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.