POLARIS AUSTRALIS (Sigma Octantis). Rivals, but no match for each other, are Polaris, the northern pole star, and Polaris Australis, which lies at the southern celestial pole. In the constellation of Octans (the Octant) and named in modern times, it is better known as Sigma Octantis, after the 18th letter of the Greek alphabet. At the dim end of fifth magnitude (5.47, nearly sixth), appearing 25 times fainter than Polaris, the southern version is little discussed or examined. A close look, however, reveals a neat symmetry between the two, as both are class F subtly-pulsating variable stars: Polaris class F7, Sigma Oct hotter, F0; Polaris an oddball Cepheid pulsator, "Australis" a Delta Scuti star. There the similarity ends, however, as Polaris is a more distant (430 light years) luminous (2500 times solar) supergiant, while closer (270 light years) Sigma struggles just to be a giant. 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 much for a star called a "giant." An equatorial rotation velocity of at least 128 kilometers per second gives way to a rotation period of under 1.5 days. Theory then tells of a star of 2.0 to 2.1 solar masses that has either just given up hydrogen fusion (or will do so shortly) and is just beginning to expand into a "real" giant (consistent with the still- large rotation velocity, as expansion slows rotation). The star is thus really more a subgiant than a true giant. The star's metal content seems high, perhaps as much as 1.8 times that of the Sun. Delta Scuti stars are lower-level dwarf-subgiant-giant versions of genuine supergiant Cepheids like Delta Cephei that pulsate by a few hundredths to a few tenths of a magnitude with multiple periods of under a day. Only one such period has been identified for Sigma Oct, a variation of 0.03 magnitudes (about 3 percent) over 2.3 hours. The two polar markers share similar offsets from their respective poles, Polaris at 3/4 degree, Sigma at just over a full degree. However, as a result of precession (the 26,000 year wobble of the Earth's axis), Polaris is getting closer to the North Celestial Pole (rather the other way around) and will pass at a minimum angle of 1/4 degree in 2105, while Sigma Oct is moving away, its minimum separation from the Southern Pole of about 3/4 degree having been passed around 1872. (Sigma Octantis is featured in Jim Kaler's " The Hundred Greatest Stars".)
Written by Jim Kaler. Return to STARS.