DELTA SGE (Delta Sagittae). Delta Sagittae (of no proper name), is a fourth magnitude (on the bright side of fourth, 3.82) reddish star smack in the middle of the classical figure of Sagitta, the Arrow. It is best known as a close double (separated by spectroscopy) that consists of a class M (M2) bright giant and a much more modest class A (A0, possible B9.5) ordinary hydrogen- fusing dwarf in a 3725 day (10.20 year) orbit. The duplicity and the cool, fuzzy "surface" of the giant leads to considerable uncertainty in deriving stellar parameters. Direct parallax gives a distance of 448 light years. A rather uncertain temperature of 3630 Kelvin, to account for invisible infrared light (and a three percent correction for dimming by interstellar dust), leads to a total luminosity of about 2800 times that of the Sun. Ignoring the much dimmer dwarf leads to a mass roughly three times that of the Sun. The angular diameter depends on the wavelength of observation, but the best measure give a radius of 125 times that of the Sun (60 percent the size of Earth's orbit). Radius and temperature then conspire to give a luminosity 2400 times solar, so we are clearly not that far off. However, not only do we have spectroscopic orbital velocities available, but also (in spite of the closeness) we have direct measures of the orbit from sophisticated interferometry. Analysis of both kinds of data gives a mean separation of 8.8 Astronomical Units, respective masses of 3.8 and 2.9 solar for the M giant and A dwarf, respective radii of 152 and 2.6 times that of the Sun, and a significantly larger distance of 565 light years. (From the dwarf, the giant would be appear some 14 times bigger than does the Sun in our sky.) These then give respective luminosities of about 4500 and 60 Suns, about right for the class. The M giant is clearly highly evolved, and is most likely in the second stage of gianthood with a dead carbon-oxygen core. Typically, the star is unstable and variable, changing by a couple tenths of a magnitude over periods of 25 and 1000 days.
Delta Sge The visual variations of Delta Sagittae are plotted against the Julian date from March 10, 1986, to November 17, 1999. Add 2440000 for a running count of days from January 1, 4713 BC. (From J. R. Percy, J. B. Wilson, and G. W. Henry in the Publications of the Astronomical Society of the Pacific, vol 113, p. 983, 2001.)
A mass loss rate of about a hundredth of a millionth of a solar mass per year (hundreds of thousands of times that of the Sun) will soon accelerate, the lost envelope converting the star into a modest white dwarf with a mass three-quarters that of the current Sun.
Written by Jim Kaler 6/23/07. Return to STARS.