Movatterモバイル変換
[0]ホーム
GAMMA DOR (Gamma Doradus).Dorado, theSwordfish, extends from deep southern skies almost to the southpole itself. Only its most northerly stars are visible fromlatitudes above 30 degrees north. Of these, fourth magnitude(4.25) Gamma Dor, ranking number three in the constellation, is themost northerly. Relatively close to us, only 66 light years away,Gamma Dor outwardly appears as a modest class F (F4) giant with asurface temperature of 7000 Kelvin, a modest luminosity only 6times that of theSun, and a radius a mere1.6 solar, not much for a purported giant star. In fact, theluminosity and temperature tell of a 1.6 solar mass star that hasactually yet to become a giant, and is still fusing hydrogen intohelium in its core. It also falls into the minority ranks ofsingle stars, having no companion yet detected. While appearingunassuming, the star harbors a marvelous secret that has made itfamous. A large number of the set of dwarf stars that fall into atemperature region between 7500 and 8500 Kelvin (and extendingupward to lower temperature and higher luminosity) subtly vary bya few hundredths of a magnitude with multiple periods. CalledDelta Scuti stars, after the prototype, they are most brightlyrepresented byCaph, Beta Cassiopeiae. They are low mass versions of the famed Cepheid variables,represented byDelta Cephei,Eta Aquilae, andMekbuda (Zeta Geminorum). A group ofdwarfs and small giants that seem to have similar properties fallsin a region of lower temperature and luminosity where they seem tohave no business varying, but they do, but with notably longerperiods. Chief among the dozen or so known is Gamma Doradus, andthe group is therefore known as the Gamma Dor stars. The mechanismfor variation is different than it is for the Delta Scuti stars,and it is possible for a star to be both. Gamma Dor itselfchatters away over a range of a few hundredths of a magnitude withtwo closely spaced periods of 17.5 and 18.1 hours. Though notablycloser, Gamma Dor lies close in the sky toAlpha Horologii, the two providing a fineexample of the contrasting styles of stars in the nightly sky.
[8]ページ先頭