ANKAA (Alpha Phoenicis). Well into the southern hemisphere lies the modern constellation Phoenix, the mythical Phoenix or Firebird, named by southern hemisphere explorers and first noted in Bayer's Uranometria. The name of the bright second magnitude (2.39) luminary derives from a late application of the Arabic for the marvelous bird. Phoenix has a longer history, however, as the stars in the area were considered by the Arabs to be a "boat," giving Ankaa a second name, "Nair al Zaurak," the "bright one of the boat," a portion of the name ("Zaurak") now applied to Gamma Eridani. Ankaa, a class K (K0) giant, is among the common kinds of stars that make so much of the constellation patterns. At a distance of 77 light years, it radiates 86 solar luminosities with a soft yellow-orange light from a surface estimated to have a temperature of about 4800 Kelvin (no accurate determinations are readily available). With a mass of around 2.5 times that of the Sun and a radius 13 times solar, the star seems to be in its short-term but stable core-helium fusing stage, from which it will eventually become a brighter and larger red giant, at which point it will expel its outer shell and become a tiny white dwarf, the fate of all such "K giants." In spite of its large size, Ankaa has a rotation speed similar to (and maybe even greater than) that of the Sun (about 2 kilometers per second at the equator), a legacy of the star's origin as a hot class A or even a class B star (these classes fast spinners). Even so, Ankaa might take up to a year to make a full rotation, the spin responsible for some activity that makes the star a modest X-ray source. Ankaa's space velocity, however, is high, 88 kilometers per second relative to the Sun. The star's future behavior will be witnessed by a low-mass companion about which nothing is known, one of the few that is detected in two ways, by the spectrograph through the orbital velocity it imposes on the bright star, and directly. Orbiting eccentrically at an average distance of some 7 astronomical units (35 percent farther than Jupiter is from the Sun), the companion takes 10.5 years to make a full circuit.
Written by Jim Kaler. Return to STARS.