ASMIDISKE (Xi Puppis). This star, easy to find in Puppis (the Stern of Argo) just to the east of the bright triangle that makes the lower portion of Canis Major, epitomizes the confusion that attends to star names. Given Greek letter Xi by the explorers of the southern celestial hemisphere, its proper name Asmidiske was both mis-spelled and improperly stolen from "Aspidiske" (from Greek, meaning "little shield"), which belongs to Iota Carinae (of Carina, Argo's Keel). Asmidiske makes up for this minor insult by being a magnificent, rather rare, kind of supergiant of sunlike color. Originally classed as a "warm G" (G3) lesser supergiant, it is now considered a cooler G (G6, temperature 4990 Kelvin) brighter supergiant. The star is so far away that its distance is uncertain, but it lies in the neighborhood of 1350 light years, making it 8300 times more luminous than the Sun, from which we calculate a radius 120 times solar, or 0.57 Astronomical Units (which would take the star between the orbits of Mercury and Venus. Asmidiske is rather rapidly evolving, though it is not possible to tell just how. It may be a 10 solar mass star with a dead helium core that is growing and cooling a its surface to become a red supergiant, or it may be an 8 solar mass core-helium burner that has already been a red supergiant and is "looping back" to higher temperatures. Even then, we cannot tell whether it is in a (surface) warming or cooling phase. Whatever its internal condition, it is just to the cool side of being a Cepheid variable, a pulsating star like Delta Cephei or Eta Aquilae. It may already have been one, or it may (on an astronomical time scale) shortly become one. Given the star's great luminosity, the pulsation period will be (or was) rather long, around 20 days. The star is reputed to be "super-metal- rich," containing 60 percent more metals (relative to dominant hydrogen) than the Sun. There is also some evidence that Asmidiske is accompanied by a close-in companion that lies about 2 Astronomical Units away and orbits with a period of a year. Farther out, about 5 seconds of arc away, is a thirteenth magnitude star that is very much like the Sun and that orbits at least 2000 AU away and takes at least 26,000 years to make a full circuit around the inner pair. From Asmidiske proper, the sunlike companion would be about as bright as a gibbous Moon, while from the sunlike companion, Asmidiske would shine with the light of 1000 full moons, the close inner companion 3 minutes of arc away at best.
Written by Jim Kaler. Return to STARS.