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".)