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.