ZETA DEL (Zeta Delphini). After the Summer Triangle of Deneb, Vega, and Altair, one of the great treats of the northern summer sky is the string of four small constellations that hover near them. From Albireo at the head of Cygnus (the foot of the Northern Cross), look to the southeast and north of Altair to find modern and obscure Vulpecula (the Fox), then ancient Sagitta (the arrow, which quite looks like an arrow), Delphinus (the Dolphin, which looks like a hand with a finger pointing more or less south), and finally faint Equuleus (the Little Horse, the big Horse, Pegasus, to the northeast of it). Not quite part of Delphinus's traditional five- star grouping (made of Alpha through Epsilon), fifth magnitude (4.68) Zeta Delphini's position just half a degree almost due west of the constellation's luminary, fourth magnitude (almost third) Rotanev (Beta Del), gives it a place of some prominence. At a distance of 220 (give or take 4) light years and at the edge of the Milky Way, the white class A (A3) hydrogen fusing dwarf is dimmed by about 10 percent by intervening interstellar dust. With a surface temperature of 8450 Kelvin, the great bulk of the star's radiation lies in the optical spectrum, so little further correction is needed to calculate a luminosity of 53 times that of the Sun and a consequent radius of 3.5 times solar. The theory of stellar structure and evolution then give it a mass of 2.4 times solar and show that it is near the end of its 630 or so million year hydrogen fusing lifetime. There seem to be no companions. The star's most outstanding feature is its rather rapid equatorial rotation speed of at least 108 kilometers per second, which gives it a rotation period of under 1.6 days. The speed is enough to keep the stellar atmosphere stirred up to the point that there is no separation of elements (due to radiative lofting and gravitational settling) as is common in similar stars that spin more slowly. The metal content is pretty close to the solar value. If nothing else, Zeta Del reminds us just how many class A dwarfs there are in the sky, their brightness making them seem far to exceed their actual numbers. Remove them and the constellation patterns would be greatly altered, though Delphinus would survive, as among its brightest seven stars, Zeta is the only one of its class A dwarf kind. Oddly, Zeta Sagittae next door is also an A3 dwarf. There are no Zeta stars in either Vulpecula (which stops at Alpha) or Equuleus (Epsilon).

Written by Jim Kaler 11/8/13. Return to STARS.