DELTA EQU (Delta Equulei). Many are the bright luminous stars, those that make the outlines of the constellations, in general much more brilliant in absolute terms than is the Sun. Stars are so far away that they have to be luminous for us to see them without optical aid. Few are the naked eye stars that have less radiative power than our own star. To see the huge numbers of lesser lights -- like Proxima Centauri -- we need telescopes. It is then with some fascination that we see a few stars that even approach the solar luminosity, and here is not one, but two, Delta of Equuleus being a close pair of stars not much above a solar mass, a double-Sun of sorts, one a class F5 hydrogen-fusing dwarf, the other G0 (the Sun G2), with respective temperatures of roughly 6600 and 6000 Kelvin. Combined, the two shine at almost fifth magnitude (4.49) within their dim constellation even though they are but 60 light years away, each one contributing about half the combined light. Orbiting closely with a period of only 5.7 years, Delta Equ was (until the advent of modern technology) the visual double (one separated by eye at the telescope) with the shortest-known period, and one of the only ones observed both visually and with the spectrograph. (Nearby is a 10th magnitude "C" component that is just a line-of sight coincidence.) There is some disagreement of the separation, different studies placing them 4.3 to 4.7 Astronomical Units apart (just short of Jupiter's 5.2 AU distance from the Sun). Delta Equ's binary orbit, which is rather eccentric and that carries the two stars 1.44 times farther apart than average and then to 56 percent of average, gives masses of 1.17 and 1.22 solar in the first study, 1.59 and 1.66 in the other. In either case, the luminosities come in at only 2.17 and 2.23 solar. The luminosities and temperatures combined with the theory of evolution predict masses in good accord with those derived from the orbit. Science fiction writers seem intrigued by what life would be like on a planet orbiting a double star. Can such happen? Indeed, in the case of the wide double 16 Cygni it does, as a planet is known to orbit the "B" component. But here are two stars quite close together. Studies show that a planet could orbit either of the two, or be in a distant orbit about the pair. If we pick the larger masses, a planet could have a stable orbit within 0.66 AU of either of the stars, taking just under half a year to orbit. Its inhabitants would then see the other as a brilliant second Sun that when above the horizon would make "night" impossible. A planet could also be in a stable 37-year orbit if it were to be at least 16 AU from the stellar pair's orbital center. Though icy cold, an astronaut on such a distant planet would see a pair of suns going through their own orbit, making quite the sight as they first pass close to each other and then stretch to up to 15 degrees apart, each looking like a brilliant star.
Written by Jim Kaler. Return to STARS.