GAMMA DOR (Gamma Doradus). Dorado, the
Swordfish, extends from deep southern skies almost to the south
pole itself. Only its most northerly stars are visible from
latitudes above 30 degrees north. Of these, fourth magnitude
(4.25) Gamma Dor, ranking number three in the constellation, is the
most northerly. Relatively close to us, only 66 light years away,
Gamma Dor outwardly appears as a modest class F (F4) giant with a
surface temperature of 7000 Kelvin, a modest luminosity only 6
times that of the Sun, and a radius a mere
1.6 solar, not much for a purported giant star. In fact, the
luminosity and temperature tell of a 1.6 solar mass star that has
actually yet to become a giant, and is still fusing hydrogen into
helium in its core. It also falls into the minority ranks of
single stars, having no companion yet detected. While appearing
unassuming, the star harbors a marvelous secret that has made it
famous. A large number of the set of dwarf stars that fall into a
temperature region between 7500 and 8500 Kelvin (and extending
upward to lower temperature and higher luminosity) subtly vary by
a few hundredths of a magnitude with multiple periods. Called
Delta Scuti stars,
after the prototype, they are most brightly
represented by Caph, Beta Cassiopeiae.
They are low mass versions of the famed Cepheid variables,
represented by Delta Cephei, Eta Aquilae, and Mekbuda (Zeta Geminorum). A group of
dwarfs and small giants that seem to have similar properties falls
in a region of lower temperature and luminosity where they seem to
have no business varying, but they do, but with notably longer
periods. Chief among the dozen or so known is Gamma Doradus, and
the group is therefore known as the Gamma Dor stars. The mechanism
for variation is different than it is for the Delta Scuti stars,
and it is possible for a star to be both. Gamma Dor itself
chatters away over a range of a few hundredths of a magnitude with
two closely spaced periods of 17.5 and 18.1 hours. Though notably
closer, Gamma Dor lies close in the sky to Alpha Horologii, the two providing a fine
example of the contrasting styles of stars in the nightly sky.