides/celestial_navigation/polaris-and-circumpolar-stars)
12. [Vixen SPHINX SXD Instruction Manual](https://www.manualshelf.com/manual/vixen/sphinx-sxd/1-instruction-manual-english/page-56.html)
13. [Polar-scope field diagram, Kapitel 5 Anwendung](https://gzhls.at/blob/ldb/1/3/9/e/058d73b151c8563a875b5560e394e3e7312d.pdf)
14. [A Proposed Definition of a Planet, IAU announcement](https://www.iau.org/static/archives/announcements/pdf/ann19029a.pdf)
15. [Constellations: Octans 'the Octant', h2g2](https://h2g2.com/edited_entry/A41684457)
16. [Allen's Star Names: Octans, LacusCurtius](https://penelope.uchicago.edu/Thayer/E/Gazetteer/Topics/astronomy/_Texts/secondary/ALLSTA/Octans%2A.html)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Stellar astrophysics, structure, evolution, and variables › Pulsating variables › Delta Scuti and related dwarfs*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license