Sigma Octantis

Sigma Octantis is a solitary star in the Octans constellation that forms the pole star of the Southern Hemisphere. Its name is also written as σ Octantis, abbreviated as Sigma Oct or σ Oct, and it is officially named Polaris Australis (pou'lɛərIs_ɔː'streilIs). The star is positioned one degree away from the southern celestial pole of the Southern Hemisphere, lying in nearly opposite direction to the North Star on the celestial sphere. Located approximately 294light-years from Earth, it is classified as a subgiant with a spectral type of F0 III. Sigma Octantis has an apparent magnitude of 5.5, but is slightly variable and is classified as a Delta Scuti variable. σ Octantis (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. In 2016, the IAU organized a Working Group on Star Names (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. It is the southernmost named star. With a spectral class of F0IV, Sigma Octantis appears to be a subgiant, although it has also been classified as F0III. Evolutionary models place it at the very end of its main sequence life with an age of about 900 million years. It has expanded somewhat to a size 4.4 that of the Sun and emits 44 times as much electromagnetic radiation from its photosphere at an effective temperature of 7,415K. Sigma Octantis is a Delta Scuti variable, varying by about 0.03 magnitudes every 2.33 hours. It is thought to pulsate only in the fundamental mode. 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.