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.
Written by Jim Kaler. Return to STARS.