Editing Sigma Octantis

{{short description|Star in the constellation Octans}}
{{Starbox begin
 | name = Sigma Octantis
}}
{{Starbox image
| image = 
{{Location mark
 | image=Octans IAU.svg | alt= | float=center | width=250
 | label= | position=right
 | mark=Red circle.svg | mark_width=10 | mark_link=σ Oct
 | x=364 | y=484
}}
| caption=Location of σ Octantis (circled)
}}
{{Starbox observe
 | epoch = [[J2000.0]]
 | equinox = [[J2000.0]] ([[International Celestial Reference System|ICRS]])
 | constell = [[Octans]]
 | ra = {{RA|21|08|46.86357}}<ref name=dr3/>
 | dec = {{DEC|−88|57|23.3983}}<ref name=dr3/>
 | appmag_v = 5.47<ref name=Johnson1966/>
}}
{{Starbox character
 | class = F0&nbsp;IV<ref name=houk1975/>
 | b-v = +0.26<ref name=Johnson1966/>
 | u-b = +0.13<ref name=Johnson1966/>
 | variable = [[Delta Scuti variable|δ Sct]]<ref name=Rodriguez2000/>
}}
{{Starbox astrometry
 | radial_v = +11.9<ref name=Wilson1953/>
 | prop_mo_ra = +26.323<ref name=dr3/>
 | prop_mo_dec = +4.721<ref name=dr3/>
 | parallax = 11.1005
 | p_error = 0.0616
 | parallax_footnote = <ref name=dr3/>
 | absmag_v = {{Val|0.86|0.09}}<ref name=Antonello1997/>
}}
{{Starbox detail
 | source = 
 | mass = 1.59<ref name=David2015/>
 | radius = 4.4<ref name=dr3/>
 | luminosity = 44<ref name=dr3/>
 | temperature = {{Val|7415|252|fmt=commas}}<ref name=David2015/>
 | gravity = 3.71<ref name=David2015/>
 | metal_fe = −0.5<ref name=dr3/>
 | rotational_velocity = 145<ref name=Rodriguez2000/>
 | age_myr = 912<ref name=David2015/>
}}
{{Starbox catalog
 | names = {{odlist | name=Polaris Australis | B=σ&nbsp;Oct | CP=−89°47 | FK5=923 | HD=177482 | HIP=104382 | HR=7228 | SAO=258857 }}<ref name=SIMBAD/>
}}
{{Starbox reference
| Simbad = HD+177482
}}
{{Starbox end}}

'''Sigma Octantis''' is a solitary<ref name=Eggleton2008/> [[star]] in the [[Octans]] [[constellation]] that forms the [[pole star]] of the [[southern celestial hemisphere|Southern Hemisphere]]. Its name is also written as '''σ Octantis''', abbreviated as '''Sigma Oct''' or '''σ Oct''', and it is officially named '''Polaris Australis''' ({{IPAc-en|p|ou|'|l|ɛər|I|s|_|ɔː|'|s|t|r|ei|l|I|s}}).<ref name="IAU-LSN"/> The star is positioned one degree away from the southern [[celestial pole]] of the [[southern celestial hemisphere|Southern Hemisphere]], lying in nearly opposite direction to the [[North Star]] on the [[celestial sphere]].

Located approximately {{val|294|ul=light-years}} from Earth, it is classified as a [[subgiant]] with a [[Stellar classification#Spectral types|spectral type]] of F0&nbsp;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|thumb|left|A [[light curve]] for Sigma Octantis, plotted from ''[[Transiting Exoplanet Survey Satellite|TESS]]'' data<ref name=MAST/>]]

''σ Octantis'' ([[Latinisation of names|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.<ref name=Kaler2006/> In 2016, the IAU organized a [[IAU Working Group on Star Names|Working Group on Star Names]] (WGSN)<ref name="WGSN"/> 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.<ref name="IAU-LSN"/> It is the southernmost named star.

==Properties==
With a spectral class of F0IV,<ref name=houk1975/> Sigma Octantis appears to be a [[subgiant]], although it has also been classified as F0III.<ref name=Malaroda1975/> [[Stellar evolution|Evolutionary models]] place it at the very end of its [[main sequence]] life with an age of about 900 million years.<ref name=dr3/><ref name=David2015/>  It has expanded somewhat to a size 4.4 [[solar radius|that of the Sun]] and emits 44 times as much [[electromagnetic radiation]] from its [[photosphere]] at an [[effective temperature]] of {{val|7,415|fmt=commas|ul=K}}.

Sigma Octantis is a [[Delta Scuti variable]], varying by about 0.03&nbsp;magnitudes every 2.33&nbsp;hours.<ref name=Rodriguez2000/>  It is thought to pulsate only in the [[fundamental mode]].<ref name=ibvs/>

==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 [[Axial precession|precession of the equinoxes]].

[[File:Octans trapezoid.GIF|thumb|left|The south celestial pole. The trapezoidal asterism at the centre includes Sigma Octantis.]]

At magnitude&nbsp;+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]].<ref name=Jumk/>
Because of this, the [[constellation]] [[Crux]] is often preferred for determining the position of the [[Celestial pole|South Celestial Pole]].<ref name=CSIRO/>
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 (astronomy)|asterism]]: Sigma, [[Chi Octantis|Chi]], [[Tau Octantis|Tau]], and [[Upsilon Octantis]] are all stars of around magnitude&nbsp;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 [[Harvard Revised Photometry Catalogue|1908 Revised Harvard Photometry]] catalogue. [[Alpha Ursae Minoris|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."<ref name=rhpc/>

== In culture ==

Sigma Octantis is the dimmest star to be represented on a national flag. It appears on the [[flag of Brazil]], symbolising the [[Federal District (Brazil)|Brazilian Federal District]].<ref name=FOTW/>
{{clear left}}

==See also==
* [[Polar alignment]]
*[[Polaris]]

{{-}}
==References==
{{reflist|refs=

<ref name=CSIRO>{{cite web
 | url         = http://www.csiro.au/helix/sciencemail/activities/crux.html
 | title       = Finding South | publisher   = [[CSIRO]] | date        = 26 July 2013
 | accessdate  = 26 July 2013 | url-status     = dead
 |archiveurl  = https://web.archive.org/web/20130814041220/http://www.csiro.au/helix/sciencemail/activities/crux.html
 |archivedate = 14 August 2013
}}</ref>

<ref name=Jumk>{{cite web
 | url = http://jumk.de/astronomie/special-stars/sigma-octantis.shtml
 | title = Sigma Octantis | publisher = Jumk.De | date = 26 July 2013
 | accessdate = 26 July 2013}}</ref>

<ref name="WGSN">{{cite web
 | url=https://www.iau.org/science/scientific_bodies/working_groups/280/
 | title=IAU Working Group on Star Names (WGSN)|accessdate=22 May 2016}}</ref>

<ref name="IAU-LSN">{{cite web
 | url=https://www.iau.org/public/themes/naming_stars/ |title=Naming Stars
 | publisher=IAU.org |access-date=16 December 2017}}</ref>

<ref name=MAST>{{cite web
 | title=MAST: Barbara A. Mikulski Archive for Space Telescopes
 | url=https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html
 | publisher=Space Telescope Science Institute |access-date=8 December 2021}}</ref>

<ref name=Kaler2006>{{cite book
 | title=The Hundred Greatest Stars
 | first1=James B. | last1=Kaler
 | publisher=Springer Science & Business Media
 | year=2006 | isbn=0387216251 | page=171
 | url=https://books.google.com/books?id=jmoQBwAAQBAJ&pg=PA171 }}</ref>

<ref name=Wilson1953>{{cite journal
 | last=Wilson | first=Ralph Elmer | year=1953
 | title=General catalogue of stellar radial velocities
 | journal=Washington | pages=0
 | publisher=Carnegie Institution of Washington
 | bibcode=1953GCRV..C......0W }}</ref>

<ref name=Antonello1997>{{cite journal
 | title=Luminosity and related parameters of δ Scuti stars from HIPPARCOS parallaxes. General properties of luminosity
 | last1=Antonello | first1=E. | last2=Mantegazza | first2=L.
 | journal=Astronomy and Astrophysics
 | volume=327 | pages=240–244 | date=November 1997
 | bibcode=1997A&A...327..240A }}</ref>

<ref name=Eggleton2008>{{cite journal
 | last1=Eggleton | first1=P. P. | last2=Tokovinin | first2=A. A.
 | title=A catalogue of multiplicity among bright stellar systems
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=389 | issue=2 | pages=869–879 | date=September 2008
 | doi=10.1111/j.1365-2966.2008.13596.x | doi-access=free | bibcode=2008MNRAS.389..869E
 | arxiv=0806.2878 | s2cid=14878976 }}</ref>

<ref name=David2015>{{cite journal
 | last1=David | first1=Trevor J. | last2=Hillenbrand | first2=Lynne A.
 | title=The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets
 | journal=The Astrophysical Journal
 | volume=804 | issue=2 | pages=146 | year=2015
 | bibcode=2015ApJ...804..146D | doi=10.1088/0004-637X/804/2/146
 | arxiv=1501.03154 | s2cid=33401607 }}</ref>

<ref name=Rodriguez2000>{{cite journal
 | display-authors=1 | last1=Rodríguez | first1=E.
 | last2=López-González | first2=M. J. | last3=López de Coca | first3=P.
 | title=A revised catalogue of δ Sct stars
 | journal=Astronomy and Astrophysics Supplement
 | volume=144 | pages=469–474 | date=June 2000
 | issue=3 | doi=10.1051/aas:2000221 | bibcode=2000A&AS..144..469R | doi-access=free | hdl=10261/226673 | hdl-access=free }}</ref>

<ref name=Johnson1966>{{cite journal | display-authors=1
 | last1=Johnson | first1=H. L. | last2=Iriarte | first2=B.
 | last3=Mitchell | first3=R. I. | last4=Wisniewskj | first4=W. Z.
 | title=UBVRIJKL photometry of the bright stars
 | journal=Communications of the Lunar and Planetary Laboratory
 | volume=4 | issue=99 | year=1966
 | page=99 | bibcode=1966CoLPL...4...99J }}</ref>

<ref name=FOTW>{{cite web
 | title=Astronomy of the Brazilian Flag
 | url=https://flagspot.net/flags/br_astro.html
 | publisher=FOTW Flags Of The World website }}</ref>

<ref name=SIMBAD>{{cite simbad
 | title=sig Oct | accessdate=2017-08-10 }}</ref>

<ref name=dr3>{{cite Gaia DR3|6341181494973204096}}</ref>

<ref name=rhpc>{{cite journal
 | title=Revised Harvard Photometry
 | last1=Pickering | first1=Edward Charles
 | journal=Annals of the Astronomical Observatory of Harvard College
 | volume=50 | pages=2 | year=1908
 | bibcode=1908AnHar..50....1P }}</ref>

<ref name=houk1975>{{cite journal
 | title=Michigan catalogue of two-dimensional spectral types for the HD stars
 | volume=1 | last1=Houk | first1=Nancy
 | journal=University of Michigan Catalogue of Two-Dimensional Spectral Types for the Hd Stars. Volume I. Declinations -90_ to -53_Ƒ0 | location=Ann Arbor, Michigan
 | publisher=Dept. of Astronomy, University of Michigan
 | year=1975 | bibcode=1975mcts.book.....H }}</ref>

<ref name=ibvs>{{cite journal |bibcode=1982IBVS.2084....1T |title=On the Radial Pulsations of the delta Scuti Stars sigma Octantis and B Octantis |last1=Tsvetkov |first1=Ts. G. |journal=Information Bulletin on Variable Stars |year=1982 |volume=2084 |page=1 }}</ref>

<ref name=Malaroda1975>{{citation
 | last=Malaroda | first=S.
 | title=Study of the F-type stars. I. MK spectral types
 | journal=Astronomical Journal | postscript=.
 | volume=80 | pages=637–641 |date=August 1975
 | doi=10.1086/111786 | bibcode=1975AJ.....80..637M | doi-access=free }}</ref>
 
}}

{{Pole star}}
{{Stars of Octans|state=collapsed}}

{{Authority control}}

{{DEFAULTSORT:Sigma Octantis}}
[[Category:Delta Scuti variables]]
[[Category:F-type subgiants]]
[[Category:Southern pole stars]]
[[Category:Stars with proper names|Polaris Australis]]
[[Category:Bayer objects|Octantis, Sigma]]
[[Category:Octans]]
[[Category:Durchmusterung objects]]
[[Category:Henry Draper Catalogue objects|177482]]
[[Category:Hipparcos objects|104382]]
[[Category:Bright Star Catalogue objects|7228]]