DELTA DEL (Delta Delphini). Delphinus,
the Dolphin, looks more like a celestial hand with its finger
pointing to the southwest toward Aquila
and Sagittarius. In spite of its Greek
letter (no proper name), fourth magnitude (4.43) Delta Del ranks
fifth in brightness in the constellation, following Beta, Alpha, Gamma, and
Epsilon. Bayer probably named
the stars of the exquisite box that makes the hand first before
going on to Epsilon Del. Delta is not one star, however, but two that orbit every 40.58 days,
its identical members too close together to see separately, the
binarity detectable only spectroscopically. Though
officially classed as a type A (A7) giant star, its anomalously low
temperature (probably of both stars) of 7000 Kelvin fits more with
a secondary classification as an F0 subgiant, the difference the
result of an odd metal abundance. From the binary's distance of
203 light years, we deduce a combined luminosity of 51 times that
of the Sun, and given the similarity of the
stars, similar individual luminosities of 25.5 solar. Luminosity
and temperature in turn lead to radii of 3.4 solar, masses of 2
solar, and a confirmation that the stars are really class F
subgiants that have just ended core hydrogen fusion (or at least will
shortly), with ages just over a billion years. Given the 40-plus
day period, they must be only 0.37 Astronomical Units apart, about
Mercury's distance from the Sun. They are both slightly variable.
One (arbitrarily called "B") seems to be a Delta Scuti type star, one that chatters
away by about a tenth of a magnitude with multiple periods of
0.156, 0.136, and 0.153 days. The other ("A") is more a
straightforward single-period subtle variable with a period of 0.10
days (separating the two quite difficult). Delta Del's most
prominent claim to fame is its role as prototype of a technical
subclass of modest "metallic line stars" that have somewhat
peculiar chemical compositions caused by diffusion of elements in
stable stellar atmospheres. Unfortunately for our star, the term
is now seldom ever invoked. While the pair seems destined to
become a simple double white
dwarf, the two will probably interfere with each other during
their giant evolution, making it difficult, if not impossible, to
predict their exact fates.
Written by Jim Kaler 8/03/07. Return to STARS.