Movatterモバイル変換

[0]ホーム

DELTA SGE (Delta Sagittae). Delta Sagittae (of no proper name), isa fourth magnitude (on the bright side of fourth, 3.82) reddishstar smack in the middle of the classical figure ofSagitta, the Arrow. It is best known asa closedouble (separated byspectroscopy) that consists of aclass M (M2) brightgiant anda much more modest class A (A0, possible B9.5) ordinary hydrogen-fusing dwarf in a 3725 day (10.20 year) orbit. The duplicity andthe cool, fuzzy "surface" of the giant leads to considerableuncertainty in deriving stellar parameters. Directparallax gives a distance of448 light years. A rather uncertain temperature of 3630 Kelvin, toaccount for invisible infrared light (and a three percentcorrection for dimming by interstellar dust), leads to a totalluminosity of about 2800 times that of theSun. Ignoring the much dimmer dwarf leads toa mass roughly three times that of the Sun. The angular diameterdepends on the wavelength of observation, but the best measure givea radius of 125 times that of the Sun (60 percent the size ofEarth's orbit). Radius and temperature then conspire to give aluminosity 2400 times solar, so we are clearly not that far off. However, not only do we have spectroscopic orbital velocitiesavailable, but also (in spite of the closeness) we have directmeasures of the orbit from sophisticated interferometry. Analysisof both kinds of data gives a mean separation of 8.8 AstronomicalUnits, respective masses of 3.8 and 2.9 solar for the M giant andA dwarf, respective radii of 152 and 2.6 times that of the Sun, anda significantly larger distance of 565 light years. (From thedwarf, the giant would be appear some 14 times bigger than does theSun in our sky.) These then give respective luminosities of about4500 and 60 Suns, about right for the class. The M giant isclearly highly evolved, and is most likely in thesecond stage of gianthood witha dead carbon-oxygen core. Typically, the star is unstable andvariable, changing by a couple tenths of a magnitude over periodsof 25 and 1000 days.

The visual variations of Delta Sagittae are plotted against theJulian date from March 10, 1986, to November 17, 1999. Add 2440000for a running count of days from January 1, 4713 BC. (From J. R.Percy, J. B. Wilson, and G. W. Henry in thePublications of theAstronomical Society of the Pacific, vol 113, p. 983, 2001.)

A mass loss rate of about a hundredth of a millionth of a solarmass per year (hundreds of thousands of times that of the Sun) willsoon accelerate, the lost envelope converting the star into amodestwhite dwarf witha mass three-quarters that of the current Sun.

Written byJim Kaler 6/23/07. Return toSTARS.

[8]ページ先頭