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Sigma Octantis

Sigma Octantis (σ Octantis), also known as Polaris Australis, is a solitary star in the constellation Octans that serves as the current southern pole star, positioned approximately 1° from the South Celestial Pole and too faint for reliable navigation.^[1] With an apparent visual magnitude of 5.42, it is a Delta Scuti variable star of spectral type F0IV, located at a distance of 90 parsecs (about 294 light-years) from the Sun.^[2] As a subgiant, it has an estimated mass of 1.6 to 2.1 solar masses, a radius of 3.7 to 4.4 solar radii, a luminosity 34 to 44 times that of the Sun, and a surface temperature of approximately 7,300 Kelvin, giving it a yellow-white appearance.^[3]^[4] This F-type subgiant is receding from the Solar System at a radial velocity of +11.9 km/s and exhibits small pulsations with a variability amplitude of 0.03 magnitudes over about 2.3 hours, characteristic of Delta Scuti stars.^[2] Its proper motion is 26.3 mas/year in right ascension and 4.7 mas/year in declination, causing it to slowly drift relative to the pole over millennia.^[2] Unlike the brighter northern counterpart Polaris, Sigma Octantis lacks a suitable alignment for precise polar alignment in the south, but it remains a key reference for southern sky observers.^[1]

Nomenclature and identification

Bayer designation

The Bayer designation for the star is σ Octantis, where σ denotes the Greek letter sigma, the 18th letter in the Greek alphabet, signifying its sequential position among the brighter stars cataloged in the constellation Octans.^[5] This naming convention follows the system introduced by German astronomer Johann Bayer in his 1603 star atlas Uranometria, which assigned Greek letters (starting with alpha for the brightest star) followed by the genitive form of the constellation name to identify prominent stars visible to the naked eye.^[6] However, as Octans is a modern southern constellation without pre-existing assignments from Bayer's era—due to its creation by French astronomer Nicolas-Louis de Lacaille during his 1750–1752 expedition to the Cape of Good Hope—the Greek letter designations for its stars were applied later by Lacaille himself.^[7] Lacaille first depicted Octans (originally "l'Octans de Réflexion") in his 1756 planisphere and formalized it in his 1763 catalogue Coelum Australe Stelliferum, where he assigned σ to this particular star as part of his effort to systematically name southern stars using Bayer's method, thereby integrating the new constellation into established astronomical nomenclature.^[7] This post-1752 cataloging addressed the absence of ancient or early modern labels for southern skies, ensuring comprehensive identification for observers in the hemisphere.^[5]

Alternative names

Sigma Octantis is most commonly referred to as Polaris Australis, a name denoting its status as the southern analog to the northern pole star Polaris, owing to its close alignment with the south celestial pole. This name was officially approved by the International Astronomical Union (IAU) in 2016.^[3]^[8] This informal designation highlights its navigational significance in the Southern Hemisphere, where it serves as a reference point despite not being perfectly aligned with the pole.^[9] In various astronomical catalogs, the star bears additional identifiers, including HR 7228 from the Harvard Revised Catalogue, HD 177482 from the Henry Draper Catalogue, and HIP 104382 from the Hipparcos Catalogue.^[10] These designations facilitate precise referencing in stellar databases and research. The formal name "Sigma Octantis" originates from the Bayer system, where "sigma" is the 18th letter of the Greek alphabet, marking the star's relative brightness within its constellation, and "Octantis" derives from Octans, the Latin term for the octant—a reflecting instrument used for measuring angular distances, invented around 1730.^[11] Octans itself is a modern constellation, created by French astronomer Nicolas-Louis de Lacaille in 1752 based on his southern sky observations, and thus lacks any ancient mythological associations.^[12]

Stellar properties

Physical characteristics

Sigma Octantis is classified as an F0IV yellow-white subgiant star, with a surface temperature of approximately 7280 K.^[3]^[13] Its physical parameters include a radius of 3.7 solar radii, a mass of approximately 2.0 solar masses, and a luminosity 34 times that of the Sun.^[3] The star's metallicity is enhanced, with an iron abundance [Fe/H] ≈ +0.25, corresponding to about 1.8 times the solar value.^[3] As a subgiant, Sigma Octantis is evolving off the main sequence toward the red giant phase, with an estimated age of 2-3 billion years based on evolutionary models consistent with its mass and spectral properties.^[3] Its average apparent visual magnitude is 5.42, though it shows slight variability.^[3]

Variability

Sigma Octantis is classified as a Delta Scuti variable star, a subtype of pulsating variables typically found among subgiants and giants in the instability strip, exhibiting both radial and non-radial oscillations driven by the kappa mechanism in helium ionization zones.^[14] These pulsations arise from partial ionization of helium in the star's outer layers, leading to opacity changes that cause periodic expansion and contraction of the stellar envelope. The star's brightness fluctuates with a small amplitude of approximately 0.03 magnitudes, corresponding to an apparent magnitude range of roughly 5.44 to 5.50, and a primary pulsation period of about 0.1 days (around 2.3 hours).^[3] This low-amplitude variation makes it challenging to detect without photometric instruments, as the changes are subtle and require precise measurements over multiple cycles. Like many Delta Scuti stars, Sigma Octantis shows multiple oscillation modes, though only a dominant single period has been firmly identified in observations. Its variability was first identified in 1978 through photometric observations by McInally and Austin, who reported an initial period of 0.064 days based on limited data from two nights. Subsequent studies confirmed the Delta Scuti nature and refined the period to around 0.097 days with an amplitude of 0.025 magnitudes in visual light, using more extensive monitoring. Recent analyses with Transiting Exoplanet Survey Satellite (TESS) data have further characterized its pulsations without evidence of hybrid modes with gamma Doradus-type variations.^[14]

Astrometry and position

Coordinates and distance

Sigma Octantis possesses equatorial coordinates in the J2000.0 epoch of right ascension 21ʰ 08ᵐ 46.86ˢ and declination −88° 57′ 23″, positioning it approximately 1° angular distance from the south celestial pole. These coordinates are derived from high-precision astrometric observations in the Gaia Data Release 3 catalog. In galactic coordinates, the star lies at longitude l = 303.91° and latitude b = −27.71°, situating it within the southern galactic hemisphere relative to the Milky Way's plane.^[15] The distance to Sigma Octantis is determined from its trigonometric parallax of 11.1005 ± 0.0616 milliarcseconds as measured by the Gaia mission in Data Release 3, yielding 90.1 ± 0.5 parsecs or equivalently 294 ± 2 light-years. This places the star at a moderate distance within the solar neighborhood, allowing for reliable parallax-based ranging. The corresponding absolute visual magnitude is calculated as +0.65 ± 0.09, reflecting its intrinsic luminosity when corrected for interstellar extinction and distance effects.^[16]^[15] With an apparent visual magnitude of 5.42, Sigma Octantis is marginally visible to the naked eye under dark sky conditions in the southern hemisphere, requiring minimal light pollution for detection. Due to its high southern declination, the star traces a circumpolar path for observers located south of approximately 1° north latitude, remaining above the horizon throughout the night.^[15]

Proper motion and space velocity

Sigma Octantis has a proper motion of μ_α cos δ = +26.323 ± 0.071 mas/yr in right ascension and μ_δ = +4.721 ± 0.074 mas/yr in declination, based on astrometric measurements from the Gaia mission. These values indicate a gradual drift across the sky, with the dominant component in the right ascension direction and a smaller northward component in declination, corresponding to a total proper motion of approximately 26.7 mas/yr. Earlier observations from the Hipparcos satellite provided foundational data with proper motions of about +25.8 mas/yr in RA and +5.0 mas/yr in Dec, achieving uncertainties around 1 mas/yr, while the Tycho-2 catalogue offered complementary ground-based measurements that helped establish the reference frame for subsequent missions. The Gaia data, refined in Data Release 3, has enhanced precision to below 0.1 mas/yr for bright stars like Sigma Octantis, enabling accurate modeling of its trajectory relative to background stars. The star's radial velocity is +11.9 ± 2 km/s, determined from spectroscopic observations. When combined with the proper motion and a Gaia-derived distance of 90.1 ± 0.5 pc, this yields a transverse velocity of roughly 11.4 km/s and a total three-dimensional space velocity relative to the Sun of approximately 16 km/s. These kinematics place Sigma Octantis in the thin galactic disk, where it orbits with low eccentricity, consistent with the dynamics of nearby solar neighborhood stars. Given its modest proper motion, Sigma Octantis will slowly increase its separation from the south celestial pole over time, shifting northward by about 4.7 mas per year. In roughly 1000 years, this will result in an additional displacement of around 4.7 arcseconds from its current position, which is about 1 degree (or 3600 arcseconds) from the pole. However, Earth's axial precession, with a 26,000-year period, will dominate long-term changes by causing the celestial pole to trace a circle, ensuring Sigma Octantis never achieves perfect alignment and allowing other stars to become closer approximations to the pole star in distant epochs.

Role as southern pole star

Current alignment

Sigma Octantis is positioned approximately 1°03′ from the exact south celestial pole (SCP), with a declination of −88.96°, making it the closest naked-eye visible star to this point and a primary reference for southern celestial orientation.^[4]^[17] This angular separation places the SCP within the constellation Octans, which currently lacks a prominent marker star at the pole itself.^[18] Due to its offset from the SCP, Sigma Octantis does not remain stationary but instead describes small circles around the pole as Earth rotates, offering a rough but useful guide for determining true south in navigation.^[19] It remains visible year-round from locations in the southern hemisphere where it is circumpolar, though its apparent magnitude of 5.5 renders it faint and challenging to spot without aid, limiting its practical utility compared to brighter polar indicators.^[20] The star's alignment is influenced by Earth's axial precession, a slow wobble with a cycle of about 25,772 years that gradually shifts the SCP's position relative to the stars; currently, Sigma Octantis provides one of the better approximations for modern observers, though the pole is moving away from it over time.^[21]

Comparison to northern pole star

Sigma Octantis, with an apparent magnitude of 5.47, is significantly fainter than Polaris, which shines at magnitude 2.0, making the southern pole star barely visible to the naked eye even under dark skies and rendering it impractical for casual observation or navigation.^[3]^[22] In contrast, Polaris's brightness allows it to be seen easily from most northern latitudes, serving as a reliable reference point historically. Additionally, Polaris's status as a classical Cepheid variable star has enabled precise distance measurements through its period-luminosity relationship, contributing to the cosmic distance ladder, whereas Sigma Octantis exhibits only minor Delta Scuti variability without similar calibration utility.^[22]^[3] In terms of alignment, Sigma Octantis lies approximately 1° from the South Celestial Pole (SCP), providing a rough indicator but requiring supplementary methods like the Southern Cross for accurate southern orientation.^[23] Polaris, however, is closer at about 0.7° from the North Celestial Pole (NCP), and its greater prominence has made it a more central navigational aid throughout history despite neither being perfectly aligned.^[24] The dimness of Sigma Octantis diminishes its effectiveness compared to Polaris, which benefits from its position in the prominent Ursa Minor constellation. Astronomically, both are F-type stars, but Polaris is a complex triple system consisting of a primary Cepheid supergiant (F7Ib-II) with two companions, while Sigma Octantis is a solitary yellow-white subgiant (F0IV).^[25]^[26] Furthermore, the northern celestial pole's location near bright circumpolar constellations fostered ancient navigational traditions in Eurasian cultures, whereas the SCP's position in the faint constellation of Octans has resulted in reliance on asterisms like the Southern Cross rather than a single star for southern hemisphere guidance.^[12]^[27] There is no true southern equivalent to Polaris due to the SCP's placement amid dim stars in Octans, lacking a bright, stable marker; future precessional shifts will bring the SCP nearer to brighter stars like Gamma Chamaeleontis around 4200 AD and eventually Sirius in about 66,000 years, differing from the northern cycle's more frequent alignments with prominent stars.^[12]^[28]

Cultural and historical significance

Observational history

Due to its location near the south celestial pole, Sigma Octantis has no records of observation from pre-telescopic northern hemisphere cultures, as it is invisible from latitudes north of about 1° N.^[5] The star's first documented observations occurred during French astronomer Nicolas-Louis de Lacaille's expedition to the Cape of Good Hope from 1750 to 1752, where he mapped southern skies and created the constellation Octans in 1752 to encompass the region around the south pole. Lacaille cataloged Sigma Octantis within Octans, assigning it the Bayer designation σ, but did not initially highlight its proximity to the pole in his 1756 chart, instead focusing on the constellation's representation of the navigational octant instrument.^[5] In the 19th century, improved southern surveys began precise positional measurements of Sigma Octantis, aiding its identification as the nearest naked-eye star to the south celestial pole with early charts around the 1800s. British astronomer John Herschel included the star in his comprehensive Cape Catalogue from observations at the Royal Observatory, Cape of Good Hope, between 1834 and 1838, measuring its coordinates and magnitude as part of efforts to map over 68,000 southern objects. By the early 20th century, its role as a reference for southern photometry was established, with Sigma Octantis used to calibrate magnitudes in the 1908 Revised Harvard Photometry catalogue, which extended brightness standards to the southern hemisphere.^[29] Its status as the southern pole star gained prominence in 20th-century navigation texts, such as those from the U.S. Hydrographic Office, where it was recommended for latitude determination despite its faintness compared to Polaris. Photometric investigations in the late 1970s confirmed Sigma Octantis as a variable star, with McInally and Austin's survey at the University of Canterbury identifying it as a δ Scuti-type pulsator through photoelectric observations showing small amplitude variations. Subsequent studies built on this, refining its light curve and period. The European Space Agency's Gaia mission, launched in 2013, revolutionized its astrometry with high-precision parallax measurements; the Hipparcos satellite's 1997 data initially estimated a distance of approximately 270 light-years, revised to 281 light-years in 2007, while Gaia Data Release 3 in 2022 updated it to 294 ± 2 light-years based on a parallax of 11.1005 ± 0.0616 mas. Due to its position near the south celestial pole, Sigma Octantis has served as an approximate marker for true south in navigation for southern hemisphere sailors and explorers, particularly during Antarctic expeditions where precise orientation is critical.^[23] However, its apparent magnitude of 5.47 renders it faint and challenging to locate with the naked eye, even under optimal dark-sky conditions, leading most navigators to prefer more prominent formations like the Southern Cross for practical guidance.^[30] This limitation has historically prompted the use of instruments such as octants—reflected in the constellation's name—to measure stellar altitudes more reliably.^[5] In modern culture, Sigma Octantis is frequently highlighted in astronomy education and planetarium presentations as the "southern pole star," emphasizing its role in demonstrating celestial mechanics for southern observers.^[3] It appears in discussions of Earth's rotation, including 2020s online debates surrounding flat Earth theories, where its apparent stationarity and the circling of surrounding stars around the pole challenge alternative models, though such contexts often rely on visual simulations rather than direct observation.^[31] The star's faintness underscores its educational value in teaching the contrasts between northern and southern skies. First cataloged by Nicolas-Louis de Lacaille in his 1763 southern star survey, Sigma Octantis continues to be referenced in contemporary astronomical resources for its historical and positional significance.^[5] Today, Sigma Octantis aids pole-finding in digital tools like the Stellarium planetarium software, which simulates its position for users worldwide to practice southern alignments.^[32] It also plays a minor but essential role in space mission planning at southern observatories, where it facilitates polar alignment of telescopes and tracking mounts to minimize rotational errors during long-exposure observations.^[31]

References

[1]
Constellation: Octans
### Facts about Sigma Octantis and its Role as Southern Pole Star
[2]
Sigma Oct
### Summary of Sigma Octantis Data
[3]
Sigma Octantis - JIM KALER
With a temperature estimated at 7280 Kelvin, the star radiates at a rate just 34 times that of the Sun, which gives it a radius of only 3.7 times solar, not ...Missing: data facts
[4]
Star Sigma Octantis - Stellar Catalog
Yellow giant Sigma Octantis is located 294 light years away from the Sun. It is a single star of spectral class F0 III, that has 159 % of solar mass.
[5]
Octans - Star Tales - Ian Ridpath
Sixth-magnitude Sigma (σ) Octantis, currently the closest naked-eye star to the south celestial pole, received its Greek-letter designation in Lacaille's star ...Missing: Bayer history
[6]
Book Review: Uranometria von Johann Bayer 1603. - NASA ADS
... designate each star. Johannes Bayer was not the first to try this, but his Uranometria of 1603 with its handsomely engraved figures and Greek letter tags on ...
[7]
Lacaille's Southern Star Catalog
Feb 19, 2021 · In the manner of Bayer, Lacaille assigns Greek and Latin letters to the stars in the new constellations and to those in the old ones that didn't ...
[8]
South Star: Sigma Octantis (Polaris Australis)
Jun 28, 2022 · Sigma Octantis is the current South Star, the nearest visible star to the south celestial pole. It is a yellow giant located 294 light-years away.<|control11|><|separator|>
[9]
sigma Octantis
### Alternative Names and Identifiers for Sigma Octantis
[10]
Octans - Constellations of Words
Formed by La Caille in 1752 in recognition of the octant invented in 1730 by John Hadley. The octant, a forerunner of the sextant (constellation Sextans) is a ...
[11]
Octans | Southern Sky, Constellation, Stars - Britannica
The French astronomer Nicolas Louis de Lacaille formed this constellation in 1754. It represents the octant, a navigational instrument that was replaced by the ...Missing: Bayer designation
[12]
Sigma Octantis
### Physical and Stellar Parameters for Sigma Octantis
[13]
Gaia Data Release 3 (Gaia DR3) - ESA Cosmos
Gaia DR3, released June 13, 2022, includes object classifications, astrophysical parameters, and new data products, building upon the previous release.Gaia DR3 events · Gaia DR3 stories · Gaia DR3 previews · Gaia DR3 software toolsMissing: Sigma Octantis
[14]
sigma Octantis - ADS - Astrophysics Data System
We examine data from three Sectors of observations from NASA's Transiting Exoplanet Survey Satellite (TESS) for the delta Scuti star sigma Octantis = HD 177482.
[15]
https://simbad.cds.unistra.fr/simbad/sim-id?Ident=Sigma+Octantis
[16]
Polaris Australis Star Facts (Sigma Octantis) - Universe Guide
Jan 11, 2025 · Polaris Australis colour is yellow - white. Polaris Australis temperature is in the range of between 6,000 and 7,500 Kelvin. Based on the ...
[17]
Octans and Apus circle the south celestial pole - EarthSky
Sep 25, 2024 · Octans and Apus (the Octant and the Bird of Paradise, respectively) lie far south. In fact, Octans is home to the south celestial pole.
[18]
The changing pole star - Explaining Science
Sep 25, 2020 · The nearest star visible to the naked eye is the star Sigma Octantis ... Polaris has a magnitude of 1.98 (which makes it the forty eighth ...
[19]
Pole Stars - Cosmic Reflections
Mar 26, 2018 · Sigma Octantis at magnitude 5.5 is not easy to see with the unaided eye, and being 1.1 degrees away from the SCP doesn't win it any awards.<|control11|><|separator|>
[20]
Earth's Axial Precession - FlatEarth.ws
Axial precession is the change in Earth's rotational axis. It is a slow change ... The closest star to the south celestial pole is currently Sigma Octantis ...
[21]
Polaris | Location, Constellation, & Facts | Britannica
Oct 3, 2025 · The apparent visual magnitude of the Polaris system is 2.00. It is about 447.6 light-years from Earth, and, because it is the closest Cepheid ...
[22]
Southern star and how to find south in the Southern Hemisphere
May 3, 2025 · Sigma Octantis is the south pole star. It's located in the constellation Octans and is the closest star to the south celestial pole.Missing: future position
[23]
Polaris: How to find the North Star | Space
Jan 24, 2022 · Polaris is located at a distance of 323 light-years from Earth ... Only about 0.7 degree separates Polaris from the North Celestial Pole ...
[24]
Polaris - JIM KALER
About 18 seconds of arc away lies an eighth magnitude F3 1.4 (or so) solar mass dwarf, which at the distance of Polaris must be at least 2400 Astronomical ...
[25]
[astro-ph/0609759] Polaris: Mass and Multiplicity - arXiv
Sep 27, 2006 · Polaris, the nearest and brightest classical Cepheid, is a member of at least a triple system. It has a wide (18'') physical companion, the F- ...
[26]
https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=Sigma+Octantis
[27]
Sirius is a future southern Pole Star - EarthSky
Jan 30, 2025 · Meeus wrote that Sirius will take its turn as the southern Pole Star some 60,000 years from now, around the year 66270. In that year, Sirius ...
[28]
https://earthsky.org/tonight/sirius-future-south-pole-star/
[29]
The Stars of the Carina Constellation - Sky & Telescope
Mar 15, 2024 · At the moment the southern title is held by Sigma Octantis, a dim star (magnitude 5.5) that is nowhere near as handy for finding the pole as ...
[30]
[PDF] ST-Feb-2022.pdf - Sky & Telescope
Feb 2, 2022 · We've been there at the star party when dew is fogging up the view of the lunar eclipse through the hole in the clouds with a line of ...
[31]
Stars – Australian Indigenous Astronomy
Aboriginal and Islander people paid special significance to all stars, from the brightest in the sky to those barely visible. Each community has their own views ...Missing: Sigma Octantis references pole
[32]
Stellarium Astronomy Software
Stellarium is a planetarium software that shows exactly what you see when you look up at the stars. It's easy to use, and free.Screenshot gallery · Stellarium 25.2 · Stellarium 25.1 · Stellarium 24.1Missing: Sigma Octantis pole planning southern