Sigma Octantis is a star of magnitude 5.47 in the constellation Octans, the Octant. It is most notable for being the current south pole star, and is sometimes called Polaris Australis, though this name is certainly a modern invention.
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 the southern-hemisphere counterpart to Polaris, the current North Star. However, Sigma Octantis is 25 times fainter than Polaris, and barely visible to the naked eye. In fact, Sigma Octantis is the dimmest star to be represented on a national flag, symbolizing the capital city Brasilia on the flag of Brazil.
Precession and Observation
σ Oct 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. Its minimum separation from the true southern pole, about 3/4°, occurred around 1872.
Because of the faintness of σ Oct, the constellation Crux (the Southern Cross) is often preferred for determining the position of the South Celestial Pole. Once σ Oct's approximate position has been determined by using the Southern Cross, it can be verified using the trapezoid-shaped asterism of σ, χ, τ, and υ Oct - all stars of around magnitude 5.5.
Properties, Evolution, and Variability
Sigma Octantis is approximately 270 light years from Earth, and is classified as an F0 III giant. With a temperature estimated at 7280 K, the star has a luminosity of 34 suns, and a radius only 3.7 times solar. The star's metal content seems high, perhaps as much as 1.8 times the Sun's. An equatorial rotation velocity of at least 128 km/sec gives it a rotation period of under 1.5 days. The star contains 2.0 to 2.1 solar masses; it is close to finishing hydrogen fusion, and is just beginning to expand into a true giant. This is consistent with its still-high rotational velocity, as expansion slows rotation.
σ Oct is also a δ Scuti variable, varying by about 0.03 magnitudes over 2.3 hours. δ Scuti stars are lower-level dwarf/subgiant/giant versions of genuine supergiant Cepheids. Thus in many ways, the star is thus really more a subgiant than a true giant.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]