NU OCT (Nu Octantis). As if fourth magnitude (3.76) Nu Octantis,
the luminary of Octans (the Octant, one of
the sky's several navigational
instruments), is not already obscure enough for being in such
a dim constellation, it sinks further into obscurity by being quite
overwhelmed by the fame of the constellation's best-known star, the
fifth magnitude (5.47, nearly sixth) southern pole star, Sigma
Octantis. Way down the Greek letter list,
Nu Oct oddly beats out the next star down, fifth magnitude (4.15)
Beta Oct, while the Alpha star, usually a constellation's
brightest, is a full magnitude (5.15) fainter yet. In spite of its
seeming anonymity, which is enhanced by being yet one more yellow-
orange class K (K0) giant, Nu
Oct harbors a pleasant surprise by providing us with a wonderful
chance to see what is going to happen to the Sun. For a giant, its character is modest
indeed. As befits this humble star, there is no temperature
measure, so 4700 Kelvin must be adopted from its K-giant class.
From its distance of 69 light years (at least we have that much),
the apparent magnitude, and the amount of infrared radiation
implied by temperature, we calculate a luminosity only 17 times
that of the Sun, making Nu Oct one of the least radiant of
celestial giants. Consistently, Nu Oct is relatively small, its
remarkably low radius of 6.3 times solar hardly befitting a star
called a "giant." The reasons involve mass and evolutionary
status. From the theory of stellar structure and evolution, Nu
Octantis weighs in at almost exactly one solar mass (perhaps just
a bit more). After giving up hydrogen fusion in its core some two
billion years ago, it began to swell some and to cool as a
subgiant. It is now in the process of expanding much more
vigorously and brightening. In a sense we are watching the "birth"
of a genuine giant, one that in "just" 100 million years or so will
become more than 60 times brighter and 15 times larger than it is
today (before it settles down to fuse its helium into carbon and
oxygen). Assuming a solar mass, the star's current age would come
in at 12.1 billion years (less if greater mass).
Given that the Sun has a well-determined
age (from radioactive dating of meteorites) of 4.6 billion years,
Nu Oct provides a brilliant lesson in what our Sun will look like
in another 7.5 billion. But in Nu Octantis's case, we probably
need not worry about what is going to happen to its planets, as the
star has a binary companion
close enough to it to make a planetary system unlikely (though
given the odd planetary systems astronomers
are finding, who knows?). The dim companion, about which nothing
is known, orbits with a period of 2.8 years and lies somewhere
between one and two Astronomical Units from Nu Oct proper (that is,
within the orbital radii of Earth and Mars). The binarity,
however, does not preclude the star's real story. If you can't
yourself wait another 7.5 billion years to follow the solar tale,
look to the luminary of the Octant.