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.