Sigma Octantis

Starbox begin
name=Sigma Octantis Starbox image


caption=Octans and around, with Sigma Octantis in the middle marked in cyan.
(Click on the image to see it full size.) Starbox observe
epoch=J2000
ra=21^h 08^m 46.8^s
dec=-88° 57′ 23″
appmag_v=5.42
constell=Octans Starbox character
class=F0 III
b-v=0.27
u-b=0.13
variable=Delta Scuti Starbox astrometry
radial_v=+11.9
prop_mo_ra=25.96
prop_mo_dec=5.02
parallax=12.07
p_error=0.48
dist_ly=270
dist_pc=83
absmag_v=0.83 Starbox catalog
names=Polaris Australis, HR 7228, HD 177482, SAO 258857, FK5 923, HIP 104382. Starbox reference
Simbad = HD+177482

Sigma Octantis (σ Oct / σ Octantis) is a magnitude 5.6 star in the constellation Octans most 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 capital city Brasilia. [cite web
title=Astronomy of the Brazilian Flag
url=http://www.fotw.net/flags/br_astro.html
publisher=FOTW Flags Of The World website] Its position near the southern celestial pole makes it the southern hemisphere's pole star.

outhern Pole Star

Sigma Octantis is the southern pole star, whose counterpart to 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.

Sigma Octantis is sometimes called Polaris Australis, or South Star, though it has never been officially recognised as such and this name is certainly a modern invention.

Locating Sigma Octantis

At magnitude 5.5, Sigma Octanis is barely visible to the naked eye, making it a rather poor pole star, especially by comparison with the much brighter and more easily visible Polaris. Because of this, the Crux constellation is often preferred for determining the position of the South Celestial Pole.

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 Octantis, Kappa Octantis, Tau Octantis and Upsilon Octantis are all stars of around magnitude 5.5, and form the distinctive shape of a trapezoid.

References

* cite web | title=HD 177482 -- Variable Star | work=SIMBAD Astronomical Database | url=http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=Sigma+Octantis&NbIdent=1&Radius=10&Radius.unit=arcmin&CooFrame=FK5&CooEpoch=2000&CooEqui=2000&output.max=all&o.catall=on&output.mesdisp=N&Bibyear1=1983&Bibyear2=2005&Frame1=FK5&Frame2=FK4&Frame3=G&Equi1=2000.0&Equi2=1950.0&Equi3=2000.0&Epoch1=2000.0&Epoch2=1950.0&Epoch3=2000.0
accessdate=2005-11-05