DELTA TRI (Delta Trianguli). Tucked as it is just above Gamma Trianguli (in Triangulum, the Triangle), fainter Delta Tri is not
given much attention, except that with 7 Tri (just southwest of
Gamma) it helps make a line-of-sight triple star, the three having
nothing to do with each other. At fifth magnitude (4.87), Delta is
roughly midway in brightness between the other two. It is,
however, by far the closest of them, lying a mere 35 light years
from Earth. Its relative faintness is the result of the intrinsic
low luminosity of a solar type star, actually two of them, one
shining visually at magnitude 5.2, the other at 6.5, 3.3 times
fainter (the two inseparable by eye). In close orbit about each
other every 10.0201 days, these two suns, each less luminous than
our own Sun, is a science fiction writer's
dream. The primary star, like the Sun, is class G (G0, at 5900
Kelvin somewhat warmer than our G2, 5780 Kelvin Sun), the other
around G9 (5300 Kelvin). Both are fairly young, hydrogen-fusing
dwarfs. Their total luminosities are respectively 0.8 and 0.3
times that of the Sun (the allowance for invisible infrared
radiation making the secondary star a bit more respectable), their
radii 0.86 and 0.64 solar, their masses (estimated from
luminosities and temperatures) 1.0 and 0.8 solar. Other analyses
give somewhat different results. While the pair cannot be split
visually, sophisticated interferometer observations have
established the orbit. They move about each other in circular
paths at a separation of 0.106 Astronomical units, only 28 percent
Mercury's distance from the Sun. From the orbit, their combined
masses are 1.60 times that of the Sun, somewhat less than the 1.8
solar found from their evolutionary status. The brighter may be as
much as 6 times as visually luminous as the fainter, which would
lessen the mass of the secondary and bring the two estimates of
total mass into better agreement. The system is rather like Alpha Centauri, except here a planet
(were there one) would have to orbit the pair rather than the
individuals (as the components of the Alpha Cen system are widely
separated). To achieve the same irradiance we have on Earth, an
orbiting planet would have to lie at a distance only four percent
farther than we are from the Sun. From there we would see two
yellow suns looping around each other, separated by as much as 6
degrees (just a bit greater angle than separates the front bowl
stars of the Big Dipper). Given a 24 hour rotation period of our
hypothetical planet, a person at the equator would see one rise or
set at most half an hour after the other. A third star, of 14th
magnitude, lies a minute of arc away from the pair. Alas, at a
distance of 360 light years, 10 times farther than Delta Tri, the
alignment is a mere coincidence.