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.
Written by Jim Kaler. Return to STARS.