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.